Central ring structure identified in one of the world's best-preserved impact craters - Gebhardt, A.C.; Niessen, F.; Kopsch, C. (Geology, 2006)
Seismic refraction and reflection data were acquired in 2000 and 2003 to study the morphology and sedimentary fill of the remote El'gygytgyn crater (Chukotka, northeastern Siberia; diameter 18 km). These data allow a first insight into the deeper structure of this unique impact crater. Wide-angle data from sonobuoys reveal a five-layer model: a water layer, two lacustrine sedimentary units that fill a bowl-shaped apparent crater morphology consisting of an upper layer of fallback breccia with P-wave velocities of ∼3000 m/s, and a lower layer of brecciated bedrock (velocities >3600 m/s). The lowermost layer shows a distinct anticline structure that, by analogy with other terrestrial and lunar craters of similar size, can be interpreted as a central ring structure. The El'gygytgyn crater exhibits a well-expressed morphology that is typical of craters formed in crystalline target rocks. © 2006 Geological Society of America.
cited By 38
Keywords: Central ring structure; El' gygytgyn; Impact crater; Seismic reflection; Seismic refraction; Anticline structure; Crystalline target rocks; El' gygytgyn; Five layer models; Impact craters; Sedimentary units; Seismic reflections; Seismic refraction; Crystalline rocks; Geologic models; Geomorphology; Lakes; Sedimentology; Seismic waves; Seismology; Crystalline rocks; Lakes; Morphology; Refraction; Sedimentology; Seismic waves; Structural geology; Seismology; impact structure; seismic reflection; seismic refraction; bedrock; breccia; caldera; crater; crater lake; fill; lacustrine deposit; P-wave; preservation; seismic reflection; seismic refraction; Chukchi; Elgygytgyn Lake; Russian Federation

Dynamics of explosive volcanism at Unzen volcano: An experimental contribution - Scheu, B.; Spieler, O.; Dingwell, D.B. (Bulletin of Volcanology, 2006)
Knowledge of the dynamics of magma fragmentation is necessary for a better understanding of the explosive behaviour of silicic volcanoes. Here we have measured the fragmentation speed and the fragmentation threshold of five dacitic samples (6.7-53.5 vol% open porosity) from Unzen volcano, Kyushu, Japan. The measurements were carried out using a shock-tube-based fragmentation apparatus modified after Alidibirov and Dingwell (1996a,b). The results of the experimental work confirm the dominant influence of porosity on fragmentation dynamics. The velocity of the fragmentation front increases and the value of the fragmentation threshold decreases with increasing porosity. Further, we observe that the fragmentation speed is strongly influenced by the initial pressure difference and the texture of the dacite. At an initial pressure difference of 30 MPa, the fragmentation speed varies from 34 m/s for the least porous sample to 100 m/s for the most porous sample. These results are evaluated by applying them to the 1990-1995 eruptive activity of Unzen volcano. Emplacements of layered lava dome lobes, Merapi-type pyroclastic flows and minor explosive events dominated this eruption. The influence of the fragmentation dynamics on dome collapse and Vulcanian events is discussed. © Springer-Verlag 2006.
cited By 51
Keywords: explosive volcanism; fragmentation; lava dome; volcanology; Asia; Eurasia; Far East; Japan; Kyushu; Nagasaki [Kyushu]; Unzen Volcano

Cenozoic exhumation and deformation of northeastern Tibet and the Qinling: Is Tibetan lower crustal flow diverging around the Sichuan basin? - Enkelmann, Eva; Ratschbacher, Lothar; Jonckheere, Raymond; Nestler, Ralf; Fleischer, Manja; Gloaguen, Richard; Hacker, Bradley R.; Zhang, Yue Qiao; Ma, Yin-Sheng (Bulletin of the Geological Society of America, 2006)
Apatite fission-track thermochronology data elucidate the cooling/ exhumation history of the Qinling (Qin Mountains), which contain a Paleozoic-Mesozoic orogenic collage north of the Sichuan Basin and northeast of the Tibetan Plateau. In particular, we examine the extent to which the Qinling were affected by the rising plateau. The northern and eastern Qinling show continuous cooling and slow exhumation since the Cretaceous. In contrast, in the southwestern Qinling, rapid cooling initiated at 9-4 Ma, a few million years later than in the eastern Tibetan Plateau. A compilation of major Cenozoic faults in the eastern Tibetan Plateau and the Qinling, and their kinematic and dynamic characterization, shows that deformation in the Qinling has predominantly been strike slip. Active sinistral and dextral strike-slip faults delineate an area of eastward rock flow and bound the area of rapid late Cenozoic cooling outlined by apatite fission-track thermochronology. These data can be interpreted to indicate that lower crustal flow has been diverted around the Longmen Shan and beneath the southwestern Qinling, causing active plateau uplift in this area. Alternatively, northeastern Tibet may be growing eastward faster in the western Qinling than the entire South China Block is extruding to the east. © 2006 Geological Society of America.
Keywords: Asia; China; Eurasia; Far East; Qinling Mountains; Sichuan Basin; Xizang; Deformation; Dynamics; Geochronology; Kinematics; Seismic prospecting; Structural geology; Cenozoic exhumation; Fission track; Plateau growth; Qinling; Tibet; Cenozoic; deformation; exhumation; fission track dating; lower crust; plateau; structural geology; thermochronology; Geological surveys

Cenozoic deposits of the underwater Akademicheskii Ridge in Lake Baikal - Kashik, S.A.; Lomonosova, T.K. (Lithology and Mineral Resources, 2006)
This paper presents data on the lithological composition of Cenozoic deposits penetrated for the first time by boreholes BDP-96-1, BDP-96-2, and BDP-98 down to a depth of 600 m on the underwater Akademicheskii Ridge in Lake Baikal. The deposits are subdivided into the upper (Angara) and lower (Barguzin) sequences, which span the Middle Miocene-Holocene period. They formed under different climatic conditions and tectonic settings. Sources of the terrigenous material were also different. Outbursts of diatom-and mineral formation in Lake Baikal can be related to not only climatic fluctuations in the Miocene-Holocene, but also the endogenous activity. By the analogy with the World Ocean, underwater gas-hydrothermal fluid discharge detected at the water-bottom interface in this lake may be accompanied by the formation of diatomaceous oozes and ferromanganese nodule fields and the concentration of rare elements. © Pleiades Publishing, Inc. 2006.

Building a natural earthquake laboratory at focal depth (DAFSAM-NELSAM project, South Africa) - Reches, Z. (Scientific Drilling, 2006)
cited By 12

Anisotropy in the shallow crust observed around the San Andreas fault before and after the 2004 M 6.0 parkfield earthquake - Cochran, E.S.; Li, Y.-G.; Vidale, J.E. (Bulletin of the Seismological Society of America, 2006)
Local seismic arrays were deployed at two locations along the San Andreas fault (SAF) near Parkfield, California, before and after the 2004 M 6.0 Parkfield earthquake. Using local earthquakes we determine the anisotropic field within 12 km of the main trace of the SAF at the two array locations separated by 12 km. The initial array, near the SAFOD site, was deployed for six weeks in October and November 2003, and the second array, located near the town of Parkfield, was deployed for 3 months following the 28 September 2004 M 6.0 Parkfield earthquake. We find the fast shear-wave polarization direction nearly fault-parallel (N40°W) for stations on the main fault trace and within 100 m to the southwest of the SAP at both array locations. These fault-parallel measurements span the 100- to 150-m-wide zone of pervasive cracking and damage interpreted from fault-zone-trapped waves associated with the main fault core (Li et al., 2004, 2006). Outside of this zone, the fast orientations are scattered with some preference for orientations near N10°E, roughly parallel to the regional maximum horizontal compressive stress direction (σh). In addition, fast directions are preferentially oriented parallel to a northern branch of the SAF recorded on stations in the 2004 Parkfield deployment. The measured anisotropy is likely due to a combination of stress-aligned microcracks away from the fault and shear fabric within the highly evolved fault core. The majority of our measurements are taken outside of the main fault core, and we estimate the density of microcracks from the measured delay times. Apparent crack densities are approximately 3%, with large scatter. The data suggest weak depth dependence to the measured delay times for source depths between 2 and 7 km. Below 7-km source depth, the delay times do not correlate with depth suggesting higher confining pressure is forcing the microcracks to close. No coseismic variation in the anisotropic parameters is observed, suggesting little to no influence on measured splitting due to the 2004 M 6.0 Parkfield earthquake. However, the premainshock and postmainshock data presented here are from arrays separated by 12 km, limiting our sensitivity to small temporal changes in anisotropy.
cited By 68
Keywords: Anisotropic parameters; Postmainshock data; Shallow crust; Shear-wave polarization; Anisotropy; Earthquake effects; Microcracks; Polarization; Shock waves; Compressive stress; Earthquakes; Seismic waves; Shear waves; Tectonics; Earthquakes; Seismology; anisotropy; crust; earthquake event; fault zone; microcrack; Parkfield earthquake 2004; S-wave; San Andreas Fault; trapped wave; coseismic process; earthquake; seismic anisotropy; California; North America; Parkfield; United States; Fast shear-wave polarization; Fault-parallel measurements; Fault-zone-rapped waves; San Andreas fault (SAF); Shallow crust

Analyses of shocked quartz at the global K-P boundary indicate an origin from a single, high-angle, oblique impact at Chicxulub - Morgan, J.; Lana, C.; Kearsley, A.; Coles, B.; Belcher, C.; Montanari, S.; D'iaz-Mart'inez, E.; Barbosa, A.; Neumann, V. (Earth and Planetary Science Letters, 2006)
The precise cause and timing of the Cretaceous-Paleocene (K-P) mass extinction 65 Ma ago remains a matter of debate. Many advocate that the extinction was caused by a meteorite impact at Chicxulub, Mexico, and a number of potential kill-mechanisms have been proposed for this. Although we now have good constraints on the size of this impact and chemistry of the target rocks, estimates of its environmental consequences are hindered by a lack of knowledge about the obliquity of this impact. An oblique impact is likely to have been far more catastrophic than a sub-vertical one, because greater volumes of volatiles would have been released into the atmosphere. The principal purpose of this study was to characterize shocked quartz within distal K-P ejecta, to investigate whether the quartz distribution carried a signature of the direction and angle of impact. Our analyses show that the total number, maximum and average size of shocked quartz grains all decrease gradually with paleodistance from Chicxulub. We do not find particularly high abundances in Pacific sites relative to Atlantic and European sites, as has been previously reported, and the size-distribution around Chicxulub is relatively symmetric. Ejecta samples at any one site display features that are indicative of a wide range of shock pressures, but the mean degree of shock increases with paleodistance. These shock- and size-distributions are both consistent with the K-P layer having been formed by a single impact at Chicxulub. One site in the South Atlantic contains quartz indicating an anomalously high average shock degree, that may be indicative of an oblique impact with an uprange direction to the southeast ± 45°. The apparent continuous coverage of proximal ejecta in this quadrant of the crater, however, suggests a relatively high impact angle of > 45°. We conclude that some of the more extreme predictions of the environmental consequences of a low-angle impact at Chicxulub are probably not applicable. © 2006 Elsevier B.V. All rights reserved.
cited By 51

An effective medium inversion algorithm for gas hydrate quantification and its application to laboratory and borehole measurements of gas hydrate-bearing sediments - Chand, S.; Minshull, T.A.; Priest, J.A.; Best, A.I.; Clayton, C.R.I.; Waite, W.F. (Geophysical Journal International, 2006)
The presence of gas hydrate in marine sediments alters their physical properties. In some circumstances, gas hydrate may cement sediment grains together and dramatically increase the seismic P- and S-wave velocities of the composite medium. Hydrate may also form a load-bearing structure within the sediment microstructure, but with different seismic wave attenuation characteristics, changing the attenuation behaviour of the composite. Here we introduce an inversion algorithm based on effective medium modelling to infer hydrate saturations from velocity and attenuation measurements on hydrate-bearing sediments. The velocity increase is modelled as extra binding developed by gas hydrate that strengthens the sediment microstructure. The attenuation increase is modelled through a difference in fluid flow properties caused by different permeabilities in the sediment and hydrate microstructures. We relate velocity and attenuation increases in hydrate-bearing sediments to their hydrate content, using an effective medium inversion algorithm based on the self-consistent approximation (SCA), differential effective medium (DEM) theory, and Biot and squirt flow mechanisms of fluid flow. The inversion algorithm is able to convert observations in compressional and shear wave velocities and attenuations to hydrate saturation in the sediment pore space. We applied our algorithm to a data set from the Mallik 2L-38 well, Mackenzie delta, Canada, and to data from laboratory measurements on gas-rich and water-saturated sand samples. Predictions using our algorithm match the borehole data and water-saturated laboratory data if the proportion of hydrate contributing to the load-bearing structure increases with hydrate saturation. The predictions match the gas-rich laboratory data if that proportion decreases with hydrate saturation. We attribute this difference to differences in hydrate formation mechanisms between the two environments. © 2006 The Authors Journal compilation © 2006 RAS.
cited By 54
Keywords: algorithm; borehole geophysics; elastic wave; gas hydrate; inverse problem; marine sediment; P-wave; S-wave; wave attenuation

A reconstruction of the climate and vegetation of northeastern Siberia based on lake sediments - Lozhkin, A.V.; Anderson, P.A. (Paleontological Journal, 2006)
Detailed palynological analysis of glacial, tectonic, and crater lakes of northeastern Siberia reveals continuous records of the changing vegetation during one or several climatic cycles of the Pleistocene and in the Holocene. The most continuous records in the mountain areas of the region are those of Lake Elikchan-4 (northern Okhotsk Sea Region). Pollen records of Lake El-gygytgyn, which was formed by the impact of a meteorite in the northern Chukchi Peninsula, reflect the response of land vegetation to the global climatic impact during the last 300 ka. © Pleiades Publishing, Inc. 2006.
cited By 14

A multiscale study of the mechanisms controlling shear velocity anisotropy in the San Andreas Fault Observatory at Depth - Boness, N.L.; Zoback, M.D. (Geophysics, 2006)
We present an analysis of shear velocity anisotropy using data in and near the San Andreas Fault Observatory at Depth (SAFOD) to investigate the physical mechanisms controlling velocity anisotropy and the effects of frequency and scale. We analyze data from borehole dipole sonic logs and present the results from a shear-wave-splitting analysis performed on waveforms from microearthquakes recorded on a downhole seismic array. We show how seismic anisotropy is linked either to structures such as sedimentary bedding planes or to the state of stress, depending on the physical properties of the formation. For an arbitrarily oriented wellbore, we model the apparent fast direction that is measured with dipole sonic logs if the shear waves are polarized by arbitrarily dipping transversely isotropic (TI) structural planes (bedding/ fractures). Our results indicate that the contemporary state of stress is the dominant mechanism governing shear velocity anisotropy in both highly fractured granitic rocks and well-bedded arkosic sandstones. In contrast, within the finely laminated shales, anisotropy is a result of the structural alignment of clays along the sedimentary bedding planes. By analyzing shear velocity anisotropy at sonic wavelengths over scales of meters and at seismic frequencies over scales of several kilometers, we show that the polarization of the shear waves and the amount of anisotropy recorded are strongly dependent on the frequency and scale of investigation. The shear anisotropy data provide constraints on the orientation of the maximum horizontal compressive stress SHmax and suggest that, at a distance of only 200 m from the San Andreas fault (SAF), SHmax is at an angle of approximately 70° to the strike of the fault. This observation is consistent with the hypothesis that the SAF is a weak fault slipping at low levels of shear stress. © 2006 Society of Exploration Geophysicists.
cited By 113
Keywords: Compressive stress; Earthquakes; Granite; Sandstone; Seismic waves; Shale; Shear waves; Seismic array; Seismic frequencies; Shear velocity anisotropy; Shear-wave-splitting analysis; Sonic wavelengths; Seismology; earthquake mechanism; faulting; microearthquake; S-wave; seismic anisotropy; seismic velocity; seismology; waveform analysis

A field guide to the central, creeping section of the San Andreas fault and the San Andreas Fault Observatory at Depth - Rymer, M.J.; Hickman, S.H.; Stoffer, P.W. (GSA Field Guides, 2006)
This field trip is along the central section of the San Andreas fault and consists of eight stops that illustrate surface evidence of faulting, in general, and features associated with active fault creep, in particular. Fault creep is slippage along a fault that occurs either in association with small-magnitude earthquakes or without any associated large-magnitude earthquakes. Another aspect of the trip is to highlight where there are multiple fault traces along this section of the San Andreas fault zone in order to gain a better understanding of plateboundary processes. The first stop is along the Calaveras fault, part of the San Andreas fault system, at a location where evidence of active fault creep is abundant and readily accessible. The stops that follow are along the San Andreas fault and at convenient locations to present and discuss rock types juxtaposed across the fault that have been transported tens to hundreds of kilometers by right-lateral motion along the San Andreas fault. Stops 6 and 7 are examples of recent studies of different aspects of the fault: drilling into the fault at the depth of repeating magnitude (M) 2 earthquakes with the San Andreas Fault Observatory at Depth (SAFOD) and the geological, geophysical, and seismological study of M 6 earthquakes near the town of Parkfield. Along with the eight official stops on this field trip are 12 rolling stops - sites of geologic interest that add to the understanding of features and processes in the creeping section of the fault. Many of the rolling stops are located where stopping is difficult to dangerous; some of these sites are not appropriate for large vehicles (buses) or groups; some sites are not appropriate for people at all. We include photographs of or from many of these sites to add to the reader's experience without adding too many stops or hazards to the trip. An extensive set of literature is available for those interested in the San Andreas fault or in the creeping section, in particular. For more scientifically oriented overviews of the fault, see Wallace (1990) and Irwin (1990); for a more generalized overview with abundant, colorful illustrations, see Collier (1999). Although the presence of small sections of the San Andreas fault was known before the great 1906 San Francisco earthquake, it was only after that event and subsequent geologic investigations reported in Lawson (1908) that showed the fault as a long structure, extending all the way from east of Los Angeles into northern California. Prentice (1999) described the importance of the 1908 Lawson report and how it pivotally influenced the understanding of the San Andreas. Hill (1981) presented a wonderful introduction to the evolution of thought on the San Andreas. Geologic maps and maps of the most recently active fault trace in the creeping section, or large parts of it, include those by Brown (1970), Dibblee (1971, 1980), and Wagner et al. (2002); detailed geologic maps are discussed at various stops in this guide. Various aspects of the creeping section of the San Andreas fault have been the focus of many geologic field trips in the past few decades. Guidebooks for some of those trips include those by Gribi (1963a, 1963b), Brabb et al. (1966), Rogers (1969), Bucknam and Haller (1989), Harden et al. (2001), and Stoffer (2005). The creeping section of the San Andreas fault zone lies between areas that experienced large-displacement surface breakage during great earthquakes in 1857 and 1906 (Fig. 1, inset). Burford and Harsh (1980) divided the creeping section into three segments: (1) a northwest section where the creep rate increases to the southeast in step-like increments, (2) a central section where the creep rate is relatively constant at a maximum value of ∼30 mm/yr (∼1.2 in/yr), and (3) a southeast section where the creep rate decreases to the southeast (Fig. 2). The rate of slip along the creeping section of the fault zone has been measured using creepmeters, alignment arrays, and laser distance-measuring devices. The aperture of measurements over which these measurements are made ranges from 10 m (∼33 ft) (creepmeters) to 100 m (∼330 ft) (alignment arrays) to kilometers and tens of kilometers (laser measuring devices). Creepmeter and alignment-array measurements are here termed near- fault measurements; laser measurements over distances of 1-2 km (∼0.6-1.2 mi) are termed intermediate-scale measurements; laser measurements over tens of kilometers (miles) are termed broadscale measurements. Comparisons among near-fault, intermediate-scale, and broadscale measurements and geologic maps show that the northwest part of the creeping section of the fault is composed of two narrow zones of active deformation, one along the San Andreas fault and one along the Calaveras-Paicines fault, whereas the central and southeast sections are both composed of a single relatively narrow zone of deformation. The southeast section is transitional to a locked zone southeast of Cholame; a locked fault is one that slips only in association with a moderate to large earthquake. Throughout the creeping section of the San Andreas fault zone, broadscale measurements generally indicate more deformation than near-fault and intermediate-scale measurements, which are in reasonably close agreement except at Monarch Peak (Mustang Ridge), near the center of the creeping section and our Stop 5 (Figs. 1 and 2). Features that we see on this trip include offset street curbs, closed depressions (sag ponds), fault scarps (steep slopes formed by movement along a fault), a split and displaced tree, offset fence lines, fresh fractures, and offset road lines (Fig. 3 is a sketch showing some of the landforms that represent deformation by an active fault). We also see evidence of long-term maturity of the San Andreas fault, as indicated by fault features and displaced rock types (Fig. 4). Finally, we will visit sites of ongoing research into the processes associated with earthquakes and their effects. Discussions include drilling into the San Andreas fault at the SAFOD drill site and the 2004 Parkfield earthquake and its effects and implications. © 2006 Geological Society of America. All rights reserved.
cited By 6
Keywords: Creep; Curbs; Earthquakes; Infill drilling; Locks (fasteners); Observatories; Strike-slip faults; Creeping section; Fault structure; Hollister; Parkfield; SAFOD; San Andreas fault; Structural geology

14c in methane and dic in the deep terrestrial subsurface: implications for microbial methanogenesis - Slater, G.F.; Lippmann-Pipke, J.; Moser, D.P.; Reddy, C.M.; Onstott, T.C.; Lacrampe-Couloume, G.; Lollar, B. (Geomicrobiology Journal, 2006)
A comparison between the14C content of the methane and dissolved inorganic carbon (DIC) in deep, terrestrial subsurface systems was used to assess the timing of microbial methanogenesis contributing to gases in fracture water samples from three mines in the Witwatersrand Basin, South Africa. The results demonstrated that the majority of methane was produced over geologic timescales. In four of the samples, the methane contained no significant radiocarbon, indicating that the estimated 90% microbial methane in these samples was produced in the geologic past by indigenous microbial communities. In two samples from different mines, methaneΔ14C levels indicated a primarily ancient origin for the microbial methane with the potential for more recent contributions from ongoing indigenous microbial activities constrained to between 0 and40%, and 0 and 24%, respectively. Microbiological evidence for methanogenic archaea was observed in both of these samples. One sample had a Δ14C CH4 that was higher than the corresponding DIC, indicating an extreme decoupling between these species and raising concerns over the representative quality of this sample. The variations in the Δ14C of DIC and CH4 between and within mines demonstrate the need for a thorough assessment of each sample to obtain an accurate understanding of the role and timing of microbiological gas production in these complex, heterogeneous, terrestrial subsurface systems. The approach detailed here introduces timing as a new and widely applicable signature for the recognition of a major geochemical marker of indigenous life in the deep subsurface. © Taylor & Francis.
cited By 4
Keywords: carbon isotope; dissolved inorganic carbon; methane; methanogenesis; microbial activity; microbiology; Africa; South Africa; Southern Africa; Sub-Saharan Africa; Witwatersrand; Archaea

Magnetostratigraphy of the Cretaceous/Tertiary boundary and early Paleocene sedimentary sequence from the Chicxulub Impact Crater - Rebolledo-Vieyra, M.; Urrutia-Fucugauchi, J. (earth, planets and space, 2006)
We report on the magnetostratigraphy of the Chicxulub crater impact breccias and first 15 meters of the Paleocene sedimentary sequence recovered in three boreholes of the UNAM Scientific Drilling Program. Three geomagnetic polarity zones are documented in the impact breccias and sedimentary sequence, which span from chron 29R to 28N. For the 15 m interval they represent 2.5 Ma, which yields low apparent sedimentary rates for boreholes UNAM-5 (110 km from the center of the crater) and UNAM-7 (127 km from the center of the crater). The carbonate sedimentary sequence can be associated to a shallow basin depositional environment. In these boreholes the thickness between the 29R and the 29N chrons is just 0.5 m, suggesting that during the 100 ka from the K/T boundary to the polarity transition sediments were not deposited or eroded. Within borehole UNAM-6 (152 km from the center of the crater) it appears that sediments containing chron 29N are missing, the lack of the upper breccias, the long duration of a reversal event within the base of the sequence and low apparent sedimentary rate of 3.3 m/Ma, suggests a hiatus within the impact breccias and the basal Paleocene sedimentary sequence. Magnetic susceptibility logs confirm absence of the upper breccias at UNAM-6 borehole. Magnetic susceptibility values increase towards the base of the sequence, suggesting that basement and melt clasts were subjected to a low temperature hydrothermal alteration. © 2006, The Seismological Society of Japan, Society of Geomagnetism and Earth, Planetary and Space Sciences, The Volcanological Society of Japan, The Geodetic Society of Japan, The Japanese Society for Planetary Sciences. All rights reserved.
cited By 15

Drilling to the core of Japan's Unzen Volcano - Nakada, S.; Uto, K.; Sakuma, S.; Eichelberger, J.; Shimizu, H. (Geodrilling International, 2006)
To understand the structure and growth history of Unzen Volcano in Japan and to clarify the eruption mechanisms of SiO 2-rich viscous magmas, the Unzen Scientific Drilling Project (USDP), a six-year program consisting of two phases, began in April 1999. In the first phase, two holes were drilled into the volcano's flank (USDP-1 and -2 wells). In the second phase, drilling penetrated the magma conduit that fed a lava dome at the summit during the 1991-95 eruption. Through directional drilling, samples of the lava dike believed to have fed the 1991-95 eruption were recovered. The lava dike sample was unexpectedly altered, suggesting that circulation of hydrothermal fluids rapidly cools the conduit region of even very active volcanoes. It is likely that seismic signals monitored before the emergence of lava dome reflected fracturing of the country rocks, caused by veining as volatiles escaped predominantly upward, not outward, from the rising magma.
cited By 0
Keywords: drilling

Amplitude and frequency anomalies in regional 3D seismic data surrounding the Mallik 5L-38 research site, Mackenzie Delta, Northwest Territories, Canada - Riedel, M.; Bellefleur, G.; Dallimore, S.R.; Taylor, A.; Wright, J.F. (Geophysics, 2006)
Amplitude and frequency anomalies associated with lakes and drainage systems were observed in a 3D seismic data set acquired in the Mallik area, Mackenzie Delta, Northwest Territories, Canada. The site is characterized by large gas hydrate deposits inferred from well-log analyses and coring. Regional interpretation of the gas hydrate occurrences is mainly based on seismic amplitude anomalies, such as brightening or blanking of seismic energy. Thus, the scope of this research is to understand the nature of the amplitude behavior in the seismic data. We have therefore analyzed the 3D seismic data to define areas with amplitude reduction due to contamination from lakes and channels and to distinguish them from areas where amplitude blanking may be a geologic signal. We have used the spectral ratio method to define attenuation (Q) over different areas in the 3D volume and subsequently applied Q-compensation to attenuate lateral variations ofdispersive absorption. Underneath larger lakes, seismic amplitude is reduced and the frequency content is reduced to 20-40 Hz, which is half the original bandwidth. Traces with source-receiver pairs located inside of lakes show an attenuation factor Q of ∼ 40, approximately half of that obtained for source-receiver pairs situated on deep, continuous permafrost outside of lakes. Deeper reflections occasionally identified underneath lakes show low-velocity-related pull-down. The vertical extent of the washout zones is enhanced by acquisition with limited offsets and from processing parameters such as harsh mute functions to reduce noise from surface waves. The strong attenuation and seismic pull-down may indicate the presence of unfrozen water in deeper lakes and unfrozen pore water within the sediments underlying the lakes. Thus, the blanking underneath lakes is not necessarily related to gas migration or other in situ changes in physical properties potentially associated with the presence of gas hydrate. © 2006 Society of Exploration Geophysicists.
cited By 21
Keywords: Drainage; Lakes; Permafrost; Seismic waves; Spectrum analysis; Surface waves; Well logging; Gas hydrate deposits; Seismic amplitude; Seismic data; Seismology; amplitude; data interpretation; seismic attenuation; seismic data; seismic migration; seismology; spectral analysis; surface wave; three-dimensional modeling; well logging; Canada; Mackenzie Delta; North America; Northwest Territories

Energy release and heat generation during the 1999 Ms7.6 Chi-Chi, Taiwan, earthquake - Wang, J.-H. (Journal of Geophysical Research: Solid Earth, 2006)
On 20 September 1999, the Ms7.6 Chi-Chi earthquake raptured the Chelungpu fault in central Taiwan. Integrating observed and inversed results of source parameters, the fracture energy, Eg. and frictional energy, Ef, on the fault and its northern and southern segments are estimated. Together with given values of strain energy, ΔE, and seismic radiation energy, Es, the seismic efficiency, i.e., η = Es/ΔE, and the radiation efficiency, i.e., ηR = Es/(Es + Eg), are evaluated. The average fracture energy per unit area, G, is also calculated from Eg. The frictional heat caused by dynamic frictional stress is calculated from Ef. Results show a marked difference in source properties between the two segments. The average frictional and ambient stress levels on the two segments are estimated. The total energy budget of and heat generated by the earthquake are elucidated based on a two-dimensional faulting model with frictional heat. Both observed and calculated results suggest the possible existence of fluids, which produced suprahydrostatic gradients, on the fault during faulting. Lubrication and thermal fluid pressurization might play a significant role on rupture. Copyright 2006 by the American Geophysical Union.
cited By 24
Keywords: Chi-Chi earthquake 1999; earthquake magnitude; earthquake rupture; energy budget; faulting; seismicity; source parameters; Asia; Eurasia; Far East; Taiwan

Hydrates as an energy source - Risks and opportunities - Dallimore, S. (Society of Petroleum Engineers, 68th European Association of Geoscientists and Engineers Conference and Exhibition, incorporating SPE EUROPEC 2006, EAGE 2006: Opportunities in Mature Areas, 2006)
Gas hydrates are solid crystalline substances consisting of natural gas (typically methane) and water molecules, that remain stable under conditions of relatively cold formation temperatures and high pressures. Significant natural gas hydrates deposits occur in the Arctic in association with areas of thick permafrost, and in many marine environments around the world. Their energy potential is significant as the in place estimates of gas hydrate are thought to significantly exceed conventional natural gas resource estimates. While gas hydrates could represent a vast unconventional, environmentally-friendly natural gas resource, the challenges to realise their energy potential remain significant. They occur in remote frontier settings, new exploration methods are required to locate concentrated deposits, and technologies to produce them have not proven. Success will likely require a combination of science, technology and favourable economics. Interest in gas hydrates as an energy resource has grown in recent years with many countries having established national research and development programs. Scientific and exploratory drilling has taken place in Japan, Canada and the USA, and field programs are also planned in the near future offshore of India and China. In 2002, a seven member research partnership conducted small-scale production testing at the Mallik site in Canada's Northwest Territories. Two 1188 m science observation wells and a 1166 m production research well were drilled and instrumented to examine gas production and the physical behaviour of reservoir sediments in response to depressurization and thermal stimulation. Other aspects of the program included the collection of gas hydrate core samples, downhole geophysical logging and extensive laboratory and modeling studies. The results of the production testing and scientific investigations have been released Geological Survey of Canada Bulletin that includes 62 scientific papers and an extensive data base. Copyright 2006, European Association of Geoscientists and Engineers.
cited By 0
Keywords: Crystalline materials; Database systems; Energy resources; Permafrost; Pressurization; Risk assessment; Cold formation temperatures; Downhole geophysical logging; Natural gas hydrates; Natural gas resources; Gas hydrates

Establishing the link between the Chesapeake Bay impact structure and the North American tektite strewn field: The Sr-Nd isotopic evidence - Deutsch, A.; Koeberl, C. (Meteoritics and Planetary Science, 2006)
The Chesapeake Bay impact structure, which is about 35 Ma old, has previously been proposed as the possible source crater of the North American tektites (NAT). Here we report major and trace element data as well as the first Sr-Nd isotope data for drill core and outcrop samples of target lithologies, crater fill breccias, and post-impact sediments of the Chesapeake Bay impact structure. The unconsolidated sediments, Cretaceous to middle Eocene in age, have εSrt=35.7Ma of +54 to +272, and εNdt=35.7Ma ranging from -6.5 to - 10.8; one sample from the granitic basement with a TNdCHUR model age of 1.36 Ga yielded an εSrt=35.7Ma of + 188 and an εNdt=35.7Ma of -5.7. The Exmore breccia (crater fill) can be explained as a mix of the measured target sediments and the granite, plus an as-yet undetermined component. The post-impact sediments of the Chickahominy formation have slightly higher TNdCHUR model ages of about 1.55 Ga, indicating a contribution of some older materials. Newly analyzed bediasites have the following isotope parameters: +104 to +119 (εSrt=35.7Ma), -5.7 (εNdt=35.7Ma), 0.47 Ga (TSsUR), and 1.15 Ga (TNdCHUR), which is in excellent agreement with previously published data for samples of the NAT strewn field. Target rocks with highly radiogenic Sr isotopic comparison, as required for explaining to isotopic characteristic of Deep Sea Drilling Project (DSDP) site 612 tektites, were not among the analyzed sample suite, Based on the new isotope data, we exclude any relation between the NA tektites and the Popigai impact crater, athough they have identical ages within 2σ errors. The Chesapeake Bay structure, however, is now clearly constrained as the source crater for the North American tektites, although the present data set obviously does not include all target lithologies that have contributed to the composition of the tektites. © The Meteoritical Society, 2006.
cited By 44
Keywords: crater; Deep Sea Drilling Project; impact structure; isotopic composition; neodymium; strontium; tektite

Magma mingling as indicated by texture and Sr / Ba ratios of plagioclase phenocrysts from Unzen volcano, SW Japan - Browne, B.L.; Eichelberger, J.C.; Patino, L.C.; Vogel, T.A.; Uto, K.; Hoshizumi, H. (Journal of Volcanology and Geothermal Research, 2006)
Textural and geochemical characteristics of plagioclase phenocrysts from the eruptive products of Mount Unzen (SW Japan) record repeated intrusions of basaltic magma into a dacitic host magma chamber over the lifetime of the volcano. An important aspect of this mixing style is the exchange of phenocrysts between the intruding basalt magma and host dacite magma, and the effect that this wide-reaching mixing style has on the mineralogical diversity of the erupted products. Plagioclase phenocrysts that originally crystallized from the host dacite magma are identified by oscillatory zoning patterns, low An content cores (An45 to An60), and low Sr / Ba ratios. Host-derived plagioclase phenocrysts are engulfed during intrusion of basaltic magma, evidenced by their presence in basaltic to andesitic enclaves. In response to changes in temperature and composition of the surrounding melt, the engulfed plagioclases develop resorption zones, which are composed of a densely packed network of micron-sized glass inclusions and high An content plagioclase (An72-An92) with high Sr / Ba ratios that match those of plagioclase microphenocrysts inherent to the enclave-forming magma. Over time, host-derived plagioclase phenocrysts that were once engulfed during replenishment events are recycled back to the host as enclaves disaggregate (e.g.[ Clynne, M.A., 1989. The disaggregation of quenched magmatic inclusions contributes to chemical diversity in silicic lavas of Lassen Peak, California. Bull New Mexico Bureau of Mines and Mineral Resources, 131: 54]). An eruption of andesite lava with no enclaves, something particularly unique for Unzen, occurred in 1663. Similar to enclaves, all plagioclase phenocrysts in this lava flow are surrounded by resorption zones suggesting that the 1663 lava may represent a magma that was erupted after thoroughly mixing with the intruding basaltic. Using experimentally calibrated crystallization rates, we estimate that phenocrysts exist in the Unzen chamber a minimum of 0.5-3 months between the time of their encounter with a basaltic intrusion and eruption. © 2006 Elsevier B.V. All rights reserved.
cited By 92
Keywords: Crystallization; Geochemistry; Laser ablation; Mineralogy; Trace elements; Enclaves; Magma mixing; Plagioclase; Unzen volcano; Volcanoes; andesite; lava flow; magma; phenocryst; plagioclase; volcanic eruption; Asia; Eurasia; Far East; Japan; Kyushu; Nagasaki [Kyushu]; Unzen Volcano

Long-term sustainability of a high-energy, low-diweniff crystal-biome - Lin, L.-H.; Wang, P.-L.; Rumble, D.; Lippmann-Pipke, J.; Boice, E.; Pratt, L.M.; Lollar, B.S.; Brodie, E.L.; Hazen, T.C.; Andersen, G.L.; DeSantis, T.Z.; Moser, D.P.; et al. (Science, 2006)
Geochemical, microbiological, and molecular analyses of alkaline saline groundwater at 2.8 kilometers depth in Archaean metabasalt revealed a microbial biome dominated by. a single phylotype affiliated with thermophilic sulfate reducers belonging to Firmicutes. These sulfate reducers were sustained by geologically produced sulfate and hydrogen at concentrations sufficient to maintain activities for millions of years with no apparent reliance on photosynthetically derived substrates.
cited By 287
Keywords: Basalt; Geochemistry; Groundwater; Photosynthesis; Reduction; Salts; Metabasalt; Microbial biomes; Microbiology; ground water; hydrogen; sulfate; Archean; biome; groundwater; high energy environment; hydrogen; metabasalt; molecular analysis; salinity; sulfate; sulfate-reducing bacterium; Archean; article; biome; environmental sustainability; Firmicutes; geochemical analysis; microbiology; photosynthesis; priority journal; Bacteria; Biodiversity; DNA; Ribosomal; Ecosystem; Gold; Hydrogen; Mining; Oligonucleotide Array Sequence Analysis; Oxidation-Reduction; Phylogeny; RNA; Ribosomal; 16S; South Africa; Sulfates; Temperature; Thermodynamics; Time; Water Microbiology; Firmicutes

Leaky mode: A horizontal seismic attenuation mechanism in a gas hydrate-bearing sediment - Zanoth, S.R.; Saenger, E.H.; Kr{uger, O.S.; Shapiro, S.A. (SEG Technical Program Expanded Abstracts, 2006)
This paper is concerned with the leaky mode, a possible horizontal attenuation phenomenon of seismic waves in a gas hydrate-bearing sediment layer. This attenuation mechanism in horizontal direction occurs when a high-velocity layer is embedded in a low-velocity zone. This is a typical situation for gas hydrate occurrences. To quantify th is mechanism a digital rock model based on the crosswell-data of the Mallik 2002 Gas Hydrate Research Well Program is created. In our elastic simulations we can exclude attenuation mechanism like scattering loss or intrinsic absorption. We will demonstrate that the leaky mode is a significant horizontal attenuation mechanism which cannot be neglected. © 2005 Society of Exploration Geophysicists.
cited By 0
Keywords: Geophysical prospecting; Hydration; Petroleum prospecting; Seismology; Gas hydrate bearing sediments; High velocity; Horizontal attenuation; Intrinsic absorptions; Leaky modes; Low velocity zones; Scattering loss; Seismic attenuation; Gas hydrates

Lake Qinghai scientific drilling project - Zhisheng, An; Li, Ai; Yougui, Song; Colman, Steven M (Scientific Drilling, 2006)

Investigating maar formation and the climate history of southern argentina-the potrok aike maar lake sediment archive drilling project (PASADO) - Zolitschka, Bernd; Corbella, Hugo; Maidana, Nora; Ohlendorf, Christian (Scientific Drilling, 2006)

Intercorrelation of down-core variations of the high-resolution magnetic susceptibility for CON01-603 and CON01-605 - Demory, F; Nowaczyk, N; Witt, A; Oberh{ansli, Hedi (GFZ Data Services, 2006)

Inclination, declination and the reversal angle of the ChRM for CON01-603-2 - Demory, François; Nowaczyk, N; Witt, Annette; Oberh{ansli, Hedi (GFZ Data Services, 2006)

In situ measurement of the hydraulic diffusivity of the active Chelunepu Fault, Taiwan - Doan, M.L.; Brodsky, E.E.; Kano, Y.; Ma, K.F. (Geophysical Research Letters, 2006)
[1] Hydraulic diffusivity controls fluid pressure and hence affects effective normal stress during rupture. Models suggest a particularly spectacular example of fluid pressurization during the Mw = 7.6 1999 Chichi earthquake when pressurization may have reduced high-frequency shaking in the regions of large slip if the fault was sufficiently sealed. We investigate in situ hydraulic diffusivity which is the key parameter in such models through a cross-hole experiment. We find a diffusivity of D = (7 ±1) × 10-5 m2/s, which is a low value compatible with pressurization of the Chelungpu fault during the earthquake. In most poroelastic media, the hydraulic storativity 5 lies between 10-7 and 10 -5, so that the transmissivity T along the fault zone is comprised between 10-11 m2/s and 10-9 m2/s. The corresponding permeability (10-18-10-16 m2) is at most one hundred times larger than the value obtained on core samples from the host rock. The fault zone is overpressurized by 0.06 to 6 MPa, which is between 0.2% and 20% of the lithostatic pressure. Copyright 2006 by the American Geophysical Union.
cited By 66
Keywords: Carrier communication; Core samples; Diffusion; Earthquakes; Fluid dynamics; Mathematical models; Pressure effects; Stress analysis; Fault zone; Hydraulic diffusivity; Lithostatic pressure; Poroelastic media; Hydraulic fracturing; crosshole seismic method; diffusion; earthquake; fault; fluid pressure; hydraulic conductivity; poroelasticity; Asia; Eurasia; Far East; Taiwan

ICDP Workshop on the Emerging Modern Aerobic Earth System - Melezhik, Victor A; Lepland, Aivo (Scientific Drilling, 2006)

Hysteresis measurements (uncorrected and corrected for the paramagnetic influence) for VER98-1-1 at core depth of 240.3 cm. - Demory, François; Oberh{ansli, Hedi; Nowaczyk, N; Gottschalk, Matthias; Wirth, R; Naumann, R (GFZ Data Services, 2006)

Late Quaternary climate-induced lake level variations in Lake Pet'en Itz'a, Guatemala, inferred from seismic stratigraphic analysis - Anselmetti, F.S.; Ariztegui, D.; Hodell, D.A.; Hillesheim, M.B.; Brenner, M.; Gilli, A.; McKenzie, J.A.; Mueller, A.D. (Palaeogeography, Palaeoclimatology, Palaeoecology, 2006)
We used seismic images and sedimentary data from piston cores to conduct a sequence stratigraphic analysis of sediments in Lake Pet'en Itz'a, northern Guatemala. Our results document lake level fluctuations in this lowland Neotropical region that were related to glacial-to-interglacial climate change during the Late Pleistocene. A bathymetric survey of Lake Pet'en Itz'a (area = 100 km2) revealed a maximum water depth of ∼160 m and the existence of a deep cryptodepression that extends 50 m below modern sea level. The great depth suggests that the basin held water even during arid conditions associated with full glacial periods. Lake Pet'en Itz'a may thus possess the only long continuous lacustrine sediment record of Late Pleistocene environmental and climate change in the lowland Neotropics. Two seismic reflection campaigns imaged the subsurface basin sediments that overlie basement. The sediment package was divided into four major seismic sequences (T, G, R, and B). Sequences are separated by unconformities that represent depositional cycles related to lake level fluctuations. Sediments of the uppermost sequence (T) were recovered and radiocarbon-dated in Kullenberg piston cores taken along a water depth transect. Seismic profiles reveal a basin-wide paleoshoreline just below sequence T at ∼56 m below present lake level. This constructional feature formed during a lowstand of the last glacial period when the lake was reduced to only ∼13% of its present volume. In cores taken landward of the paleoshoreline, Late Glacial-age deposits consist of paleosols, indicating subaerial exposure. Basinward of the shoreline, sediments are composed of dense gypsum sands and interbedded silty clays, reflecting authigenic gypsum formation under arid climate conditions. The top of the soil horizon and cessation of gypsum precipitation are represented by a strong seismic reflection (t). It marks the base of the uppermost seismic sequence T and is dated in several cores between ∼11.1 and 10.2 cal kyr BP. Lake level rose quickly at this time in response to a shift from arid-to-humid climate conditions at the Late Glacial/Early Holocene transition. We infer a similar sediment response to climate variations in the older stratigraphic sequences (G, R, and B), related to earlier glacial-to-interglacial and stadial-to- interstadial cycles. Older sequences are also distinguished from one another by erosional unconformities that probably represent major lake level falls. Future recovery of the older stratigraphic record by drilling in Lake Pet'en Itz'a will provide ages for these older units and enable us to test the depositional model inferred from seismic stratigraphy. © 2005 Elsevier B.V. All rights reserved.
Keywords: Central America; Guatemala [Central America]; climate variation; lacustrine deposit; paleoclimate; Quaternary; seismic stratigraphy

HOTSPOT: The snake river scientific drilling project-tracking the yellowstone hotspot through space and time - Shervais, John W.; Branney, Michael J.; Geist, Dennis J.; Hanan, Barry B.; Hughes, Scott; Prokopenko, Alexsander A.; Williams, Douglas F. (Scientific Drilling, 2006)

Generation of Porphyritic and Equigranular mafic enclaves during magma recharge events at Unzen volcano, Japan - Browne, B.L.; Eichelberger, J.C.; Patino, L.C.; Vogel, T.A.; Dehn, J.; Uto, K.; Hoshizumi, H. (Journal of Petrology, 2006)
Mafic to intermediate enclaves are evenly distributed throughout the dacitic 1991-1995 lava sequence of Unzen volcano, Japan, representing hundreds of mafic recharge events over the life of the volcano. This study documents the morphological, textural, chemical, and petrological characteristics of the enclaves and coexisting silicic host lavas. The eruptive products described in this study appear to be general products of magma mingling, as the same textural types are seen at many other volcanoes. Two types of magmatic enclaves, referred to as Porphyritic and Equigranular, are easily distinguished texturally. Porphyritic enclaves display a wide range in composition from basalt to andesite, are glass-rich, spherical and porphyritic, and contain large, resorbed, plagioclase phenocrysts in a matrix of acicular crystals and glass. Equigranular enclaves are andesitic, non-porphyritic, and consist of tabular, medium-grained microphenocrysts in a matrix glass that is in equilibrium with the host dacite magma. Porphyritic enclaves are produced when intruding basaltic magma engulfs melt and phenocrysts of resident silicic magma at their mutual interface. Equigranular enclaves are a product of a more prolonged mixing and gradual crystallization at a slower cooling rate within the interior of the mafic intrusion. © The Author 2005. Published by Oxford University Press. All rights reserved.
cited By 73
Keywords: enclave; mafic rock; magmatism; petrogenesis; Asia; Eurasia; Far East; Japan; Kyushu; Nagasaki [Kyushu]; Unzen Volcano

HIRM record for CON01-603-2 - Demory, François; Oberh{ansli, Hedi; Nowaczyk, N; Witt, A (GFZ Data Services, 2006)

Fracture analysis in the south-western Corinth rift (Greece) and implications on fault hydraulic behavior - Micarelli, Luca; Moretti, Isabelle; Jaubert, Manon; Moulouel, Hakim (Tectonophysics, 2006)
This paper reviews the data concerning the fracture network and the hydraulic characteristics of faults in an active zone of the Gulf of Corinth. Pressure gap measured through fault planes shows that in this area the active normal faults (Aigion, Helike) act, at least temporarily and locally, as transversal seal. The analysis of the carbonate cements in the fractures on both the hangingwall and the footwall of the faults also suggests that they have acted as local seals during the whole fault zone evolution. However, the pressure and the characteristics of the water samples measured in the wells indicate that meteoric water circulates from the highest part of the relief to the coast, which means it goes through the fault zones. Field quantitative analysis and core studies from the AIG-10 well have been performed to define both regional and fault-related fracture networks. Then laboratory thin section observations have been done to recognize the different fault rocks characterizing the fault zone components. These two kinds of approach give information on the permeability characteristics of the fault zone. To synthesize the data, a schematic conceptual 3D fluid flow modeling has been performed taking into account fault zone permeability architecture, sedimentation, fluid flow, fault vertical offset and meteoric water influx, as well as compaction water flow. This modeling allows us to fit all the data with a model where the fault segments act as a seal whereas the relays between these segments allow for the regional flow from the Peloponnese topographic highs to the coast. © 2006 Elsevier B.V. All rights reserved.
Keywords: Gulf of Corinth; Ionian Sea; Mediterranean Sea; active fault; damage mechanics; fault zone; fracture network; normal fault

Frictional strength heterogeneity and surface heat flow: Implications for the strength of the creeping San Andreas fault - d'Alessio, M.A.; Williams, C.F.; B{urgmann, R. (Journal of Geophysical Research: Solid Earth, 2006)
Heat flow measurements along much of the San Andreas fault (SAF) constrain the apparent coefficient of friction (μapp) of the fault to <0.2, much lower than laboratory-derived friction values for most geologic materials. However, heat flow data are sparse near the creeping section of the SAF, a frictional asperity where the fault slips almost exclusively by aseismic creep. We test the hypothesis that the creeping section has a substantially higher or lower μ app than adjacent sections of the SAF. We use numerical models to explore the effects of faults with spatially and temporally heterogeneous frictional strength on the spatial distribution of surface heat flow. Heat flow from finite length asperities is uniformly lower than predicted by assuming an infinitely long fault. Over geologic time, lateral offset from strike-slip faulting produces heat flow patterns that are asymmetric across the fault and along strike. We explore a range of asperity sizes, slip rates, and displacement histories for comparing predicted spatial patterns of heat flow with existing measurements. Models with μapp ∼ 0.1 fit the data best. For most scenarios, heat flow anomalies from a frictional asperity with μapp > 0.2 should be detectable even with the sparse existing observations, implying that μapp for the creeping section is as low as the surrounding SAF. Because the creeping section does not slip in large earthquakes, the mechanism controlling its weakness is not related to dynamic processes resulting from high slip rate earthquake ruptures. Copyright 2006 by the American Geophysical Union.
cited By 28
Keywords: creep; earthquake mechanism; earthquake rupture; flow pattern; friction; heat flow; San Andreas Fault

Faulting style and stress field investigations for swarm earthquakes in NE Baveria/Germany - The transition between Vogtland/NW-Bohemia and the KTB-site - Ibs-von Seht, M.; Plenefisch, T.; Schmedes, E. (Journal of Seismology, 2006)
A seismicity and stress field analysis of a region in NE Bavaria reveals a complex picture of seismic dislocation. The magnitudes are generally low, the strongest event recorded had a magnitude of 2.3. In the southern part of the area investigated, earthquakes occur very rarely. During the observation period of approximately four years, only four events, two of them forming a doublet, were recorded. Hypocentral depths in the southern part are considerably great (15 to 17 km) and indicate a mafic lower crust. The seismicity of the Marktredwitz area, located in the western extension of the Eger rift, is dominated by earthquake swarms that are strongly clustered in space and time. The swarms occurred at depths between 10 and 14 km. Precise relative relocations show clear planar arrangements of the hypocentres and enable to identify the orientation of active fault planes. A comparison of the structural and geomorphological settings reveals major similarities in the occurrence of earthquake swarms compared to the situation in the bordering Vogtland/NW-Bohemia swarm area. Focal mechanisms cover a wide range of faulting styles. Normal fault, strike slip and reverse fault mechanisms as well as movements along sub-horizontal planes were found. The focal mechanisms were used to invert for the stress field. The inversion results reveal an ambiguity for the state of stress in the area of investigation and allow two different interpretations: A clockwise rotation of the stress field from North to South as well as a predominance of two slightly different stress regimes are possibilities. © Springer Science+Business Media, Inc. 2006.
Keywords: Bavaria; Central Europe; Eurasia; Europe; Germany; earthquake magnitude; earthquake swarm; focal mechanism; seismicity; stress field

Frictional strength of cuttings and core from SAFOD drillhole phases 1 and 2 - Tembe, S.; Lockner, D.A.; Solum, J.G.; Morrow, C.A.; Wong, T.-F.; Moore, D.E. (Geophysical Research Letters, 2006)
We investigated the frictional properties of drill cuttings and core obtained from 1.85-3.1 km true vertical depth in the SAFOD scientific borehole in central California. Triaxial frictional sliding experiments were conducted on samples from primary lithologic traits and significant shear zones, including the inferred active trace of the San Andreas fault. The samples were deformed at room temperature under constant effective normal stresses of 10, 40, and 80 MPa with axial shortening rates of 0.01-1.0 μm s-1. The weakest samples were from shale, claystone, and siltstone units with friction coefficient μ = 0.4-0.55. Stronger samples were from quartzo-feldspathic rocks with μ ≥ 0.6. Materials tested from two shear, zones at 2560 and 3067 m measured depth had μ = 0.4-0.55 and velocity strengthening behavior consistent with fault creep at depths <4 km. The coefficient of friction for bulk samples from the inferred trace of the San Andreas fault was ∼0.6. Copyright 2006 by the American Geophysical Union.
cited By 45
Keywords: Boreholes; Core samples; Friction; Lithology; Tectonics; Drill cuttings; Triaxial frictional sliding; Core analysis; borehole geophysics; creep; cutting; effective stress; fault; friction; San Andreas Fault; shear zone; sliding; California; North America; San Andreas; United States

Geochemical identification of projectiles in impact rocks - Tagle, R.; Hecht, L. (Meteoritics and Planetary Science, 2006)
The three major geochemical methods for impactor identification are evaluated with respect to their potential and limitations with regards to the precise detection and identification of meteoritic material in impactites. The identification of a projectile component in impactites can be achieved by determining certain isotopic and elemental ratios in contaminated impactites. The isotopic methods are based on Os and Cr isotopic ratios. Osmium isotopes are highly sensitive for the detection of minute amounts of extraterrestrial components of even ≪0.05 wt% in impactites. However, this only holds true for target lithologies with almost no chemical signature of mantle material or young mantle-derived mafic rocks. Furthermore, this method is not currently suitable for the precise identification of the projectile type. The Cr-isotopic method requires the relatively highest projectile contamination (several wt%) in order to detect an extraterrestrial component, but may allow the identification of three different groups of extraterrestrial materials, ordinary chondrites, an enstatite chondrites, and differentiated achondrites. A significant advantage of this method is its independence of the target lithology and post-impact alteration. The use of elemental ratios, including platinum group elements (PGE: Os, Ir, Ru, Pt, Rh, Pd), in combination with Ni and Cr represents a very powerful method for the detection and identification of projectiles in terrestrial and lunar impactites. For most projectile types, this method is almost independent of the target composition, especially if PGE ratios are considered. This holds true even in cases of terrestrial target lithologies with a high component of upper mantle material. The identification of the projectile is achieved by comparison of the projectile elemental ratio derived from the slope of the mixing line (target-projectile) with the elemental ratio in the different types of possible projectiles (e.g., chondrites). However, this requires a set of impactite samples of various degree of projectile contamination. © The Meteoritical Society, 2006.
cited By 69

Geophysical survey on the tectonic and sediment distribution of Qinghai Lake basin - An, ZhiSheng; Wang, Ping; Shen, Ji; Zhang, Yixiang; Zhang, Peizhen; Wang, Sumin; Li, Xiaoqiang; Sun, Qianli; Song, YouGui; Al, Li; Zhang, Yechun; Jiang, Shaoren; et al. (Science in China, Series D: Earth Sciences, 2006)
The intensive geophysical survey of Qinghai Lake reveals the buried depth of lake sediments and their distribution features. The result indicates that there are three important interphases of Qinghai Lake sediments: T1 is the rife interphase of the lake, above which sediments are spread all over the lake basin with roughly the same thickness; T5 is the interphase from which the neotectonic sedimentary cycle begins, and its above sedimentary environment is relatively stable; Tg is the base of the lake basin. Five west-northwest (WNW) fault belts defined the tectonic structure of Qinghai Lake basin: the central hunch around Haixin Shan with two subbasins both in its north and south. The thickness of the lake sediments varies at different places, the thickest sediments are found within the two subbasins. According to the depth that the Sparker System can reach, sediment in the northern subbasin is deeper than 560 m, while sediment in the southern subbasin is deeper than 700 m. The correlation between the seismic sequence stratigraph and the lithology of onshore core shows that Qinghai Lake sediments consist of muddy silt, clay silt, silty clay, gravel silty clay, etc. © Science in China Press 2006.
Keywords: Asia; China; Eurasia; Far East; Qinghai; Qinghai Lake; Clay; Geophysical prospecting; Lithology; Sedimentology; Sediments; Seismology; Stratigraphy; Tectonics; lacustrine deposit; lithology; neotectonics; sediment thickness; sedimentary sequence; sedimentary structure; seismic survey; Fault belts; Neotectonic sedimentary cycle; Lakes

Geothermal Fluid Flow Model in Shimabara Peninsula - Fujimitsu, Y.; Ehara, S.; Oki, R. (Journal of the Geothermal Research Society of Japan, 2006)
The Unzen Scientific Drilling Project (USDP) had been conducted by the Science and Technology Agency (FY1999-2000) and the Ministry of Education, Culture, Sports, Science and Technology, Japan (FY2001-2004). In this project, one of the objectives of our study was to construct a comprehensive hydrothermal model of beneath Unzen Volcano by using a numerical simulation. Four large geothermal systems are known in the Shimabara Peninsula (Obama hot springs, Unzen fumarolic field, Shimabara hot springs and the West Unzen High Temperature Body [WUHTB]). Three pressure source locations (“Sources A”, “B” and “C” from shallow to the deep) were determined by geodetic data during the 1990-95 eruption. Source C is located at about 8 km deep at WUHTB, and is considered to be a magma reservoir. We attempted to explain the existence mechanism of the four geothermal systems from the large-scale structures (the topography of the Shimabara Peninsula and Unzen Graben) and the various heat sources. We first set a heat source around Source C and changed its position and size. This numerical model produced the upflow zones at the Obama and Shimabara hot springs and WUHTB; however the Unzen fumarolic field became a recharge area. This result indicated that it would be difficult to develop the Unzen fumarolic field only by Source C ; therefore, we set another heat source just beneath the fumarolic field. Consequently, two heat sources beneath WUHTB and the Unzen fumarolic field are involved in the formation of the four hydrothermal systems in the Shimabara Peninsula. Especially, the heat source beneath the Unzen fumarolic field is essential to generate heat discharges at the fumarolic field. © 2006, THE GEOTHERMAL RESEARCH SOCIETY OF JAPAN. All rights reserved.
cited By 2

Ground ice and slope sediments archiving late Quaternary paleoenvironment and paleoclimate signals at the margins of El'gygytgyn Impact Crater, NE Siberia - Schwamborn, G.; Meyer, H.; Fedorov, G.; Schirrmeister, L.; Hubberten, H.-W. (Quaternary Research, 2006)
An accumulation terrace close to the El'gygytgyn Impact Crater in northeastern Siberia contains stratigraphic and periglacial evidence of the paleoenvironmental and paleoclimatic history and permafrost dynamics during late Quaternary time. A succession of paleo active-layer deposits that mirror environmental changes records periods favorable for the establishment and growth of ice-wedge polygonal networks and sediment variations. These two elements of the periglacial landscape serve as complementary paleoenvironmental archives that can be traced back to ∼ 14,000 cal yr BP. The slope sediments and the ground ice contained therein have prominent relative maxima and minima in properties (grain size, total organic content, oxygen isotopes). They document a regional early Holocene thermal maximum at about 9000 cal yr BP, followed by a transition to slightly cooler conditions, and a subsequent transition to slightly warmer conditions after about 4000 cal yr BP. Results from sedimentary analysis resemble morphological and geochemical (oxygen and hydrogen isotopes) results from ice wedge studies, in which successive generations of ice-wedge polygonal networks record warmer winters in late Holocene time. Moreover, peaks of light soluble cation contents and quartz-grain surface textures reveal distinct traces of cryogenic weathering. We propose a conclusive sedimentation model illustrating terrace formation in a permafrost terrain. © 2006 Elsevier B.V. All rights reserved.
cited By 52
Keywords: environmental change; frozen ground; Hypsithermal; paleoclimate; paleoenvironment; periglacial landform; permafrost; Eurasia; Siberia

High magnetic susceptibility of fault gouge within Taiwan Chelungpu fault: Nondestructive continuous measurements of physical and chemical properties in fault rocks recovered from Hole B, TCDP - Hirono, T.; Lin, W.; Yeh, E.-C.; Soh, W.; Hashimoto, Y.; Sone, H.; Matsubayashi, O.; Aoike, K.; Ito, H.; Kinoshita, M.; Murayama, M.; Song, S.-R.; et al. (Geophysical Research Letters, 2006)
The Taiwan Chelungpu-fault Drilling Project (TCDP) was undertaken in 2002 to investigate the faulting mechanism of the 1999 Taiwan Chi-Chi earthquake. Hole B penetrated the Chelungpu fault, and recovered core samples from between 948.42 m and 1352.60 m depth. Three zones, marked 1136mFZ, 1194mFZ and 1243mFZ, were recognized in the core samples as active fault-zones within the Chelungpu fault. Multi-Sensor Core Logger measurements revealed lower densities and higher magnetic susceptibilities within the black gouge zones in all three fault zones. Even though the fault zone that slipped during the 1999 earthquake has not been identified, higher magnetic susceptibilities indicate that frictional heating has taken place in the Chelungpu fault. Copyright 2006 by the American Geophysical Union.
cited By 79
Keywords: Earthquakes; Heating; Magnetic susceptibility; Project management; Sensors; Tribology; Chemical properties; Fault rocks; Geomorphology; Chi-Chi earthquake 1999; fault gouge; magnetic susceptibility; physicochemical property; rock; Asia; Eurasia; Far East; Taiwan

Particle size and energetics of gouge from earthquake rupture zones - Wilson, B.; Dewers, T.; Reches, Z.; Brune, J. (Nature, 2005)
Grain size reduction and gouge formation are found to be ubiquitous in brittle faults at all scales, and most slip along mature faults is observed to have been localized within gouge zones. This fine-grain gouge is thought to control earthquake instability, and thus understanding its properties is central to an understanding of the earthquake process. Here we show that gouge from the San Andreas fault, California, with ∼160 km slip, and the rupture zone of a recent earthquake in a South African mine with only ∼0.4 m slip, display similar characteristics, in that ultrafine grains approach the nanometre scale, gouge surface areas approach 80 m2g-1, and grain size distribution is nonfractal. These observations challenge the common perception that gouge texture is fractal and that gouge surface energy is a negligible contributor to the earthquake energy budget. We propose that the observed fine-grain gouge is not related to quasi-static cumulative slip, but is instead formed by dynamic rock pulverization during the propagation of a single earthquake.
cited By 234
Keywords: Comminution; Earthquakes; Fractals; Grain size and shape; Rocks; Earthquake instability; Energetics; Fine-grain gouge; Gouge formation; Particle size analysis; earthquake; particle size; article; earthquake; energy consumption; energy transfer; mining; particle size; priority journal; rock; surface property

Paleolimnological records from Asian lacustrine systems: Archives of large-scale seasonal atmospheric circulation changes in the Northern Hemisphere - Prokopenko, A.; Catto, N. (Quaternary International, 2005)
Keywords: atmospheric circulation; lacustrine deposit; paleolimnology

Oxygen isotope geochemistry of rocks from the Pre-Pilot Hole of the Chinese Continental Scientific Drilling Project (CCSD-PPH1) - Zhang, Ze M.; Rumble, D.; Liou, J.G.; Xiao, Yi L.; Gao, Yong J. (American Mineralogist, 2005)
The Qinglongshan O- and H-isotope anomaly occurs within a coesite-bearing, eclogite-facies regional metamorphic belt in Eastern China near Donghai in Jiangsu province. The anomaly is defined by low values of δ18O and δD. Garnets from eclogite have δ18O as low as -11‰ and rutiles are -15‰ (VSMOW). Phengites have δD of -120‰ (VSMOW). The anomaly is Neoproterozoic in age. Surface outcrops of coesite-eclogite-facies rocks with unusually low δ18O and δD values extend over an area of at least 1600 km2. The Chinese Continental Scientific Drilling project has made it possible to investigate the depth of the Qinglongshan anomaly and to measure an O-isotope profile across a garnet peridotite body sandwiched between crustal rocks. New O-isotope analyses of minerals separated from drill core gneisses, eclogites, amphibolite, and quartzite verify that the Qinglongshan O-isotope anomaly extends to a depth of at least 432 m. Crustal rocks with unusually low δ18O form both the hanging- and foot-walls of a 100 m thick, fault-bounded, garnet peridotite body intersected by the drill. Minerals of the garnet peridotite body have δ18O values indistinguishable from mantle nodules and megacrysts. The garnet peridotites may have originated from enriched mantle sources in sub-cratonic lithosphere, isolated from mantle convection. This origin is consistent with the collision of the Archean Sino-Korean craton with the Yangtze plate, consequent subduction, and UHP metamorphism. The garnet peridotite was transported from the mantle and emplaced into UHP crustal rocks during Triassic subduction and cratonic collision.
Keywords: Asia; China; Donghai; Eastern Hemisphere; Eurasia; Far East; Jiangsu; World; Drills; Exploratory geochemistry; Garnets; Infill drilling; Isotopes; Metamorphic rocks; Oxide minerals; Oxygen; Silica; Structural geology; Coesite-eclogite; Continental scientific drillings; Garnet peridotite; Isotope anomalies; Jiangsu province; Mantle convection; Metamorphic belts; Sino-Korean craton; Chinese Continental Scientific Drilling Project; collision zone; metamorphic rock; oxygen isotope; Core drilling

Overview of production test results for the mallik 2002 gas hydrate production research well program - Inoue, T. (Nihon Enerugi Gakkaishi/Journal of the Japan Institute of Energy, 2005)
This paper presents an overview of the pressure drawdown test and the thermal stimulation test at the Mallik 2002 gas hydrate research well, conducted in February and March 2002. Pressure drawdown tests were conducted on methane hydrate, free gas, and water intervals, using Schlumberger's Modular Formation Dynamics Tester (MDT) wireline tool. The thermal stimulation test was designed to increase the in-situ temperature of a well-defined and constrained methane hydrate reservoir above the methane hydrate stability point, while maintaining constant pressure. The objectives of the production tests were to confirm the feasibility of production of natural gas from methane hydrate deposits by depressurization and thermal stimulation, and to collect sufficient data to determine relevant methane hydrate formation properties. The production tests were successful in that significant amounts of the in-situ dissociation properties, as well as other scientific measurements, were obtained.
cited By 2
Keywords: Methane; Natural gas; Natural gas well drilling; Production; Mallik; Canada; MDT; Methane hydrates; Pressure drawdown tests; Production tests; Thermal stimulation tests; Gas hydrates

Orientation of three-component geophones in the San Andreas Fault observatory at depth Pilot Hole, Parkfield, California - Oye, V.; Ellsworth, W.L. (Bulletin of the Seismological Society of America, 2005)
To identify and constrain the target zone for the planned SAFOD Main Hole through the San Andreas Fault (SAF) near Parkfield, California, a 32-level three-component (3C) geophone string was installed in the Pilot Hole (PH) to monitor and improve the locations of nearby earthquakes. The orientation of the 3C geophones is essential for this purpose, because ray directions from sources may be determined directly from the 3D particle motion for both P and S waves. Due to the complex local velocity structure, rays traced from explosions and earthquakes to the PH show strong ray bending. Observed azimuths are obtained from P-wave polarization analysis, and ray tracing provides theoretical estimates of the incoming wave field. The differences between the theoretical and the observed angles define the calibration azimuths. To investigate the process of orientation with respect to the assumed velocity model, we compare calibration azimuths derived from both a homogeneous and 3D velocity model. Uncertainties in the relative orientation between the geophone levels were also estimated for a cluster of 36 earthquakes that was not used in the orientation process. The comparison between the homogeneous and the 3D velocity model shows that there are only minor changes in these relative orientations. In contrast, the absolute orientations, with respect to global North, were significantly improved by application of the 3D model. The average data residual decreased from 13° to 7°, supporting the importance of an accurate velocity model. We explain the remaining residuals by methodological uncertainties and noise and with errors in the velocity model.
cited By 14
Keywords: Calibration; Data reduction; Earthquakes; Installation; California; USA; Pilot Hole (PH); San Andreas Fault (SAF); Three-component (3C) geophones; Seismology; geophone; orientation; seismometry; California; North America; Parkfield; United States; Western Hemisphere; World

Occurrence and structural characterization of gas hydrates associated with a cold vent field, offshore Vancouver Island - Lu, H.; Moudrakovski, I.; Riedel, M.; Spence, G.; Dutrisac, R.; Ripmeester, J.; Wright, F.; Dallimore, S. (Journal of Geophysical Research: Solid Earth, 2005)
Gas hydrate samples recovered from a cold vent field offshore Vancouver Island were studied in detail both by macroscopic observations and instrumental methods (powder X-ray diffraction method (PXRD), nuclear magnetic resonance (NMR), and Raman spectroscopy). It was found that gas hydrates were massive from 2.64 to 2.94 m below seafloor (mbsf), elongated, nodular and tabular from 4.60 to 4.81 mbsf, and vein-like from 5.48 to 5.68 mbsf, showing a trend of decreasing hydrate content with increasing depth. All samples were determined to be structure I hydrate from PXRD, NMR, and Raman spectroscopies. The hydration numbers were estimated to be 6.1 ± 0.2 on average as determined from the methane distribution over the cage sites from NMR and Raman analytical results. Estimates of conversion levels indicated that ∼78% of the water in the massive samples was hydrate, down to a low value of ∼0.4% for the pore hydrate samples. The results are compared with measurements on synthetic hydrates and samples recovered from below the permafrost on the Mallik site. Differences in methane content and lattice parameters for synthetic and natural samples are relatively minor. Additional work is needed to address the presence of minor gas components and the heterogeneity of natural hydrate samples. Copyright 2005 by the American Geophysical Union.
cited By 59
Keywords: cold seep; gas hydrate; British Columbia; Canada; North America; Vancouver Island

Occurrence and geology of natural gas hydrate distributed in the mallik area, Northwest Territory, Canada - Uchida, T. (Nihon Enerugi Gakkaishi/Journal of the Japan Institute of Energy, 2005)
The Mallik 5L-38 gas hydrate production research well was drilled to 1166 m early in 2002, and abundant gas-hydrate-bearing cores were successfully retrieved from a variety of sediments. Gas hydrate-dominant layers were identified at depths of 889 to 1108 m. Gas-hydrate-bearing strata typically range from 10 cm to a few meters in thickness, and gas hydrate saturations in most gas hydrate layers were quite high. Pore-space hydrate is very small in size and fills the intergranular porosity of sandy sediments. The X-ray CT images of gas-hydrate-bearing sediments always show very uniform CT values. Those values in some parts of the sediment samples suggest that the methane hydrate contents range from 23 to 36 vol.% of the sample, which indicate high methane hydrate saturations in the intergranular porosity of the sand samples. Most of the gas hydrates fill the intergranular pore systems of sands, which are derived from channels and crevasse splay/levee deposits. Gas hydrate is less frequently found in siltstones and mudstones from interdistributary bay and overbank deposits. Measurements of water permeability were undertaken, initially keeping the hydrate stable at 10 °C by pressurizing, and then depressurizing gradually. Initial water permeabilities of gas hydrate/ice filled sands range from 1 to 5 millidarcy. Pore-space hydrate was observed to occur primarily in fine- to medium-grained arenite sands. X-ray diffraction and Raman spectroscopy measurements showed that pore-space hydrates contained in sands are mainly Structure I. P-wave velocities of those sands, measured over the interval -20 to 5 °C, decrease from 4 to 3 km/sec as water ice melted, S-wave velocities decrease from about 2.5 km/sec at -20 °C to about 1.5 km/sec at 5°C. Measurements of mechanical strength and electrical resistivity were also undertaken.
cited By 0
Keywords: Computerized tomography; Deposits; Electric resistance; Gas hydrates; Geology; Grain size and shape; Natural gas well drilling; Porosity; Raman spectroscopy; Sediments; Strength of materials; X ray diffraction; Mallik; Canada; Methane hydrate; P- & S-wave velocity; Pore-space hydrates; Natural gas

Numerical modeling of the largest terrestrial meteorite craters - Ivanov, B.A. (Solar System Research, 2005)
Multi-ring impact basins have been found on the surfaces of almost all planetary bodies in the Solar system with solid crusts. The details of their formation mechanism are still unclear. We present results of our numerical modeling of the formation of the largest known terrestrial impact craters. The geological and geophysical data on these structures accumulated over many decades are used to place constraints on the parameters of available numerical models with a dual purpose: (i) to choose parameters in available mechanical models for the crustal response of planetary bodies to a large impact and (ii) to use numerical modeling to refine the possible range of original diameters and the morphology of partially eroded terrestrial craters. We present numerical modeling results for the Vredefort, Sudbury, Chicxulub, and Popigai impact craters and compare these results with available geological and geophysical information. © 2005 Pleiades Publishing, Inc.
cited By 153

Non-dilatant deformation and failure mechanism in two Long Valley Caldera rocks under true triaxial compression - Chang, C.; Haimson, B. (International Journal of Rock Mechanics and Mining Sciences, 2005)
We conducted laboratory rock strength experiments in two ultra-fine-grained brittle rocks, hornfels and metapelite, which together are the major constituent of the Long Valley Caldera (California, USA) basement in the 2025-2996 m depth range. Both rocks are banded, and have very low porosity. Uniaxial compression tests at different orientations with respect to banding planes reveal that while the hornfels compressive strength is nearly isotropic, the metapelite possesses distinct anisotropy. Conventional triaxial tests in these rocks reveal that their respective strengths in a specific orientation increase approximately linearly with confining pressure. True triaxial compression experiments in specimens oriented at a consistent angle to banding, in which the magnitudes of the least (σ3) and the intermediate (σ2) principal stresses are different but kept constant during testing while the maximum principal stress is increased until failure, exhibit a behavior unlike that previously observed in other rocks under similar testing conditions. For a given magnitude of σ3, compressive strength σ1 does not vary significantly in both Long Valley rock types, regardless of the applied σ2, suggesting little or no intermediate principal stress effect. Strains measured in all three principal directions during loading were used to obtain plots of σ1 versus volumetric strain. These are consistently linear almost to the point of rock failure, suggesting no dilatancy. The phenomenon was corroborated by SEM inspection of failed specimens that showed no microcrack development prior to the emergence of one through-going shear failure plane steeply dipping in the σ3 direction. The strong dependency of compressive strength on the intermediate principal stress in other crystalline rocks was found to be related to microcrack initiation upon dilatancy onset, which rises with increased σ2 and retards the failure process. We infer that strength independence of σ2 in the Long Valley rocks derives directly from their non-dilatant deformation. © 2005 Elsevier Ltd. All rights reserved.
cited By 61
Keywords: Compressive strength; Crack initiation; Deformation; Failure analysis; Porosity; Strain; Strength of materials; Volumetric analysis; Brittle rocks; Metapelite; Rock strength; Volumetric strain; Rocks; deformation; triaxial test; volcanic rock; California; Long Valley; North America; United States; Western Hemisphere; World

Magnetization intensity mapping on Unzen Volcano, Japan, determined from high-resolution, low-altitude helicopter-borne aeromagnetic survey - Okubo, A.; Tanaka, Y.; Utsugi, M.; Kitada, N.; Shimizu, H.; Matsushima, T. (Earth, Planets and Space, 2005)
On September 18, 2002, we conducted a high-resolution, low-altitude helicopter-bome aeromagnetic survey at two flight altitudes, using spiral trajectories for the first time, over Unzen Volcano in the framework of the Unzen Scientific Drilling Project (USDP). This study obtained more detailed and new information than the previous aeromagnetic studies in Unzen volcano about the geological features, for understanding the history and eruption mechanism of the Unzen volcano. Therefore, we conducted a magnetization intensity mapping on the volcano, on the assumption that the magnetic anomalies are caused by the terrain magnetized in the same direction as the present Earth's magnetic field and the magnetization intensity varies only laterally. This map shows good agreement with the geologic features, especially the hydrothermal alteration zone and the collapsed pyroclastic deposits. In addition, even in the area covered by lavas, the magnetization intensities show various values corresponding to each eruption event. It may be considered that the differences in magnetic properties reflect different oxygen fugacity in rocks during their cooling time period. Local magnetization lows on Heisei-Shinzan suggest that the Heisei lava produced by the 1991-1995 eruption has not yet been cooled enough. Copyright © The Society of Geomagnetism and Earth, Planetary and Space Sciences (SGEPSS); The Seismological Society of Japan; The Volcanological Society of Japan; The Geodetic Society of Japan; The Japanese Society for Planetary Sciences; TERRAPUB.
cited By 5
Keywords: aeromagnetic survey; cooling; lava flow; magnetic intensity; volcano; Asia; Eastern Hemisphere; Eurasia; Far East; Japan; Kyushu; Nagasaki [Kyushu]; Unzen Volcano; World

New surveys of the Chesapeake Bay impact structure suggest melt pockets and target-structure effect - Shah, A.K.; Brozena, J.; Vogt, P.; Daniels, D.; Plescia, J. (Geology, 2005)
We present high-resolution gravity and magnetic field survey results over the 85-km-diameter Chesapeake Bay impact structure. Whereas a continuous melt sheet is anticipated at a crater this size, shallow-source magnetic field anomalies of ∼100 nT instead suggest that impact melt pooled in kilometer-scaled pockets surrounding the base of a central peak. A central anomaly of ∼300 nT may represent additional melt or rock that underwent shock-induced remagnetization. Models predict that the total volume of the melt ranges from ∼0.4 to 10 km3, a quantity that is several orders of magnitude smaller than expected for an impact structure this size. However, this volume is within predictions given a transient crater of diameter of 20-40 km for a target covered with water and sedimentary deposits such that melt fragments were widely dispersed at the time of impact. Gravity data delineate a gently sloping inner basin and a central peak via a contrast between crystalline and sedimentary rock. Both features are ovoid, oriented parallel to larger preimpact basement structures. Conceptual models suggest how lateral differences in rock strength due to these preimpact structures helped to shape the crater's morphology during transient-crater modification. © 2005 Geological Society of America.
cited By 27
Keywords: Chesapeake bay impact structures; Craters; Melt fragments; Remagnetization; Gravitation; Magnetic fields; Magnetization; Melting; Morphology; Rocks; Tectonics; gravity survey; impact structure; magnetic survey; remagnetization; shock metamorphism; Chesapeake Bay; North America; United States; Western Hemisphere; World

Morphological, structural and lithological records of terrestrial impacts: An overview - Masaitis, V.L. (Australian Journal of Earth Sciences, 2005)
Impact cratering produces not only craterform topographic features, but also structural disturbances at the site of impact, and a spectrum of transformed and newly formed rocks. The term 'coptogenesis' (from the Greek χoπτo, to destroy by shock) may be used collectively to describe the impact process-a process fundamental to all cosmic bodies. Principal coptogenic topographic features of terrestrial impact craters may be subdivided into excavational, structural and accumulative landforms, most of which subsequently experience various processes of degradation. Nevertheless, the original shape of craters may in some cases be reconstructed and compared with fresh craters on other planets. An immediate conformity between the pre-erosional topographic features of complex terrestrial craters, and the morphostructural elements of their erosional remnants, is not a standing rule. Geological observations show that the inner structure of the proximal crater fill and distal ejecta are characterised by pseudo-stratification and that these materials represent a group of facies of impact-derived and impact-related, or coptogenic, lithologies. The study of these facies allows us to distinguish various facies settings of rock-forming processes. Impact lithologies, or coptogenic rocks, may be systematised and classified using the principles adopted by igneous petrology and volcanology. Appropriate geological methods and approaches should be applied to the investigation of terrestrial impact craters, including their identification, mapping, and study of their various physiographic, structural, and lithological features. © Geological Society of Australia.
cited By 11

Modeling of acoustic wave dissipation in gas hydrate-bearing sediments - Guerin, G.; Goldberg, D. (Geochemistry, Geophysics, Geosystems, 2005)
Recent sonic and seismic data in gas hydrate-bearing sediments have indicated strong waveform attenuation associated with a velocity increase, in apparent contradiction with conventional wave propagation theory. Understanding the reasons for such energy dissipation could help constrain the distribution and the amounts of gas hydrate worldwide from the identification of low amplitudes in seismic surveys. A review of existing models for wave propagation in frozen porous media, all based on Biot's theory, shows that previous formulations fail to predict any significant attenuation with increasing hydrate content. By adding physically based components to these models, such as cementation by elastic shear coupling, friction between the solid phases, and squirt flow, we are able to predict an attenuation increase associated with gas hydrate formation. The results of the model agree well with the sonic logging data recorded in the Mallik 5L-38 Gas Hydrate Research Well. Cementation between gas hydrate and the sediment grains is responsible for the increase in shear velocity. The primary mode of energy dissipation is found to be friction between gas hydrate and the sediment matrix, combined with an absence of inertial coupling between gas hydrate and the pore fluid. These results predict similar attenuation increase in hydrate-bearing formations over most of the sonic and seismic frequency range. Copyright 2005 by the American Geophysical Union.
cited By 69

Microbial diversity in ultra-high-pressure rocks and fluids from the Chinese continental scientific drilling project in China - Zhang, Gengxin; Dong, Hailiang; Xu, Zhiqin; Zhao, Donggao; Zhang, Chuanlun (Applied and Environmental Microbiology, 2005)
Microbial communities in ultra-high-pressure (UHP) rocks and drilling fluids from the Chinese Continental Scientific Drilling Project were characterized. The rocks had a porosity of 1 to 3.5% and a permeability of ∼0.5 mDarcy. Abundant fluid and gas inclusions were present in the minerals. The rocks contained significant amounts of Fe2O3, FeO, P2O5, and nitrate (3 to 16 ppm). Acridine orange direct counting and phospholipid fatty acid analysis indicated that the total counts in the rocks and the fluids were 5.2 × 103 to 2.4 × 10 4 cells/g and 3.5 x 108 to 4.2 × 109 cells/g, respectively. Enrichment assays resulted in successful growth of thermophilic and alkaliphilic bacteria from the fluids, and some of these bacteria reduced Fe(III) to magnetite. 16S rRNA gene analyses indicated that the rocks were dominated by sequences similar to sequences of Proteobacteria and that most organisms were related to nitrate reducers from a saline, alkaline, cold habitat; however, some phylotypes were either members of a novel lineage or closely related to uncultured clones. The bacterial communities in the fluids were more diverse and included Proteobacteria, Bacteroidetes, gram-positive bacteria, Planctomycetes, and Candidatus taxa. The archaeal diversity was lower, and most sequences were not related to any known cultivated species. Some archaeal sequences were 90 to 95% similar to sequences recovered from ocean sediments or other subsurface environments. Some archaeal sequences from the drilling fluids were >93% similar to sequences of Sulfolobus solfataricus, and the thermophilic nature was consistent with the in situ temperature. We inferred that the microbes in the UHP rocks reside in fluid and gas inclusions, whereas those in the drilling fluids may be derived from subsurface fluids. Copyright © 2005, American Society for Microbiology. All Rights Reserved.
Keywords: Atmospheric Pressure; Bacteria; China; Crenarchaeota; Culture Media; DNA; Archaeal; DNA; Bacterial; DNA; Ribosomal; Ecosystem; Geologic Sediments; Geology; Minerals; Molecular Sequence Data; Phylogeny; Polymorphism; Restriction Fragment Length; RNA; Ribosomal; 16S; Sequence Analysis; DNA; Asia; China; Eastern Hemisphere; Eurasia; Far East; World; Archaea; Bacteroidetes; Candidatus; Planctomycetes; Posibacteria; Proteobacteria; Sulfolobus solfataricus; Bacteria; Bioassay; Drilling; Fatty acids; High pressure effects; Iron; Magnetite; Phosphorus compounds; Porosity; RNA; Rocks; ferric ion; magnetite; nitrate; RNA 16S; sodium chloride; microbial community; alkalinity; article; bacterial flora; bacterial gene; Bacteriodetes; bacterium; Candidatus; China; controlled study; drill; gene sequence; geographic distribution; Gram positive bacterium; microbial diversity; nonhuman; nucleotide sequence; Planctomycetes; Proteobacteria; rock; Sulfolobus solfataricus; thermophilic bacterium; unindexed sequence; Gas inclusion; Microbial diversity; Prototypes; Ultra-high-pressure (UHP) rocks; Biodiversity

Mallik 2002 gas hydrate production research well program: Numerical simulation studies for analyzing the mechanism of gas production from methane hydrate reservoirs - Kurihara, M. (Nihon Enerugi Gakkaishi/Journal of the Japan Institute of Energy, 2005)
The results of the thermal production test and Modular Formation Dynamics Tester (MDT) tests conducted at the Mallik 5L-38 gas hydrate production research well were analyzed, using the numerical simulator coded for gas hydrate reservoirs. The reservoir models were constructed as a series of grid blocks. In the simulation for the thermal production test, the reservoir model was tuned through history matching, by introducing the concept that part of the circulating hot water might have invaded into the reservoir, which resulted in the excellent agreement between observed and simulated performances. Using the history-matched reservoir model, sensitivity simulation and prediction of future performances were conducted to examine the effects of uncertain reservoir parameters and the gas hydrate dissociation/production methods on the recovery of gas from the Mallik reservoir. In the MDT test simulation, to investigate the applicability of conventional pressure transient test analysis methods, bottomhole pressure responses during MDT tests in hypothetical and actual gas hydrate zones were simulated and were then analyzed by the conventional analysis methods. This study revealed that conventional methods might indicate the average effective permeability over the area of gas hydrate dissociation while the estimation of the radius of gas hydrate dissociation was quite imprecise.
cited By 0
Keywords: Computer simulation; Mathematical models; Natural gas well drilling; Sensitivity analysis; History matching; Methane hydrates; Modular formation dynamics tester (MDT); Thermal methods; Well tests; Gas hydrates

Lead isotopes reveal bilateral asymmetry and vertical continuity in the Hawaiian mantle plume - Abouchami, W.; Hofmann, A.W.; Galer, S.J.G.; Frey, F.A.; Eisele, J.; Felgenson, M. (Nature, 2005)
The two parallel chains of Hawaiian volcanoes ('Loa' and 'Kea') are known to have statistically different but overlapping radiogenic isotope characteristics. This has been explained by a model of a concentrically zoned mantle plume, where the Kea chain preferentially samples a more peripheral portion of the plume. Using high-precision lead isotope data for both centrally and peripherally located volcanoes, we show here that the 'two trends have very little compositional overlap and instead reveal bilateral, non-concentric plume zones, probably derived from the plume source in the mantle. On a smaller scale, along the Kea chain, there are isotopic differences between the youngest lavas from the Mauna Kea and Kilauea volcanoes, but the 550-thousand-year-old Mauna Kea lavas are isotopically identical to Kilauea lavas, consistent with Mauna Kea's position relative to the plume, which was then similar to that of present-day Kilauea. We therefore conclude that narrow (less than 50 kilometres wide) compositional streaks, as well as the larger-scale bilateral zonation, are vertically continuous over tens to hundreds of kilometres within the plume.
cited By 204
Keywords: Composition; Geographical regions; Geophysics; Lead; Volcanoes; Isotopy; Lead isotopes; Mantle plume; Radiogenic isotopes; Radioisotopes; isotope; lead; isotopic composition; lava flow; lead; mantle plume; accuracy; article; chemical composition; evolution; geographic distribution; parameter; plume; priority journal; radioactivity; regression analysis; sampling; technique; theoretical model; United States; volcano; Hawaii [(ISL) Hawaiian Islands]; Hawaii [United States]; Hawaiian Islands; Mauna Kea; Mauna Loa; North America; oceanic regions; Pacific islands; Pacific Ocean; United States; Western Hemisphere; World

Late-Quaternary lowstands of lake Bosumtwi, Ghana: Evidence from high-resolution seismic-reflection and sediment-core data - Brooks, Keely; Scholz, Christopher A.; King, John W.; Peck, John; Overpeck, Jonathan T.; Russell, James M.; Amoako, Philip Y.O. (Palaeogeography, Palaeoclimatology, Palaeoecology, 2005)
Results from the first high-resolution, single-channel seismic-reflection survey of tropical Lake Bosumtwi, Ghana, and sedimentological data from a 14C-dated sediment piston core were used to revise and extend the basin's late-Quaternary lake level history. We report four seismic sequence boundaries and an exposure surface from a sediment core, which are interpreted as erosional surfaces formed at times of drastic low lake level. The youngest erosional surface occurs as much as 31 m below present lake level (bpll) and up to 0.7 m below the present sediment-water interface. This most recent unconformity observed in the seismic data is interpreted to be coeval with the basin-wide late-Holocene dry period between 0.5 and 1 cal ky BP (calendar years before present). Another exposure surface observed in a sediment core is based on an abrupt contact separating low density, wet, clay rich sediments from underlying high density, compact, silt-rich and rootlet-rich sediments, and is interpreted to have developed prior to 16.8 cal ky BP when the lake was ∼60 m bpll. Three older, erosional surfaces occur at depths of ∼92±3, 102±3, and 107±4 m bpll, suggesting numerous lowstands in Lake Bosumtwi during the late-Pleistocene. By extrapolation of average sedimentation rates (0.41 m/ky) from the upper ∼10.5 m of sediment, we estimate the ages of these older lowstands to be ∼65, ∼86, ∼108 cal ky BP. The lowstands of Lake Bosumtwi evidenced from the seismic and sediment core data are interpreted as a response to increased aridity in this part of the equatorial tropics and may correlate to other observed continent-wide shifts in African climate over the past 100 ky, and possibly to rapid climatic shifts observed at high latitudes. Determining the precise timing of these lowstands will ultimately reveal much about the drought dynamics of tropical and subtropical Africa. © 2004 Elsevier B.V. All rights reserved.
Keywords: Africa; Ashanti; Eastern Hemisphere; Ghana; Lake Bosumtwi; Sub-Saharan Africa; West Africa; World; lake level; lowstand; Quaternary; seismic survey

Late Maastrichtian and K/T paleoenvironment of the eastern Tethys (Israel): Mineralogy, trace and platinum group elements, biostratigraphy and faunal turnovers - Adatte, T.; Keller, G.; St{uben, D.; Harting, M.; Kramar, U.; Stinnesbeck, W.; Abramovich, S.; Benjamini, C. (Bulletin de la Societe Geologique de France, 2005)
The late Maastrichtian to early Danian at Mishor Rotem, Israel, was examined based on geochemistry, bulk rock and clay mineralogies, biostratigraphy and lithology. This section contains four red clay layers of suspect impact or volcanic origin interbedded in chalk and marly chalks. PGE anomalies indicate that only the K/T boundary red layer has an Ir dominated PGE anomaly indicative of an impact source. The late Maastrichtian red clays have Pd dominated PGE anomalies which coincide with increased trace elements of terrigenous and volcanogenic origins. Deccan or Syrian-Turkey arc volcanism is the likely source of volcanism in these clay layers. Glauconite, goethite and translucent amber spherules are present in the clay layers, but the Si-rich spherules reported by Rosenfeld et al. [1989] could not be confirmed. The absence of Cheto smectite indicates that no altered impact glass has been present. The red layers represent condensed sedimentation on topographic highs during sea level highstands. In the Negev area, during the late Maastrichtian, the climate ranged from seasonally wet to more arid conditions during zones CF3 and CF2, with more humid wet conditions in the latest Maastrichtian zone CF1 and in the early Danian, probably linked to greenhouse conditions. Planktic foraminifera experienced relatively high stress conditions during this time as indicated by the low species richness and low abundance of globotruncanids. Times of intensified stress are indicated by the disaster opportunist Guembelitria blooms, which can be correlated to central Egypt and also to Indian Ocean localities associated with mantle plume volcanism. Marine plankton thus support the mineralogical and geochemical observations of volcanic influx and reveal the detrimental biotic effects of intense volcanism.
cited By 22

Petrogenesis of UHP metamorphic rocks from Qinglongshan, southern Sulu, east-central China - Zhang, Zeming; Xiao, Yilin; Liu, Fulai; Liou, J.G.; Hoefs, Jochen (Lithos, 2005)
Five kinds of UHP metamorphic rocks, including eclogite, orthogneiss, paragneiss, schist and quartzite are exposed in the Qinglongshan roadcut, southern Sulu orogenic belt of eastern central China. They comprise metamorphic supracrustal rocks with bimodal volcanic characteristics and continental affinity, and granitic intrusive associations. The preservation of coesite inclusions and/or its pseudomorphs in eclogite and other rocks indicate that they have been subjected to in-situ UHP metamorphism. Four stages of metamorphism were recognized by combining petrographic observations and compositions of minerals from various UHP rocks. Prograde epidote-amphibolite facies, UHP coesite-eclogite facies, post UHP quartz-eclogite facies, and retrograde amphibolite facies assemblages delineate an inferred P-T path with a clockwise trajectory and a retrograde event characterized by the coupling of decompression with a temperature decrease. Garnet porphyroblasts in UHP eclogites display a complex growth zoning and mineral distribution, and record a crucial segment of the prograde and retrograde metamorphic evolution. The preservation of growth zoning in eclogitic and gneissic garnets suggests that the UHP rocks had a short residence time before retrograde metamorphism and a very high uplift rate in order to preserve the prograde growth zoning. © 2004 Elsevier B.V. All rights reserved.
Keywords: Asia; China; Donghai; Eastern Hemisphere; Eurasia; Far East; Jiangsu; Sulu Belt; World; metamorphism; petrogenesis; petrology; ultrahigh pressure metamorphism

Late glacial and Holocene vegetation and regional climate variability evidenced in high-resolution pollen records from Lake Baikal - Demske, Dieter; Heumann, Georg; Granoszewski, Wojciech; Nita, Małgorzata; Mamakowa, Kazimiera; Tarasov, Pavel E.; Oberh{ansli, Hedi (Global and Planetary Change, 2005)
High-resolution pollen records from Lake Baikal revealed considerable regional differences in the vegetation development and pronounced climate variability during the last glacial-interglacial transition and Holocene. Correlation between cores was successfully based on a chronology constructed from AMS 14C dating of pollen concentrates. Comparison to other radiocarbon-dated pollen sequences from the Baikal region suggests that the chronology presented is very reliable, and thus correlation to other dated events can easily be performed. Pollen indices, which reflect relative changes in major vegetation types and limitations of growing conditions by moisture availability and temperature, demonstrate near-synchronous vegetation changes, which suggest synchronous large-scale climate variation across the Baikal region. Due to the limited influence of human impact in the Lake Baikal region, the pollen data illustrate that, in the continental interior of NE Eurasia Holocene, climate variability was very pronounced. After initial warming and a strong increase in relative moisture (ca. 16 cal ka BP), the Bølling-Allerød-like event was punctuated by three cool and dry events. These events, dated between ca. 15 and 13 cal ka BP, can be compared to coolings as recorded in GISP 2 oxygen isotope records from Greenland ice cores. An expansion of Betula sect. Nanae/Fruticosae, Artemisia and Chenopodiaceae marks the Younger-Dryas (YD)-like cooling event (ca. 12.5-12 cal ka BP). High temperatures and favourable moisture conditions during the first part of Holocene favoured the optimum development of dark coniferous taiga between 11-7.5 cal ka BP in the south and 10-8 cal ka BP in the northeast. A fir and spruce decline in the southern mountains (ca. 9.5-8.5 cal ka BP) can be related to the 8.2 cal ka BP cooling event. The pronounced mid-Holocene cooling event and a transition towards dry conditions (ca. 8-7 cal ka BP) preceded the nearly synchronous regional expansion of pine taiga. Maximum distribution of Scots pine forests marks the Holocene thermal optimum (ca. 6.5-5.7 cal ka BP), which was followed by two subsequent cooling events (ca. 5.5-4.5 cal ka BP) at the Atlantic-Subboreal transition. A subsequent temperature optimum in the southeastern Baikal region ended with pronounced cooling during the Subboreal-Subatlantic transition (ca. 3-2.5 cal ka BP). A late spread of shrub alders may evidence the beginning of the Little Ice Age. © 2004 Elsevier B.V. All rights reserved.
Keywords: Eastern Hemisphere; Eurasia; Lake Baikal; Russian Federation; World; Abies; Acer pensylvanicum; Alnus; Amaranthaceae; Artemisia; Betula; Betulaceae; Picea; Pinus sylvestris; climate variation; Holocene; late glacial; paleoclimate; palynology; vegetation history

Particle size and energetics of gouge from earthquake rupture zones - Wilson, B.; Dewers, T.; Reches, Z.; Brune, J. (Nature, 2005)
Grain size reduction and gouge formation are found to be ubiquitous in brittle faults at all scales, and most slip along mature faults is observed to have been localized within gouge zones. This fine-grain gouge is thought to control earthquake instability, and thus understanding its properties is central to an understanding of the earthquake process. Here we show that gouge from the San Andreas fault, California, with ∼160 km slip, and the rupture zone of a recent earthquake in a South African mine with only ∼0.4 m slip, display similar characteristics, in that ultrafine grains approach the nanometre scale, gouge surface areas approach 80 m2g-1, and grain size distribution is nonfractal. These observations challenge the common perception that gouge texture is fractal and that gouge surface energy is a negligible contributor to the earthquake energy budget. We propose that the observed fine-grain gouge is not related to quasi-static cumulative slip, but is instead formed by dynamic rock pulverization during the propagation of a single earthquake.
cited By 239
Keywords: Comminution; Earthquakes; Fractals; Grain size and shape; Rocks; Earthquake instability; Energetics; Fine-grain gouge; Gouge formation; Particle size analysis; earthquake; particle size; article; earthquake; energy consumption; energy transfer; mining; particle size; priority journal; rock; surface property

The 40Ar/39Ar dating of core recovered by the Hawaii Scientific Drilling Project (phase 2), Hilo, Hawaii - Sharp, Warren D.; Renne, Paul R. (Geochemistry, Geophysics, Geosystems, 2005)
The Hawaii Scientific Drilling Project, phase 2 (HSDP-2), recovered core from a ∼3.1-km-thick section through the eastern flanks of Mauna Loa and Mauna Kea volcanoes. We report results of 40Ar/39Ar incremental heating by broad-beam infrared laser of 16 basaltic groundmass samples and 1 plagioclase separate, mostly from K-poor tholeiites. The tholeiites generally have mean radiogenic 40Ar enrichments of 1-3%, and some contain excess 40Ar; however, isochron ages of glass-poor samples preserve stratigraphic order in all cases. A 246-m-thick sequence of Mauna Loa tholeiitic lavas yields an isochron age of 122 ± 86 kyr (all errors 2<r) at its base. Beneath the Mauna Loa overlap sequence lie Mauna Kea's postshield and shield sequences. A postshield alkalic lava yields an age of 236 ± 16 kyr, in agreement with an age of 240 ± 14 kyr for a geochemically correlative flow in the nearby HSDP-1 core hole, where more complete dating of the postshield sequence shows it to have accumulated at 0.9 ± 0.4 m/kyr, from about 330 to <200 ka. Mauna Kea's shield consists of subaerial tholeiitic flows to a depth of 1079 m below sea level, then shallow submarine flows, hyaloclastites, pillow lavas, and minor intrusions to core bottom at 3098 m. Most subaerial tholeiitic flows fail to form isochrons; however, a sample at 984 m yields an age of 370 ± 180 kyr, consistent with ages from similar levels in HSDP-1. Submarine tholeiites including shallow marine vitrophyres, clasts from hyaloclastites, and pillow lavas were analyzed; however, only pillow lava cores from 2243, 2614, and 2789 m yield reliable ages of 482 ± 67, 560 ± 150, and 683 ± 82 kyr, respectively. A linear fit to ages for shield samples defines a mean accumulation rate of 8.6 ± 3.1 m/kyr and extrapolates to ∼635 kyr at core bottom. Alternatively, a model relating Mauna Kea's growth to transport across the Hawaiian hot spot that predicts downward accelerating accumulation rates that reach ∼20 m/kyr at core bottom (DePaolo and Stolper, 1996) is also consistent with all reliable ages except the deepest. Copyright 2005 by the American Geophysical Union.

Principal features of impact-generated hydrothermal circulation systems: Mineralogical and geochemical evidence - Naumov, M.V. (Geofluids, 2005)
Any hypervelocity impact generates a hydrothermal circulation system in resulting craters. Common characteristics of hydrothermal fluids mobilized within impact structures are considered, based on mineralogical and geochemical investigations, to date. There is similarity between the hydrothermal mineral associations in the majority of terrestrial craters; an assemblage of clay minerals-zeolites-calcite-pyrite is predominant. Combining mineralogical, geochemical, fluid inclusion, and stable isotope data, the distinctive characteristics of impact-generated hydrothermal fluids can be distinguished as follows: (i) superficial, meteoric and ground water and, possibly, products of dehydration and degassing of minerals under shock are the sources of hot water solutions; (ii) shocked target rocks are sources of the mineral components of the solutions; (iii) flow of fluids occurs mainly in the liquid state; (iv) high rates of flow are likely (10-4 to 10-3 m s-1); (v) fluids are predominantly aqueous and of low salinity; (vi) fluids are weakly alkaline to near-neutral (pH 6-8) and are supersaturated in silica during the entire hydrothermal process because of the strong predominance of shock-disordered aluminosilicates and fusion glasses in the host rocks; and (vii) variations in the properties of the circulating solutions, as well as the spatial distribution of secondary mineral assemblages are controlled by temperature gradients within the circulation cell and by a progressive cooling of the impact crater. Products of impact-generated hydrothermal processes are similar to the hydrothermal mineralization in volcanic areas, as well as in modern geothermal systems, but impacts are always characterized by a retrograde sequence of alteration minerals. © 2005 Blackwell Publishing Ltd.
cited By 91

Structure and stress state of Hawaiian island basalts penetrated by the Hawaii Scientific Drilling Project deep core hole - Morin, Roger H.; Wilkens, Roy H. (Journal of Geophysical Research: Solid Earth, 2005)
As part of the Hawaii Scientific Drilling Project (HSDP), an exploratory hole was drilled in 1993 to a depth of 1056 meters below sea level (mbsl) and a deeper hole was drilled to 3098 mbsl in 1999. A set of geophysical well logs was obtained in the deeper hole that provides fundamental information regarding the structure and the state of stress that exist within a volcanic shield. The acoustic televiewer generates digital, magnetically oriented images of the borehole wall, and inspection of this log yields a continuous record of fracture orientation with depth and also with age to 540 ka. The data depict a clockwise rotation in fracture strike through the surficial Mauna Loa basalts that settles to a constant heading in the underlying Mauna Kea rocks. This behavior reflects the depositional slope directions of lavas and the locations of volcanic sources relative to the drill site. The deviation log delineates the trajectory of the well bore in three-dimensional space. This path closely follows changes in fracture orientation with depth as the drill bit is generally prodded perpendicular to fracture strike during the drilling process. Stress-induced breakouts observed in the televiewer log identify the orientations ot the maximum and minimum horizontal principal stresses to be north-south and east-west, respectively. This stress state is attributed to the combination of a sharp break in onshore-offshore slope that reduces stress east-west and the emergence of Kilauea that increases stress north-south. Breakouts are extensive and appear over approximately 30% of the open hole. Copyright 2005 by the American Geophysical Union.
Keywords: Hawaiian Islands; oceanic regions; Pacific islands; Pacific Ocean; World; ocean island basalt; stress field; structural geology; volcanic island

Introduction to progress towards reconstructing past climate in Central Eurasia, with special emphasis on Lake Baikal - Oberh{ansli, Hedi; Mackay, Anson W. (Global and Planetary Change, 2005)
Keywords: paleoclimate

Using openhole and cased hole resistivity logs to monitor gas hydrate dissociation during a thermal test in the Mallik 5L-38 research well, Mackenzie Delta, Canada - Anderson, B.I.; Collett, T.S.; Lewis, R.E.; Dubourg, I. (SPWLA 46th Annual Logging Symposium 2005, 2005)
Gas hydrates (naturally occurring ice-like combinations of gas and water) have the potential to provide vast amounts of natural gas from the world's oceans and polar regions. However, producing gas economically from hydrates entails major technical challenges. Proposed recovery methods such as dissociating or melting gas hydrates by heating or depressurization are currently being tested. One such test was conducted recently in northern Canada by the partners in the Mallik 2002 Gas Hydrate Production Research Well Program. This paper describes how resistivity logs were used to determine the size of the annular region of gas hydrate dissociation that occurred around the wellbore during the thermal test in the Mallik 5L-38 well. An openhole logging suite, run prior to the thermal test, included array induction, array laterolog, nuclear magnetic resonance and 1.1-GHz electromagnetic propagation logs. The reservoir saturation tool was run both before and after the thermal test to monitor formation changes. A cased hole formation resistivity log was run after the test. Baseline resistivity values in each formation layer (Rt) were established from the deep laterolog data. The resistivity in the region of gas hydrate dissociation near the wellbore (Rxo) was determined from electromagnetic propagation and reservoir saturation tool measurements. The radius of hydrate dissociation as a function of depth was then determined by means of iterative forward modeling of cased hole formation resistivity tool response. Pretest computer simulations had predicted that dissociation would take place at a constant radius over the 13-ft test interval. However, the post-test resistivity modeling showed that this was not the case. The resistivity-derived dissociation radius was greatest near the outlet of the pipe that circulated hot water in the wellbore, where the highest temperatures were recorded. The radius was smallest near the center of the test interval, where a conglomerate section with low values of porosity and permeability inhibited dissociation. The free gas volume calculated from the resistivity-derived dissociation radii yielded a value within 20% of surface gauge measurements. These results show that the inversion of resistivity measurements holds promise for use in future gas hydrate monitoring. © 2005, held jointly by the Society of Petrophysicists and Well Log Analysts (SPWLA) and the submitting authors.
cited By 1
Keywords: Boreholes; Density measurement (specific gravity); Dissociation; Electric logging; Electromagnetic logging; Gages; Gases; Hydration; Iterative methods; Nuclear magnetic logging; Oil field equipment; Radioactivity logging; Software testing; Thermal logging; Cased hole formation resistivity tools; Cased-hole formation resistivities; Electromagnetic propagation; Gas-hydrate production; Hydrate dissociation; Resistivity measurement; Resistivity modeling; Technical challenges; Gas hydrates

Topography over the Chicxulub impact crater from Shuttle Radar Topography Mission data - Kinsland, G.L.; Pope, K.O.; Cardador, M.H.; Cooper, G.R.J.; Cowan, D.R.; Kobrick, M.; Sanchez, G. (Special Paper of the Geological Society of America, 2005)
Shuttle Radar Topography Mission (SRTM) data over the Chicxulub impact crater are imaged and compared to previously available topography data. While the two data sets contain different biases related to variations in terrain and vegetation cover, the correspondence of the two sets supports earlier interpretations that the complex structure of the buried crater is expressed in the topography of the northwestern Yucat'an Peninsula, M'exico. © 2005 Geological Society of America.
cited By 5

The use of field dependence of AC susceptibility for the interpretation of magnetic mineralogy and magnetic fabrics in the HSDP-2 basalts, Hawaii - Vahle, Carsten (Earth and Planetary Science Letters, 2005)
We applied the field dependence parameter χHd (%) = [(k300A/m-k30A/m)/k300A/m] × 100 given by de Wall for the subaerial and submarine basalts drilled by the 3109 m deep HSDP-2 borehole on Hawaii in order to verify the hypothesis that mainly composition controls the field dependence of AC susceptibility in titanomagnetite of natural occurrences. When we used this parameter, our data showed a significant scattering compared to data presented in earlier studies. In addition to composition, the effect of measurement temperature, grain size and anisotropy on the field dependent susceptibility were examined and found to be critical. The impact of grain size is weaker than the other effects. It cannot be totally excluded that the observed effects arise indirectly through an overlap of the other effects for the investigated basalts. The most important factor for the variation of field dependence is the degree of oxidation, causing a modification of the titanomagnetite composition or formation of titanomaghemite, and the mixing of Ti-rich with Ti-poor titanomagnetites, which strongly reduces the χHd parameter. Field dependence is not only related to titanomagnetite composition, especially for intermediate titanomagnetites with TCs between 100 and 300 °C. Temperature dependent susceptibility measurements at different field amplitudes for these intermediate types showed at constant geometry of the k(T) curve great differences in susceptibility, resulting in significant changes of the field dependence parameter over the temperature interval from - 100 to 260 °C. herefore variations of the ambient measurement temperatures are able to influence the field dependence. The second important effect is the degree of particle shape and alignment, which controls the field dependence in different orientations especially for the intermediate titanomagnetite, which is intensively intergrown with elongated hemoilmenite grains. As a consequence, samples with higher degrees of anisotropy exhibit differences of the field dependence parameter if measured parallel to kmax or kmin axis. Therefore, in addition to compositional effects and the temperature dependence, the magnetic fabric has to be considered for the interpretation of field dependent susceptibility measurements. The influence of intrinsic (Ti-content, magnetocrystalline anisotropy), and extrinsic (shape and alignment of grains) factors for the interpretation of the degree of anisotropy has to be kept in mind when interpreting AMS data in terms of strain rates experienced by moving lava during emplacement. © 2005 Elsevier B.V. All rights reserved.
Keywords: Hawaii [(ISL) Hawaiian Islands]; Hawaii [United States]; Hawaiian Islands; North America; oceanic regions; Pacific islands; Pacific Ocean; United States; Western Hemisphere; World; Basalt; Composition effects; Grain size and shape; Magnetic susceptibility; Magnetite; Mineralogy; Oxidation; Field dependence; Magnetic fabrics; Magnetic mineralogy; Titanomagnetite; basalt; magnetic field; magnetic susceptibility; titanomagnetite; Geochemistry

The outline of mallik 2002 gas hydrate production research well program - Yasuda, M. (Nihon Enerugi Gakkaishi/Journal of the Japan Institute of Energy, 2005)
It is estimated that significant amount of methane hydrate resources are deposited offshore Japan and the Research Consortium of Methane Hydrate Resources in Japan (MH21 Research Consortium) was established to undertake the Japan's Methane Hydrate Exploitation Program which was prepared by the Ministry of Economy, Trade and Industry and announced in 2001. MH21 Research Consortium planned onshore tests of gas production from gas hydrate reservoir and the Mallik 2002 Gas Hydrate Production Research Well Program was formed in which eight bodies participated from five countries. The test site was located in onshore Mackenzie delta of Northwestern Canada. The swamp feature of the site restricted the whole test operation only in winter when frozen firm ground and transportation road are formed. From December 25th 2001 to March 14th 2002, one gas production test well and two observation wells were drilled. Pressure draw down test using MDT and a hot water circulation test were tried , and the latter test yielded 468m3 dissociated gas from the hydrate reservoir, which was the first success of gas production from naturally deposited gas hydrate reservoir. In December 2003, Mallik International Symposium was held in Chiba, Japan with more than 200 researchers' participation from thirteen countries. The success of the Mallik Program was officially announced for the first time at the Symposium.
cited By 0
Keywords: Economics; Methane; Motor transportation; Natural gas; Natural gas well drilling; Technical presentations; Gas hydrate reservoir; Gas production; Mallik; MH21; Gas hydrates

The Iceland Deep Drilling Project: A search for deep unconventional geothermal resources - Fridleifsson, Gudmundur Ó.; Elders, Wilfred A. (Geothermics, 2005)

The ICDP lake Bosumtwi drilling project: A first report - Koeberl, Christian; Peck, John; King, John; Milkereit, Bernd; Overpeck, Jonathan; Scholz, Christopher (Scientific Drilling, 2005)
The 10.5 -km-diameter, 1.07-Ma Bosumtwi impact crater was the subject of a multi-disciplinary and international drilling effort of the International Continental Scientific Drilling Program (ICDP) from July to October 2004. Sixteen different holes were drilled at six locations within the lake, to a maximum depth of 540 m. A total of about 2.2 km of core material was obtained. Despite some technical and logistical challenges, the project has been very successful and it is anticipated that the first scientific results will be available in late 2005. © 2005 Copernicus GmbH.
Keywords: Coremaking; Lakes; Continental scientific drillings; Core material; Drilling projects; Impact craters; Scientific results; Infill drilling

The Expedition El'gygytgyn Lake 2003 (Siberian Arctic) - Melles, Martin; Minyuk, P; Brigham-Grette, J (Berichte zur Polar-und Meeresforschung (Reports on Polar and Marine Research), 2005)

The chemical structure of the Hawaiian mantle plume - Ren, Z.-Y.; Ingle, S.; Takahashi, E.; Hirano, N.; Hirata, T. (Nature, 2005)
The Hawaiian-Emperor volcanic island and seamount chain is usually attributed to a hot mantle plume, located beneath the Pacific lithosphere, that delivers material sourced from deep in the mantle to the surface. The shield volcanoes of the Hawaiian islands are distributed in two curvilinear, parallel trends (termed 'Kea' and 'Loa'), whose rocks are characterized by general geochemical differences. This has led to the proposition that Hawaiian volcanoes sample compositionally distinct, concentrically zoned, regions of the underlying mantle plume. Melt inclusions, or samples of local magma 'frozen' in olivine phenocrysts during crystallization, may record complexities of mantle sources, thereby providing better insight into the chemical structure of plumes. Here we report the discovery of both Kea- and Loa-like major and trace element compositions in olivine-hosted melt inclusions in individual, shield-stage Hawaiian volcanoes-even within single rock samples. We infer from these data that one mantle source component may dominate a single lava flow, but that the two mantle source components are consistently represented to some extent in all lavas, regardless of the specific geographic location of the volcano. We therefore suggest that the Hawaiian mantle plume is unlikely to be compositionally concentrically zoned. Instead, the observed chemical variation is probably controlled by the thermal structure of the plume.
cited By 104
Keywords: Crystallization; Geochemistry; Olivine; Thermal plumes; Lithospheres; Mantle plumes; Olivine phenocryst; Volcanic islands; Volcanic rocks; trace element; chemical composition; mantle chemistry; mantle plume; melt inclusion; seamount; article; chemical composition; geochemistry; geography; geology; heat; island (geological); lava; lithosphere; mantle plume; priority journal; rock; thermal structure; United States; volcano; Hawaiian Islands; oceanic regions; Pacific islands; Pacific Ocean; World

Stable isotope record of post-impact fluid activity in the core of the Yaxcopoil-1 borehole, Chicxulub impact structure, Mexico - Zurcher, L.; Kring, D.A.; Barton, M.D.; Dettman, D.; Rollog, M. (Special Paper of the Geological Society of America, 2005)
Carbon, oxygen, and hydrogen isotope results from carbonate and silicate fractions of altered core samples from the Yaxcopoil-1 borehole drilled into the 65 Ma Chicxulub impact crater provide constraints on the physico-chemical parameters of the hydrothermal solutions, and their likely origin. Yaxcopoil-1 impactites were initially permeated with calcite and halite at ambient temperature. This was followed by thermal metamorphism (diopside after igneous augite) and widespread Na-K metasomatism (feldspar after igneous plagioclase), which were overprinted by abundant lower-temperature clay and calcite. Silicate fraction isotopic values have δ 18O SMOW values between 10 and 23% indicating important isotopic exchange between impact melt (∼8%) and Cretaceous limestone (∼26%). Heavier δ 18 O values occur over depth intervals with intense feldspar alteration (813-833 m and 864-872 m). The δD SMOW values (-34 to -54%) are chiefl y infl uenced by smectite abundance and roughly mirror δ 18 O values. Carbonate fraction δ 18 O SMOW values (22-30%) are controlled by calcite contents, and several exceed the limestone signature. Most δ0.13C PDB (-1 to +2%) values also cluster around that of local limestone, but a number are signifi cantly lighter (down to -7%). Isotopic and fluid inclusion results indicate hydrothermal fluid temperatures between 270 and 100 °C, high salinities (∼20%), and minor kerogen contents. These data are compatible with mineralogical constraints, which further support an increase in oxidation state with decreasing temperature. Isotopic data point to a saline CO 2 -bearing fluid mixed with small amounts of reduced carbon, and decarbonation and infi ltration processes. Combined results are most consistent with a basinal oilfi eld saline brine that was driven by impact-induced heat. © 2005 Geological Society of America.
cited By 12

Quantitative reconstruction of the last interglacial vegetation and climate based on the pollen record from Lake Baikal, Russia - Tarasov, Pavel; Granoszewski, W.; Bezrukova, E.; Brewer, S.; Nita, M.; Abzaeva, A.; Oberh{ansli, H. (Climate Dynamics, 2005)
Changes in mean temperature of the coldest (Tc) and warmest month (Tw), annual precipitation (Pann) and moisture index (α) were reconstructed from a continuous pollen record from Lake Baikal, Russia. The pollen sequence CON01-603-2 (53°57′N, 108°54′E) was recovered from a 386 m water depth in the Continent Ridge and dated to ca. 130-114.8 ky BP. This time interval covers the complete last interglacial (LI), corresponding to MIS 5e. Results of pollen analysis and pollen-based quantitative biome reconstruction show pronounced changes in the regional vegetation throughout the record. Shrubby tundra covered the area at the beginning of MIS 5e (ca. 130-128 ky), consistent with the end of the Middle Pleistocene glaciation. The late glacial climate was characterised by low winter and summer temperatures (Tc ∼ -38 to -35°C and Tw∼11-13°C) and low annual precipitation (Pann∼300 mm). However, the wide spread of tundra vegetation suggests rather moist environments associated with low temperatures and evaporation (reconstructed α∼1). Tundra was replaced by boreal conifer forest (taiga) by ca. 128 ky BP, suggesting a transition to the interglacial. Taiga-dominant phase lasted until ca. 117.4 ky BP, e.g. about 10 ky. The most favourable climate conditions occurred during the first half of the LI. Pann reached 500 mm soon after 128 ky BP. However, temperature changed more gradually. Maximum values of Tc ∼-20°C and Tw ∼16-17°C are reconstructed from about 126 ky BP. Conditions became gradually colder after ca. 121 ky BP. Tc dropped to ∼ -27°C and T w to ∼15°C by 119.5 ky BP. The reconstructed increase in continentality was accompanied by a decrease in Pann to ∼400-420 mm. However, the climate was still humid enough (α∼0.9) to support growth of boreal evergreen conifers. A sharp turn towards a dry climate is reconstructed after ca. 118 ky BP, causing retreat of forest and spread of cool grass-shrub communities. Cool steppe dominated the vegetation in the area between ca. 117.5 ky and 114.8 ky BP, suggesting the end of the interglacial and transition to the last glacial (MIS 5d). Shift to the new glaciation was characterised by cool and very dry conditions with Tc ∼ -28 to -30°C, Tw ∼14-15°C, Pann ∼250 mm and α∼0.5. © Springer-Verlag 2005.
Keywords: Eurasia; Lake Baikal; Russian Federation; Coniferophyta; climate change; Last Interglacial; pollen; reconstruction; vegetation history; vegetation type

Spinel-bearing spherules condensed from the Chicxulub impact-vapor plume - Ebel, D.S.; Grossman, L. (Geology, 2005)
Formation of the giant Chicxulub crater off Mexico's Yucatan Peninsula coincided with deposition of the global Ir-rich Cretaceous-Tertiary (K-T) stratigraphic boundary layer ca. 65 Ma. The boundary is marked most sharply by abundant spherules containing un-altered grains of magnesioferrite spinel. Here we predict for the first time the sequential condensation of solids and liquids from the plume of vaporized rock expected from oblique K-T impacts. We predict highly oxidizing plumes that condense silicate liquid droplets bearing spinel grains whose compositions closely match those marking the actual boundary. Systematic global variations in spinel composition are consistent with higher condensation temperatures for spinels found at Atlantic and European sites than for those in the Pacific. © 2005 Geological Society of America.
cited By 45

Shear wave anisotropy observed in VSP data at the San Andreas Fault Observatory at Depth - Taylor, S.; Malin, P.; Haldorsen, J.B.U. (Society of Exploration Geophysicists - 75th SEG International Exposition and Annual Meeting, SEG 2005, 2005)
The San Andreas Fault (SAF) is the surface expression of a major plate tectonic boundary of the Pacific Plate. While widely known as the San Andreas Fault, it is probably better thought of as an extensive fault system that represents a major transform boundary between the North American and Pacific Plates. The sequence of M=6 earthquakes along the fault near Parkfield, California has stimulated an intense research effort into the seismogenic processes of brittle deformation in the upper crust along the San Andreas Fault. Part of this research effort has led to large network of surface instrumentation in the vicinity of Parkfield including seismometers, strain meters, and a variety of other geologic monitoring equipment. Recently, further instrumentation efforts have focused primarily on subsurface monitoring of seismogenic processes as part of the San Andreas Fault Observatory at Depth (SAFOD) project. In this paper, we review our recent research conducted to determine the local subsurface structure at the SAFOD site. We then present our structural interpretation of vertical seismic profiling data collected during recent crustal profiling studies. We conclude that the observed shear wave anisotropy in our VSP data is the result of aligned fracture sets which are most likely R shears. R shears are one component of conjugate Riedel fracturing associated with shear zone tectonic deformation. © Society of Exploration Geophysicists - 75th SEG International Exposition and Annual Meeting, SEG 2005. All rights reserved.
cited By 9
Keywords: Anisotropy; Deformation; Observatories; Plates (structural components); Seismic prospecting; Seismology; Shear flow; Shear waves; Strike-slip faults; Monitoring equipment; San andreas fault observatory at depth projects; Shear-wave anisotropy; Structural interpretation; Subsurface monitoring; Subsurface structures; Tectonic deformations; Vertical seismic profiling; Transform faults

Seismic structure and location of a CO2 source in the upper mantle of the western Eger (Ohře) Rift, central Europe - Geissler, Wolfram H.; K{ampf, Horst; Kind, Rainer; Br{auer, Karin; Klinge, Klaus; Plenefisch, Thomas; Hor'alek, Josef; Zedn'ik, Jan; Nehybka, Vladimir (Tectonics, 2005)
P-SV conversions provide new insights into the lithosphere of the western Eger (Ohře) Rift, a presently active CO2 emanation area, Quaternary volcanic field, and earthquake swarm region in central Europe. Gas and isotope (He and C) mapping of free gas phases in mineral springs and mofettes proved the origin Of CO2-dominated gases from a subcrustal magmatic fluid reservoir. Analyzing teleseismic data from several seismic networks in the western Bohemian Massif the source region of these gases was investigated Moho Ps conversions have 3 to 4.5 s delay. Crustal thicknesses vary between 27 and 38 km; vp/vs ratios vary between 1.63 and 1.81. Beneath the western Eger Rift an approximately 40 km wide Moho updoming up to 27 km exists. Locally observed weak conversions indicate a complex Moho transition zone in this area. A local 6 s phase possibly originates at a discontinuity in approximately 50 to 60 km depth or may represent multiples from velocity inversions at the base of the upper crust. Moho updoming and the distribution of the 6 s phase coincide with the CO2 degassing fields and the positions of Quaternary volcanoes at the surface. We hypothesize the release of CO2-dominated fluid/magma from isolated melt reservoirs in the depth range of 60 to 30 km, separation of CO2 from the melt at 29 to 21 km depths, and CO2 transport through the crust. The geophysical indications may point to presently active magmatic underplating beneath the study area, supporting the results of gas geochemical and isotope investigations. This is the first attempt that combines seismic and gas geochemical data for a tectonic model. Our model may be transferable to other continental rift areas worldwide. Copyright 2005 by the American Geophysical Union.
Keywords: Central Europe; Eurasia; Europe; Ohre River; carbon dioxide; lithospheric structure; seismic velocity; upper mantle

Scientific results of conduit drilling in the Unzen Scientific Drilling Project (USDP) - Nakada, Setsuya; Uto, Kozo; Sakuma, Sumio; Eichelberger, John C; Shimizu, Hiroshi (Scientific drilling, 2005)

Scientific results from the Mallik 2002 gas hydrate production research well program, Mackenzie Delta, northwest territories, Canada: Preface - Dallimore, S.R.; Collet, T.S.; Taylor, A.E.; Uchida, T.; Weber, M.; Chandra, A.; Mroz, T.H.; Caddel, E.M.; Inoue, T. (Bulletin of the Geological Survey of Canada, 2005)
cited By 131
Keywords: gas hydrate

Scientific drilling reached the magma path of 1990-95 eruption of Unzen Volcano - Uto, K. (AIST Today (International Edition), 2005)
The subsurface structure and magma ascending mechanism of Unzen Volcano was investigated by Unzen Scientific Drilling project (USDP) not only by scientific drillings but also related by geophysical and geochemical studies. A hole was drilled to reach the conduit of the Heisei eruption inorder to clarify the ascending and degassing process of magma and to understand the mechanism of an eruption. There was a conduit zone near the area right under Mt.Fugen at around the sea level, where old and new conduits including the Heisei conduit were concentrated within approximately 500 meters. The temperature in the conduit zone was approximately 200°C, lower than expected before the drilling resulted from quick cooling due to hydrothermal activities.
cited By 0

Recent research on the Chesapeake Bay impact structure, USA - Impact debris and reworked ejecta - Horton Jr., J.W.; Aleinikoff, J.N.; Kunk, M.J.; Gohn, G.S.; Edwards, L.E.; Self-Trail, J.M.; Powars, D.S.; Izett, G.A. (Special Paper of the Geological Society of America, 2005)
Four new coreholes in the western annular trough of the buried, late Eocene Chesapeake Bay impact structure provide samples of shocked minerals, cataclastic rocks, possible impact melt, mixed sediments, and damaged microfossils. Parautochthonous Cretaceous sediments show an upward increase in collapse, sand fluidization, and mixed sediment injections. These impact-modifi ed sediments are scoured and covered by the upper Eocene Exmore beds, which consist of highly mixed Cretaceous to Eocene sediment clasts and minor crystalline-rock clasts in a muddy quartz-glauconite sand matrix. The Exmore beds are interpreted as seawater-resurge debris flows. Shocked quartz is found as sparse grains and in rock fragments at all four sites in the Exmore, where these fallback remnants are mixed into the resurge deposit. Crystalline-rock clasts that exhibit shocked quartz or cataclastic fabrics include felsites, granitoids, and other plutonic rocks. Felsite from a monomict cataclasite boulder has a sensitive high-resolution ion microprobe U-Pb zircon age of 613 ± 4 Ma. Leucogranite from a polymict cataclasite boulder has a similar Neoproterozoic age based on muscovite 40 Ar/ 39 Ar data. Potassium-feldspar 40 Ar/ 39 Ar ages from this leucogranite show cooling through closure (∼150 °C) at ca. 261 Ma without discernible impact heating. Spherulitic felsite is under investigation as a possible impact melt. Types of crystalline clasts, and exotic sediment clasts and grains, in the Exmore vary according to location, which suggests different provenances across the structure. Fractured calcareous nannofossils and fused, bubbled, and curled dinofl agellate cysts coexist with shocked quartz in the Exmore, and this damage may record conditions of heat, pressure, and abrasion due to impact in a shallow-marine environment. © 2005 Geological Society of America.
cited By 38
Keywords: Argon; Binary alloys; Crystalline materials; Crystalline rocks; Debris; Feldspar; Fluidization; Geochronology; Lead alloys; Mica; Quartz; Rocks; Sediments; Silicate minerals; Structural geology; Zircon; Cataclastic; Chesapeake; Crater; Dinofl agellate; Ejecta; Impact; Nannofossil; Resurge; Shock; SHRIMP; Meteor impacts; abrasion; cataclasite; Cretaceous; ejecta; Eocene; fluidization; impact structure; ion microprobe; leucogranite; melt; muscovite; quartz; research; sand; zircon; Chesapeake Bay; United States

Re-evaluating the age of the Haughton impact event - Sherlock, S.C.; Kelley, S.P.; Parnell, J.; Green, P.; Lee, P.; Osinski, G.R.; Cockell, C.S. (Meteoritics and Planetary Science, 2005)
We have re-evaluated the published age information for the Haughton impact structure, which was believed to have formed ∼23 Ma ago during the Miocene age, and report new Ar/Ar laser probe data from shocked basement clasts. This reveals an Eocene age, which is at odds with the published Miocene stratigraphic, apatite fission track and Ar/Ar data; we discuss our new data within this context. We have found that the age of the Haughton impact structure is ∼39 Ma, which has implications for both crater recolonization models and post-impact hydrothermal activity. Future work on the relationship between flora and fauna within the crater, and others at high latitude, may resolve this paradox. © The Meteoritical Society, 2005.
cited By 41

Radiolytic H2 in continental crust: Nuclear power for deep subsurface microbial communities - Lin, L.-H.; Hall, J.; Lippmann-Pipke, J.; Ward, J.A.; Lollar, B.S.; DeFlaun, M.; Rothmel, R.; Moser, D.; Gihring, T.M.; Mislowack, B.; Onstott, T.C. (Geochemistry, Geophysics, Geosystems, 2005)
H2 is probably the most important substrate for terrestrial subsurface lithoautotrophic microbial communities. Abiotic H2 generation is an essential component of subsurface ecosystems truly independent of surface photosynthesis. Here we report that H2 concentrations in fracture water collected from deep siliclastic and volcanic rock units in the Witwatersrand Basin, South Africa, ranged up to two molar, a value far greater than observed in shallow aquifers or marine sediments. The high H2 concentrations are consistent with that predicted by radiolytic dissociation of H2O during radioactive decay of U, Th, and K in the host rock and the observed He concentrations. None of the other known H2-generating mechanisms can account for such high H2 abundance either because of the positive free energy imposed by the high H2 concentration or pH or because of the absence of required mineral phases. The radiolytic H 2 is consumed by methanogens and abiotic hydrocarbon synthesis. Our calculations indicate that radiolytic H2 production is a ubiquitous and virtually limitless source of energy for deep crustal chemolithoautotrophic ecosystems. Copyright 2005 by the American Geophysical Union.
cited By 161

Journey to the centre of a quake - Cohen, P. (New Scientist, 2005)
The San Andreas Fault Observatory at Depth (SAFOD) project to reveal various mysteries associated with an earthquake is discussed. By drilling directly into the fault, the researchers will be able to understand the causes of earthquakes. The results from SAFOD can help to refine the way scientists model earthquakes, and could even help determine the effective ways of predicting a quake's location, timing and size. All the activity is aimed at helping geophysicists understand the setting in which earthquakes develop and the factors that control them.
cited By 0
Keywords: Cracks; Data acquisition; Global positioning system; Imaging techniques; Methane; Observatories; Oil well drilling; Project management; Seismic waves; Seismology; Tectonics; San Andreas Fault Observatory at Depth (SAFOD) project; Stanford University; California; US Geological Survey; US National Resource Council; Earthquakes

Hydrothermal processes associated with meteorite impact structures: Evidence from three Australian examples and implications for economic resources - Pirajno, F. (Australian Journal of Earth Sciences, 2005)
Meteorite impacts cause conversion of kinetic energy into thermal energy. Part of this thermal energy is used to form a melt sheet, part is dissipated to heat the target rocks and these together with the hot rocks that elastically rebound from the depth of several kilometres (central uplift) activate hydrothermal circulation. Impact-generated hydrothermal systems have been documented from several impact structures world-wide. Three Australian examples-Shoemaker, Woodleigh and Yarrabubba-provide evidence of hydrothermal fluid flow both within and around the structures. Field observations, and petrographic and geochemical data suggest a common evolutionary trend of post-impact hydrothermal activity from early high-temperature alkali metasomatism to a later lower temperature H+ metasomatism, resulting in the overprinting by hydrous mineral assemblages. Hydrothermal systems activated by meteorite-impact events are important because they may also form economic mineral deposits, as is documented for several impact structures in the world. A working model of hydrothermal circulation in terrestrial impact structures posits two main stages: (i) initial high-temperature fluids percolate downward causing widespread alkali metasomatism of the shattered target rocks below the melt sheet, resulting in their modification to rocks of syenitic affinity; and (ii) inflow of meteoric water and progressive cooling of the melt sheet leads to a lower temperature stage, in which hydrothermal fluid flow tends to move upward, resulting in mineral assemblages and alteration patterns that resemble those of epithermal systems. In addition, these fluids can discharge at the surface as hot springs. © Geological Society of Australia.
cited By 25

Influence of climate fluctuation on clay formation in the Baikal drainage basin - Sakai, Takayuki; Minoura, Koji; Soma, Mitsuyuki; Tani, Yukinori; Tanaka, Atsushi; Nara, Fumiko; Itoh, Nobuyasu; Kawai, Takayoshi (Journal of Paleolimnology, 2005)
Sedimentary cores BDP 96 and 98 and VER 96-2 St. 3 from Academician Ridge in Lake Baikal were investigated to investigate the effect of climatic fluctuations on rock weathering and clay formation in the Baikal drainage basin. Illite, smectite, vermiculite, and kaolinite were identified as the major clay minerals in the sediments by X-ray diffraction analysis. Biotite in gravels in alluvial soils of the Baikal drainage area weathers through illite to vermiculite, smectite, and finally to kaolinite. To investigate the relationship between weathering and climate, we measured the clay content and the concentration of biogenic silica in the sediments. High surface productivity (increased biogenic silica) and high chemical weathering (decreased clay content) occurred simultaneously, showing that crustal weathering and soil formation were enhanced under warm climatic conditions. Clay formation was enhanced in the watershed from the Late Miocene to the Middle Pliocene, and mechanical weathering of rocks increased during glacial intervals after the climate began to cool in Late Pliocene time. This change in the weathering mode in the watershed reduced the nutrient flux and aquatic productivity of Lake Baikal.
Keywords: Eastern Hemisphere; Eurasia; Lake Baikal; Russian Federation; World; biogenic deposit; clay mineral; climate change; Miocene; paleolimnology; sediment core; weathering

Buried treasure - Boswell, R. (Mechanical Engineering, 2005)
The potential of offshore deposits of methane hydrates as energy resource is discussed. The efforts to develop methods that would make production of methane from hydrate both technologically feasible and economically viable are also presented. Methane hydrate is a very efficient storehouse of energy, and when dissociated, a single cubic foot of solid hydrate releases as much as 180 cubic feet of methane gas. work at the Mallik site in the Canadian Arctic has established that production of methane from hydrates is technologically feasible. The US national methane hydrate research program is now performing field and laboratory studies designed to accelerate the commercially viable production of methane from hydrate in Alaska.
cited By 1
Keywords: Heat flow; Methane hydrates; Seismic reflection; Well logs; Computer simulation; Computerized tomography; Diffusion; Dissociation; Hydrates; Mechanical permeability; Natural gas; Nuclear magnetic resonance spectroscopy; Offshore petroleum prospecting; Raman spectroscopy; Sediments; Seismic prospecting; Methane

Detrital input and early diagenesis in sediments from Lake Baikal revealed by rock magnetism - Demory, François; Oberh{ansli, Hedi; Nowaczyk, Norbert R.; Gottschalk, Matthias; Wirth, Richard; Naumann, Rudolf (Global and Planetary Change, 2005)
A rock magnetic study was performed on sediment cores from six locations in Lake Baikal. For a comprehensive approach of the processes influencing the rock magnetic signal, additional data are presented such as total organic carbon (TOC), total sulphur (TS), opal, water content and relative variations in iron and titanium measured on selected intervals. In glacial sediments, the magnetic signal is dominated by magnetite, which is considered to be of detrital origin. This predominance of magnetite is interrupted by distinct horizons of authigenic greigite, probably confined to reductive microenvironments. In interglacial stages, besides dilution by biogenic silica and a decreasing detrital input, the weakness of the rock magnetic signal is also due to a reductive dissolution of magnetic particles. The magnetic assemblage is strongly linked to the redox history of interglacial sediment. In the oxidised bottom sediments of Lake Baikal, a biogenic magnetite is observed [Peck, J.A., King, J.W., 1996. Magnetofossils in the sediments of lake Baikal, Siberia. Earth Planet. Sci. Lett. 140 (1 –4), 159 –172]. After burial under the redox front, the magnetite is preferentially dissolved, and detrital hematite remains dominant when the sedimentation rate is low and when the residence time of the magnetite close to the redox boundary is long. During these low sedimentation rate conditions, the redox front is preserved [Granina, L., M{uller, B. and Wehrli, B., 2004. Origin and dynamics of Fe and Mn sedimentary layers in Lake Baikal. Chem. Geol. 205 (1-2), 55-72]. At constant sedimentation rate and fast burial, the magnetite is preserved or transformed into greigite when sulphate-reducing conditions are reached in the sediment. In interglacial sediments, the magnetic assemblages depict changes in the sedimentation rate, which are traced using the ratio of magnetite over hematite (S-ratio). At the beginning of interglacials, the sedimentation rate is constant with an assemblage magnetite+greigite (high S-ratio), and at the end of some interglacials, the sedimentation rate decreases with a predominance of hematite (low S-ratio).
Progress towards reconstructing past climate in Central Eurasia, with special emphasis on Lake Baikal
Keywords: Lake Baikal; Late Quaternary; Rock magnetism; Diagenesis; Detrital input

Depth constraints on nonlinear strong ground motion from the 2004 Parkfield earthquake - Rubinstein, J.L.; Beroza, G.C. (Geophysical Research Letters, 2005)
We use the two target repeating earthquake sequences of SAFOD to identify time varying properties of the shallow crust in the Parkfield area at the surface and in shallow boreholes. At the surface, we find that the 2004 Parkfield earthquake caused direct S wave delays exceeding 7 ms, and S coda delays exceeding 15 ms. We attribute these delays to cracks formed or opened during the strong shaking of the Parkfield earthquake. Observations at depth show that the direct S wave arrival time was much less affected by the Parkfield earthquake. This provides evidence that damage caused by strong shaking (nonlinear strong ground motion), is limited to the very near surface (<100 m). Copyright 2005 by the American Geophysical Union.
cited By 90
Keywords: Boreholes; Cracks; Earthquakes; Geologic models; Rock mechanics; Seismic waves; Seismology; Structural geology; Depth constraints; Earthquake sequences; Ground motion; Shallow crust; Tectonics; arrival time; damage; earthquake; ground motion; S-wave; California; North America; Parkfield; United States; Western Hemisphere; World

Crustal and uppermost mantle structure of the Bohemian Massif based on CELEBRATION 2000 data - Hrubcov'a, Pavla; 'Sroda, P.; Špič'ak, A.; Guterch, A.; Grad, M.; Keller, G.R.; Brueckl, E.; Thybo, H. (Journal of Geophysical Research: Solid Earth, 2005)
The deep structure of the Bohemian Massif (BM), the largest stable outcrop of Var scan rocks in central Europe, was studied using the data of the international seismic refraction experiment Central European Lithospheric Experiment Based on Refraction (CELEBRATION) 2000. The data were interpreted by seismic tomographic inversion and by two-dimensional (2-D) trial-and-error forward modeling of P and S waves. Additional constraint on crustal structure was given by amplitude modeling using the reflectivity method and gravity modeling. Though consolidated, the BM can be subdivided into several tectonic units separated by faults, shear zones, or thrusts reflecting varying influence of the Cadomian and Variscan orogeneses: the Saxothuringian, Barrandian, Moldanubian, and Moravian. Velocity models determine three types of crust-mantle transition in the BM reflecting variable crustal thickness and delimiting contacts of tectonic units in depth. The NW area, the Saxothuringian, has a highly reflective lower crustal layer above Moho with a strong velocity contrast at the top of this layer. This reflective laminated lower crust reaches depths of 26-35 km and is characteristic for the Saxothuringian unit, which was subject to eastward subduction. The Moldanubian in the central part is characterized by the deepest (39 km) and the most pronounced Moho within the whole BM with a strong velocity contrast 6.9-8.1 kin s-1. A thick crust-mantle transition zone in the SE, with velocity increase from 6.8 to 7.8 km s-1 over the depth range of 23-40 km, seems to be the characteristic feature of the Moravian overthrusted by the Moldanubian during Variscan collision. Copyright 2005 by the American Geophysical Union.
Keywords: Bohemian Massif; Central Europe; Eurasia; Europe; crustal structure; mantle structure; seismic tomography

Climate and the collapse of Maya civilization - Peterson, Larry C.; Haug, Gerald H. (American Scientist, 2005)
During its Classic period (250-950 A.D.), Maya civilization reached a zenith. At its peak, around 750 A.D., the population may have topped 13 million. Then, between about 750 and 950 A.D., their society imploded. The Maya abandoned what had been densely populated urban centers, leaving their impressive stone edifices to fall into ruin. The demise of Maya civilization (which archaeologists call the terminal Classic collapse) has been one of the great anthropological mysteries of modern times. What could have happened? Scholars have advanced a variety of theories over the years, pinning the fault on everything from internal warfare to foreign intrusion, from widespread outbreaks of disease to a dangerous dependence on monocropping, from environmental degradation to climate change. Some combination of these and other factors may well be where the truth lies. However, in recent years, evidence has mounted that unusual shifts in atmospheric patterns took place near the end of the Classic Maya period, lending credence to the notion that climate, and specifically drought, indeed played a hand in the decline of this ancient civilization.
Keywords: Social aspects; Collapse; Maya civilization; archaeological evidence; demography; Climatology

Chicxulub impact ejecta from the Cretaceous-Paleogene (K-P) boundary in northeastern M'exico - Schulte, P.; Kontny, A. (Special Paper of the Geological Society of America, 2005)
The combined petrological and rock magnetic study of the Cretaceous-Paleogene (K-P) boundary in northeastern M'exico revealed compositionally and texturally complex Chicxulub ejecta deposits. The predominant silicic ejecta components are Fe-Mg-rich chlorite and Si-Al-K-rich glass spherules with carbonate inclusions and schlieren. Besides these silica phases, the most prominent ejecta component is carbonate. Carbonate occurs as lithic clasts, accretionary lapilli, melt globules (often with quench textures), and as microspar. The composition of the spherules provides evidence for a range of target rocks of mafi c to intermediate composition, presumably situated in the northwestern sector of the Chicxulub impact structure. The abundance of carbonate ejecta suggests that this area received ejecta mainly from shallow, carbonate-rich lithologies. Rare μm-sized metallic and sulfi dic Ni-Corich inclusions in the spherules indicate a possible contamination by meteoritic material. This complex composition underlines the similarities of ejecta in NE M'exico to Chicxulub ejecta from K-P sections worldwide. Although the ejecta display a great variability, the magnetic susceptibility, remanence, and hysteresis properties of the ejecta deposits are fairly homogeneous, with dominantly paramagnetic susceptibilities and a weak ferromagnetic contribution from hematite and goethite. The absence of spinels and the ubiquitous presence of hematite and goethite points to high oxygen fugacity during the impact process. The microfacies and internal texture of the ejecta deposits show welding and fusing of components, as well as evidence for liquid immiscibility between silicic and carbonate melts. No evidence for binary mixing of ejecta phases was found. Therefore, Chicxulub ejecta in NE M'exico probably derived from less energetic parts of the ejecta curtain. However, welding features of ejecta particles and enclosed marl clasts and/or benthic foraminifera from a siliciclastic environment suggest interaction of the-still hot-ejecta curtain with northern Mexican shelf sediments. In addition, an initial ground surge-like ejecta-dispersion mode seems possible. © 2005 Geological Society of America.
cited By 23

Chicxulub Crater Seismic Survey prepares way for future drilling - Morgan, Jo; Urrutia-Fucugauchi, Jaime; Gulick, Sean; Christeson, Gail; Barton, Penny; Rebolledo-Vieyra, Mario; Melosh, Jay (Eos, Transactions American Geophysical Union, 2005)
Sixty-five million years ago, a large meteorite hit the Earth and formed the ∼200-km-wide Chicxulub crater in Yucat'an, Mexico. The well-known, massive extinction event at the Cretaceous-Tertiary (K-T) boundary appears to have been caused, at least in part, by this impact. In the first few seconds after impact the surface of the Earth was pushed down to form a cavity ∼35 km deep, and in the next few hundred seconds this cavity collapsed to form a multi-ring basin with an inner peak ring. To examine the rings and subsurface structure of this superbly preserved impact crater, a seismic experiment was shot across the crater in January and February 2005 by a team of scientists from Mexico, the United States, and the United Kingdom (Figure 1).

Characterization of the organic matter in lignites of the Kugmallit Formation (Oligocene) at the JAPEX/JNOC/GSC et al. Mallik 5L-38 gas hydrate production research well - Haberer, RM; Mangelsdorf, Kai; Dieckmann, V; Fuhrmann, A; Wilkes, Heinz; Horsfield, Brian (Geological Survey of Canada, Bulletin, 2005)

Characterization of expandable clay minerals in Lake Baikal sediments by thermal dehydration and cation exchange - Grygar, Tomas; Bezdicka, Petr; Hradil, David; Hruskova, Michaela; Novotna, Katerina; Kadlec, Jaroslav; Pruner, Petr; Oberh{ansli, Hedi (Clays and Clay Minerals, 2005)
The sedimentary series from Academician Ridge, Lake Baikal, eastern Siberia, was examined using cation exchange capacity (CEC) to estimate the amount of expandable clay minerals (ECM) and high-temperature X-ray diffraction (HT-XRD) to determine their basic classification. The comparison of the magnetic susceptibility (MS) at sub-millennial resolution and the δ18O record of a reference Atlantic core (ODP 980) was used to create an age model. The most closely studied part of the series covered the major part of the last glacial cycle (120-20 ky BP). The HT-XRD analysis is based on monitoring the course of ECM dehydration with 5°C steps between 25 and 250°C and enabled us to improve the discrimination between ECM, chlorite and micas. The CEC obtained at millennial resolution showed that the neoformation of ECM in warmer periods of the last interglacial was either insignificant or fully compensated by their dissolution or dilution. The CEC record was correlated with the main climatic stages in the period studied. Both MS and CEC records reflected the environmental changes at about millennial resolution, including climatic instabilities between 117 and 73 ky BP (late MIS5). Copyright © 2005, The Clay Minerals Society.
Keywords: Eastern Hemisphere; Eurasia; Lake Baikal; Russian Federation; World; Dehydration; High temperature effects; Magnetic susceptibility; Mineralogy; Positive ions; Sediments; X ray diffraction analysis; clay mineral; lacustrine deposit; physical property; Cation exchange capacity; Paleoclimate; Russia; Thermal dehydration; Clay minerals

Can diagenetic processes influence the short term hydraulic behaviour evolution of a fault? - Frima, C.; Moretti, Isabelle; Brosse, Etienne; Quattrocchi, F.; Pizzino, L. (Oil and Gas Science and Technology, 2005)
The scope of this work is to quantify the impact of calcite cementation on the hydraulic behaviour of an active fault in carbonate rocks. In the studied context, the permeability mainly depends on the fracture network. Therefore the hydraulic regime depends on the way fractures become sealed by vein deposits. The work is based on the study of cores and groundwater recovered from an active normal fault in the Gulf of Corinth, the Aigion fault, in the framework of the Corinth Rift Laboratory Project. The data set includes cores, borehole images, pumping tests and in situ fluid samples. The cores show numerous recent fractures partially, or completely, sealed by recent carbonate crystals. We modelled the fluid-rock interaction to quantify the growth rate of calcite in the veins of this specific geological context. Modelling was carried out with Diaphore, a software designed to study the diagenetic evolution of porosity in hydrocarbon reservoirs. The required time to seal a 1 mm thick fracture in the case of the Aigion fault at 760 m depth was calculated to be within the range of a few hundred years, considering the composition of water sampled at this depth. As a conclusion, diagenetic processes are likely to have a key influence on the fault hydraulic behavior over the time scale of seismic cycles. However, the possibility of calcite growth strongly depends on the composition of the fluid. Tests were made with seawater and slightly higher values of pCO2, a fluid that could also be present around the Aigion fault. In this case dissolution, and therefore opening of the fractures, happens. Copyright © 2005, Institut français du p'etrole.
Keywords: Calcite; Carbon dioxide; Carbonates; Cement additives; Computer software; Crystal growth; Crystals; Fracture; Groundwater; Hydrocarbons; Mechanical permeability; Porosity; Rocks; Seismology; Carbonate crystals; Carbonate rocks; Diagenetic processes; Hydraulic behaviour; fracture network; hydraulic property; permeability; Hydraulics

Bosumtwi impact structure, Ghana: Geochemistry of impactites and target rocks, and search for a meteoritic component - Dai, Xiongxin; Boamah, Daniel; Koeberl, Christian; Reimold, Wolf Uwe; Irvine, Gordon; McDonald, Iain (Meteoritics and Planetary Science, 2005)
Major and trace element data, including platinum group element abundances, of representative impactites and target rocks from the crater rim and environs of the Bosumtwi impact structure, Ghana, have been investigated for the possible presence of a meteoritic component in impact-related rocks. A comparison of chemical data for Bosumtwi target rocks and impactites with those for Ivory Coast tektites and microtektites supports the interpretation that the Bosumtwi structure and Ivory Coast tektites formed during the same impact event. High siderophile element contents (compared to average upper crustal abundances) were determined for target rocks as well as for impactites. Chondrite-normalized (and iron meteorite-normalized) abundances for target rocks and impactites are similar. They do not, however, allow the unambiguous detection of the presence, or identification of the type, of a meteoritic component in the impactites. The indigenous siderophile element contents are high and possibly related to regional gold mineralization, although mineralized samples from the general region show somewhat different platinum-group element abundance patterns compared to the rocks at Bosumtwi. The present data underline the necessity of extensive target rock analyses at Bosumtwi, and at impact structures in general, before making any conclusions regarding the presence of a meteoritic component in impactites. © The Meteoritical Society, 2005.
Keywords: crater; impact structure; impactite; meteorite; shock metamorphism

Dielectric method of high-resolution gas hydrate estimation - Sun, Y.F.; Goldberg, D. (Geophysical Research Letters, 2005)
In-situ dielectric properties of natural gas hydrate are measured for the first time in the Mallik 5L-38 Well in the Mackenzie Delta, Canada. The average dielectric constant of the hydrate zones is 9, ranging from 5 to 20. The average resistivity is >5 ohm.m in the hydrate zones, ranging from 2 to 10 ohm.m at a 1.1 GHz dielectric tool frequency. The dielectric logs show similar trends with sonic and induction resistivity logs, but exhibits inherently higher vertical resolution (<5 cm). The average in-situ hydrate saturation in the well is about 70%, ranging from 20% to 95%. The dielectric estimates are overall in agreement with induction estimates but the induction log tends to overestimate hydrate content up to 15%. Dielectric estimates could be used as a better proxy of in-situ hydrate saturation in modeling hydrate dynamics. The fine-scale structure in hydrate zones could help reveal hydrate formation history. Copyright 2005 by the American Geophysical Union.
cited By 20
Keywords: Dielectric properties; Electric conductivity; Frequencies; Mathematical models; Natural gas; Dielectric properties; Dielectric properties of gases; Electric logging; Hydration; Induction logging; Dielectric tools; Hydrate saturation; Hydrate zones; Induction logs; Gas hydrates; Gas hydrates; clathrate; dielectric property; gas hydrate; Dielectric methods; Dielectric tools; Fine-scale structures; High resolution; Hydrate formation; Resistivity logs; Situ hydrates; Vertical resolution

Be isotopes with textural and mineralogical data of sediment from Lake Baikal (Siberia) - Sapota, Tomasz; Aldahan, Ala; Possnert, G{oran (Neues Jahrbuch fur Geologie und Palaontologie - Abhandlungen, 2005)
In this study, results of Be isotopic analyses of sediments collected from the three longest cores drilled in Lake Baikal, i.e. BDP-98 (600 m long), BDP-96 (200 m long) and BDP-93 (102 m long), are presented in relation to their lithological variation. The sediment used in the study has a relatively uniform grain size distribution and mineral composition dominated by detrital feldspars and quartz. Biogenic silica is present in all samples, with amounts up to 40 %. The limited variability in textural and mineralogical (± biogenic silica) composition of the sediment resulted in a constrained 10Be concentration distribution that is mainly related to its production and radioactive decay. Normalization of 10Be to 9Be concentrations shows a trend similar to that of the 10Be distribution. The 10Be-based chronology for the BDP-98 and BDP-96 cores provides time spans of about 8 and 5.5 Myr respectively. Estimation of the 10Be flux indicates high values in the Late Miocene, which can be related to relatively low geomagnetic field intensity during that time. © 2005 E. Schweizerbart'sche Verlagsbuchhandlung.
Keywords: Lake Baikal; Russian Federation; beryllium isotope; core analysis; detrital deposit; feldspar; isotopic analysis; mineralogy; sediment analysis

Basinward transport of Chicxulub ejecta by tsunami-induced backflow, La Popa basin, northeastern Mexico, and its implications for distribution of impact-related deposits flanking the Gulf of Mexico - Lawton, T.F.; Shipley, K.W.; Aschoff, J.L.; Giles, K.A.; Vega, F.J. (Geology, 2005)
Ejecta-bearing strata are present at the top of Cretaceous foreland-basin deposits throughout the La Popa basin in northeastern Mexico. In the southeast part of the basin, locally thick (as much as 4.6 m) ejecta-rich conglomeratic strata occupy valley-like features at a bathymetric break that separated Maastrichtian upper shoreface from lower shoreface and prodelta depositional settings. Clast-supported textures, normally graded planar conglomerate-sandstone couplets, upcurrent-dipping low-angle cross-laminae, sparse paleocurrent data, and transported fossils indicate deposition by south- to southeast-directed turbulent, supercritical flow. In the northwest part of the basin, ejecta grains are present but less common in correlative deposits. Sediment, ejecta, and organisms were eroded from shoreward environments and transported basinward by backflow of run-up surge(s) emplaced against the continent by one or several tsunami(s). High-discharge, supercritical offshore-directed flow provides a mechanism for transport of voluminous, ejecta-bearing sediment and late Maastrichtian marine organisms into deep-water Gulf of Mexico settings. © 2005 Geological Society of America.
cited By 49

Basaltic volcanism of the central and western Snake River Plain: A guide to field relations between Twin Falls and Mountain Home, Idaho - Shervais, J.W.; Kauffman, J.D.; Gillerman, V.S.; Othberg, K.L.; Vetter, S.K.; Hobson, R.V.; Zarnetske, M.; Cooke, M.F.; Matthews, S.H.; Hanan, B.B. (GSA Field Guides, 2005)
Basaltic volcanism in the Snake River Plain of southern Idaho has long been associated with the concept of a mantle plume that was overridden by North America during the Neogene and now resides beneath the Yellowstone plateau. This concept is consistent with the time-transgressive nature of rhyolite volcanism in the plain, but the history of basaltic volcanism is more complex. In the eastern Snake River Plain, basalts erupted after the end of major silicic volcanism. The basalts typically erupt from small shield volcanoes that cover up to 680 km 2 and may form elongate flows that extend 50-60 km from the central vent. The shields coalesce to form extensive plains of basalt that mantle the entire width of the plain, with the thickest accumulations of basalt forming an axial high along the length of the plain. In contrast, basaltic volcanism in the western Snake River Plain formed in two episodes: the first (ca. 7-9 Ma) immediately following the eruption of rhyolites lavas now exposed along the margins of the plain, and the second forming in the Pleistocene (δ2 Ma), long after active volcanism ceased in the adjacent eastern Snake River Plain. Pleistocene basalts of the western Snake River Plain are intercalated with, or overlie, lacustrine sediments of Pliocene-Pleistocene Lake Idaho, which filled the western Snake River Plain graben after the end of the first episode of basaltic volcanism. The contrast in occurrence and chemistry of basalt in the eastern and western plains suggest the interpretation of volcanism in the Snake River Plain is more nuanced than simple models proposed to date. © 2005 Geological Society of America.
cited By 24
Keywords: Granite; Rivers; Basalt geochemistry; Basaltic volcanism; First episodes; Lacustrine sediments; Mantle plume; Silicic volcanism; Snake river plains; Yellowstones; Basalt

An ordinary chondrite impactor for the Popigai crater, Siberia - Tagle, R.; Claeys, P. (Geochimica et Cosmochimica Acta, 2005)
With a diameter of ∼100 km, Popigai in Northern Siberia is the largest crater known in the Cenozoic. The concentrations in platinum group elements (PGE) were analyzed in twenty samples of homogeneous impact melt collected in the northwestern flank of the crater to identify the composition of the projectile. The method selected was preconcentration by NiS fire assay followed by inductively coupled plasma-mass spectrometry (ICP-MS). This technique measures all the PGE (except Os) and by using aliquots >10g, the results are highly reproducible. The major and trace element composition of the impact melt resembles that of gneissic lithologies of the Anabar shield, which are representative of the target rock. The PGE are enriched in the melt by factors of 3 to 14 compared to the main target lithology, but the meteoritic contamination is only around 0.2 wt.%. Using plots of elemental ratios such as Ru/Rh vs. Pt/Pd or Ru/Rh vs. Pd/Ir, the Popigai impactor is clearly identified as an ordinary chondrite and most likely l-chondrite. This study indicates that PGE elemental ratios allow discrimination of the type of impactor, even in the case of low meteoritic contamination. This study confirms that a significant fraction of the crater-forming projectiles presently documented could have an ordinary chondrite composition. Their probable source, the S-type asteroids, appears to form the majority of the bodies in the main asteroid belt and among Near Earth Objects (NEOs). The ordinary chondrite origin of the Popigai projectile supports an asteroidal origin for the late Eocene impacts as a plausible alternative to the comet shower scenario proposed by Farley et al. (1998). Copyright © 2005 Elsevier Ltd.
cited By 61

An olivine-free mantle source of Hawaiian shield basalts - Sobolev, A.V.; Hofmann, A.W.; Sobolev, S.V.; Nikogosian, I.K. (Nature, 2005)
More than 50 per cent of the Earth's upper mantle consists of olivine and it is generally thought that mantle-derived melts are generated in equilibrium with this mineral. Here, however, we show that the unusually high nickel and silicon contents of most parental Hawaiian magmas are inconsistent with a deep olivine-bearing source, because this mineral together with pyroxene buffers both nickel and silicon at lower levels. This can be resolved if the olivine of the mantle peridotite is consumed by reaction with melts derived from recycled oceanic crust, to form a secondary pyroxenitic source. Our modelling shows that more than half of Hawaiian magmas formed during the past 1 Myr came from this source. In addition, we estimate that the proportion of recycled (oceanic) crust varies from 30 per cent near the plume centre to insignificant levels at the plume edge. These results are also consistent with volcano volumes, magma volume flux and seismological observations.
cited By 873
Keywords: Mathematical models; Minerals; Nickel; Seismology; Silicon; Volcanoes; Magmas; Mantle; Pyroxene; Recycled oceanic crusts; Basalt; mineral; nickel; silicon; basalt; mantle; petrogenesis; petrology; article; basalt; model; plume; priority journal; sea; United States; volcano; Hawaiian Islands; oceanic regions; Pacific islands; Pacific Ocean; World

Alteration of hyaloclastites in the HSDP 2 Phase 1 Drill core: 2. Mass balance of the conversion of sideromelane to palagonite and chabazite - Walton, A.W.; Schiffman, P.; MacPherson, G.L. (Geochemistry, Geophysics, Geosystems, 2005)
The Hawaii Scientific Drilling Project 2 Phase 1 core permits study of each stage of alteration of basalt glass during burial because stages of the process are separated vertically. Previous work has shown that alteration of hyaloclastite occurs progressively. The latest stage observed in the Phase 1 core involves marginal replacement of sideromelane in shards with palagonite while simultaneously forming chabazite in pores. The basic reaction at this stage is sideromelane + components from pore waters = palagonite + chabazite + components to pore waters. Mass balance calculations show that Fe was virtually immobile in this process, being retained in palagonite. Na, Ca, Ba, P, Al, and Si were lost during palagonitization and not fully consumed in making chabazite. Mg was lost during palagonitization but retained elsewhere in smectite. K, Rb, and Sr were extracted from pore waters and enriched in the sum of the alteration products. The amount of enrichment depended upon the amount of chabazite present, which depended upon the porosity when chabazite formed. Ti, Y, U, Zr, Nb, REE, and Th were enriched in palagonite, compared to sideromelane, but were absent in chabazite. Mass balance of all phases for the entire alteration process (including earlier stages) was not possible because poorly consolidated samples do not yield accurate modal values of phases, trace element analysis of smectite was not possible, and exchange with lavas and intrusions in the succession cannot be evaluated. Calculations indicate that too little of major oxides, except Na2O, was released during palagonitization to account for the amount of smectite observed in hyaloclastites. The results of this study, and several others published in the literature, indicate that under various circumstances palagonitization will consume particular elements from pore fluid or release them to it. Such mobility implies that the hydrology of the particular system and the composition of the dissolved solids in the pore water will control whether palagonitization is a source or sink of elements. The potential exists that palagonitization of basalt glass is an important source or sink of elements for seawater and fluids circulating in the ocean crust. Copyright 2005 by the American Geophysical Union.
cited By 34

A tsunami deposit at the Cretaceous/Paleogene boundary in the Neuqu'en Basin of Argentina - Scasso, R.A.; Concheyro, A.; Aberhan, M.; Hecht, L.; Medina, F.A.; Tagle, R. (Cretaceous Research, 2005)
A coarse-grained sandstone bed of Cretaceous/Paleogene (K/Pg) boundary age occurs in a homogeneous neritic shelf mudstone sequence (Jag{uel Formation) in the Neuqu'en Basin of Argentina. This bed, 15 - 25 cm thick, contains abundant plagioclase, broken shells and sharks' teeth. Sedimentological features include an erosive base, abundant rip-up clasts, normal grading and hummocky cross-bedding. The K/Pg boundary age of the bed was confirmed by calcareous nannofossils. Similar to other sections in the Gulf Coast region and the Danish Basin, a dead zone significantly depleted in macrofossils is evident in the basal 1 m above the clastic layer. In combination, these features suggest that the clastic layer represents a tsunami deposit that was related to the Chicxulub impact event in Yucatan/Mexico. Mechanisms of tsunami wave amplification in this extremely distal and somewhat protected setting are poorly understood but the funnel-shape of the basin may have promoted the unusually strong sedimentological response. © 2005 Elsevier Ltd. All rights reserved.
cited By 35

In situ thermal conductivity of gas-hydrate-bearing sediments of the Mallik 5L-38 well - Henninges, J.; Huenges, E.; Burkhardt, H. (Journal of Geophysical Research: Solid Earth, 2005)
Detailed knowledge about thermal properties of rocks containing gas hydrate is required in order to quantify processes involving gas hydrate formation and decomposition in nature. In the framework of the Mallik 2002 program, three wells penetrating a continental gas hydrate occurrence under permafrost were successfully equipped with permanent fiber-optic distributed temperature sensing cables. Temperature data were collected over a 21-month period after completing the wells. Thermal conductivity profiles were calculated from the geothermal data as well as from a petrophysical model derived from the available logging data and application of mixing law models. Results indicate that thermal conductivity variations are mainly lithologically controlled with a minor influence from hydrate saturation. Average thermal conductivity values of the hydrate-bearing sediments range between 2.35 and 2.77 W m-1 K-1. Maximum gas hydrate saturations can reach up to about 90% at an average porosity of 0.3. Copyright 2005 by the American Geophysical Union.
cited By 77
Keywords: gas hydrate; sediment property; thermal conductivity

A case study of impact-induced hydrothermal activity: The Haughton impact structure, Devon Island, Canadian High Arctic - Osinski, G.R.; Lee, P.; Parnell, J.; Spray, J.G.; Baron, M. (Meteoritics and Planetary Science, 2005)
The well-preserved state and excellent exposure at the 39 Ma Haughton impact structure, 23 km in diameter, allows a clearer picture to be made of the nature and distribution of hydrothermal deposits within mid-size complex impact craters. A moderate- to low-temperature hydrothermal system was generated at Haughton by the interaction of groundwaters with the hot impact melt breccias that filled the interior of the crater. Four distinct settings and styles of hydrothermal mineralization are recognized at Haughton: a) vugs and veins within the impact melt breccias, with an increase in intensity of alteration towards the base; b) cementation of brecciated lithologies in the interior of the central uplift; c) intense veining around the heavily faulted and fractured outer margin of the central uplift; and d) hydrothermal pipe structures or gossans and mineralization along fault surfaces around the faulted crater rim. Each setting is associated with a different suite of hydrothermal minerals that were deposited at different stages in the development of the hydrothermal system. Minor, early quartz precipitation in the impact melt breccias was followed by the deposition of calcite and marcasite within cavities and fractures, plus minor celestite, barite, and fluorite. This occurred at temperatures of at least 200 °C and down to ∼100-120 °C. Hydrothermal circulation through the faulted crater rim with the deposition of calcite, quartz, marcasite, and pyrite, occurred at similar temperatures. Quartz mineralization within breccias of the interior of the central uplift occurred in two distinct episodes (∼250 down to ∼90 °C, and <60 °C). With continued cooling (<90 °C), calcite and quartz were precipitated in vugs and veins within the impact melt breccias. Calcite veining around the outer margin of the central uplift occurred at temperatures of ∼150 °C down to <60 °C. Mobilization of hydrocarbons from the country rocks occurred during formation of the higher temperature calcite veins (>80 °C). Appreciation of the structural features of impact craters has proven to be key to understanding the distribution of hydrothermal deposits at Haughton. © The Meteoritical Society, 2005.
cited By 86

Diatom biostratigraphy of Lake Baikal during the past 1.25 Ma: New results from BDP-96-2 and BDP-99 drill cores - Khursevich, Galina K.; Prokopenko, Alexander A.; Fedenya, Svetlana A.; Tkachenko, Lilia I.; Williams, Douglas F. (Quaternary International, 2005)
Detailed diatom zonation in the Lake Baikal holostratotype section BDP-96-2 is extended over the interval 0.85-1.25 Ma and compared with diatom assemblage composition from the new BDP-99 drill core. Interglacial diatom assemblages at these two sites over 220 km apart are of similar composition, and the same complete and continuous succession of diatom zones is observed in both drill cores over the 0-800 ka interval. We propose a correlation of Baikal diatomaceous intervals with the marine oxygen isotope stratigraphy and with European climato-stratigraphic divisions of the early Pleistocene. We then use the diatom succession in BDP-96-2 to demonstrate evidence for disconformity in BDP-99 and to establish an approximate age model for this section. Diatom assemblages characteristic of the top of the Matuyama reversed polarity interval and the upper part of the Jaramillo normal polarity subchron are not found in the BDP-99 section below the disconformity boundary. The lack of diatom zones corresponding to the MIS 21-27 of the marine oxygen isotope stratigraphy in BDP-99 section suggests that the estimated age span of missing sediment interval in BDP-99 is ca 150-160 ka. The unusual diatom assemblage of cosmopolitan species during MIS 33 in both BDP-96-2 and BDP-99 drill core sections is indicative of abrupt ecological changes in the time interval 1.12-1.08 Ma. © 2004 Elsevier Ltd and INQUA. All rights reserved.
Keywords: Eastern Hemisphere; Eurasia; Lake Baikal; Russian Federation; World; biostratigraphy; community composition; diatom; marine isotope stage; sediment core

A simulation of the hydrothermal response to the Chesapeake Bay bolide impact - Sanford, W.E. (Geofluids, 2005)
Groundwater more saline than seawater has been discovered in the tsunami breccia of the Chesapeake Bay impact Crater. One hypothesis for the origin of this brine is that it may be a liquid residual following steam separation in a hydrothermal system that evolved following the impact. Initial scoping calculations have demonstrated that it is feasible such a residual brine could have remained in the crater for the 35 million years since impact. Numerical simulations have been conducted using the code HYDROTHERM to test whether or not conditions were suitable in the millennia following the impact for the development of a steam phase in the hydrothermal system. Hydraulic and thermal parameters were estimated for the bedrock underlying the crater and the tsunami breccia that fills the crater. Simulations at three different breccia permeabilities suggest that the type of hydrothermal system that might have developed would have been very sensitive to the permeability. A relatively low breccia permeability (1 × 10-16 m2) results in a system partitioned into a shallow water phase and a deeper superheated steam phase. A moderate breccia permeability (1 × 10-15 m2 ) results in a system with regionally extensive multiphase conditions. A relatively high breccia permeability (1 × 10-14 m2 ) results in a system dominated by warm-water convection cells. The permeability of the crater breccia could have had any of these values at given depths and times during the hydrothermal system evolution as the sediments compacted. The simulations were not able to take into account transient permeability conditions, or equations of state that account for the salt content of seawater. Results suggest, however, that it is likely that steam conditions existed at some time in the system following impact, providing additional evidence that is consistent with a hydrothermal origin for the crater brine. © Blackwell Publishing Ltd.
cited By 31
Keywords: bolide; hydrothermal circulation; impact structure; Chesapeake Bay; North America; United States; Western Hemisphere; World

Drill bit seismic imaging of the San Andreas Fault System at SAFOD - Taylor, S.; Malin, P.; Shalev, E.; Haldorsen, J.B.U.; Coates, R.; Stolte, C. (Society of Exploration Geophysicists - 75th SEG International Exposition and Annual Meeting, SEG 2005, 2005)
In conjunction with the US Geological Survey, Duke University has collected several unique datasets at the site of the San Andreas Fault Observatory at Depth (SAFOD). As part of ongoing seismic studies at SAFOD, we acquired a seismic dataset which used drill bit energy as a seismic source during the summer of 2004. The purpose of this experiment was to collect an inverse vertical seismic profile from signals generated by the drill bit while the SAFOD borehole was being drilled. To develop the profile discussed in this report, we utilized a surface array of vertical component geophones that were deployed along the azimuth proposed for the deviated borehole which is now instrumented as a subsurface observatory. While a well is being drilled, the drill bit produces low level seismic energy as it crushes rock fragments and moves downward through the rock. During drilling of the SAFOD well, we simultaneously recorded these signals using accelerometers mounted on the topdrive of the drill rig as well as a 46 channel surface array of vertical component geophones. The data collected during this study has provided us with valuable seismic information which supplements our previous data of spatially coarse vertical seismic profiles. We now have seismic data based on 10m subsurface source interval to use for interpretation of the physical properties of rocks in the study area. © Society of Exploration Geophysicists - 75th SEG International Exposition and Annual Meeting, SEG 2005. All rights reserved.
cited By 4
Keywords: Bits; Boreholes; Drills; Geophysical prospecting; Infill drilling; Observatories; Seismic waves; Strike-slip faults; San Andreas fault; Seismic imaging; Seismic information; Seismic studies; Subsurface source; US Geological Survey; Vertical component; Vertical seismic profiles; Seismology

Geochemical structure of the Hawaiian plume: Sr, Nd, and Os isotopes in the 2.8 km HSDP-2 section of Mauna Kea volcano - Bryce, J.G.; DePaolo, D.J.; Lassiter, J.C. (Geochemistry, Geophysics, Geosystems, 2005)
Sr, Nd, and Os isotopic measurements were made on 110 Mauna Kea lava and hyaloclastite samples from the drillcore retrieved from the second phase of the Hawaii Scientific Drilling Project (HSDP-2). The samples come from depths of 255 to 3098 meters below sea level, span an age range from 200 to about 550-600 kyr, and represent an ordered record of the lava output from Mauna Kea volcano as it drifted a distance of about 40 km over the magma-producing region of the Hawaiian hot spot. The deepest (oldest) samples represent the time when Mauna Kea was closest to the center of the melting region of the Hawaiian plume. The Sr and Os isotopic ratios in HSDP-2 lavas show only subtle isotopic shifts over the ∼400 kyr history represented by the core. Neodymium isotopes (∈Nd values) increase systematically with decreasing age from an average value of nearly +6.5 to an average value of +7.5. This small change corresponds to subtle shifts in 87Sr/86Sr and 187Os/188Os isotope ratios, with small shifts of ∈Hf, a large shift in 208Pb/204Pb and 208Pb/207Pb values, and with a very large shift in He isotope ratios from R/RA values of about 7-8 to values as high as 25. When Mauna Kea was closest to the plume core, the magma source did not have primitive characteristics for Nd, Sr, Pb, Hf, and Os isotopes but did have variable amounts of primitive helium. The systematic shifts in Nd, Hf, Pb, and He isotopes are consistent with radial isotopic zoning within the melting region of the plume. The melting region constitutes only the innermost, highest-temperature part of the thermally anomalous plume mantle. The different ranges of values observed for each isotopic system, and comparison of Mauna Kea lavas with those of Mauna Loa, suggest that the axial region of the plume, which has a radius of ∼20 km, is a mixture of recycled subducted components and primitive lower mantle materials, recently combined during the formational stages of the plume at the base of the mantle. The proportions of recycled and primitive components are not constant, and this requires there be longitudinal (vertical) heterogeneity within the core of the plume. The remainder of the plume, outside this plume core zone, is less heterogeneous but distinct from upper mantle as represented by mid-ocean ridge basalt (MORB). The plume structure may provide a detailed view of mantle isotopic composition near the core-mantle boundary. Copyright 2005 by the American Geophysical Union.
cited By 90

Drill hole logging with infrared spectroscopy - Calvin, W.M.; Solum, J.G. (Transactions - Geothermal Resources Council, 2005)
Infrared spectroscopy has been used to identify rocks and minerals for over 40 years. The technique is sensitive to primary silicates as well as alteration products. Minerals can be uniquely identified based on multiple absorption features at wavelengths from the visible to the thermal infrared. We are currently establishing methods and protocols in order to use the technique for rapid assessment of downhole lithology on samples obtained during drilling operations. Initial work performed includes spectral analysis of chip cuttings and core sections from drill sites around Desert Peak, NV. In this paper, we report on a survey of 10,000 feet of drill cuttings, at 100 foot intervals, from the San Andreas Fault Observatory at Depth (SAFOD). Data from Blue Mountain geothermal wells will also be acquired. We will describe the utility of the technique for rapid assessment of lithologic and mineralogic discrimination.
cited By 4
Keywords: Geothermal wells; Infrared spectroscopy; Lithology; Silicates; Spectrum analysis; Well drilling; Chip cuttings; Drill hole logging; San Andreas Fault Observatory at Depth (SAFOD); Spectroscopy mineralogy; Well logging

Impactites of the Haughton impact structure, Devon Island, Canadian High Arctic - Osinski, G.R.; Spray, J.G.; Lee, P. (Meteoritics and Planetary Science, 2005)
Contrary to the previous interpretation of a single allochthonous impactite lithology, combined field, optical, and analytical scanning electron microscopy (SEM) studies have revealed the presence of a series of impactites at the Haughton impact structure. In the crater interior, there is a consistent upward sequence from parautochthonous target rocks overlain by parautochthonous lithic (monomict) breccias, through allochthonous lithic (polymict) breccia, into pale grey allochthonous impact melt breccias. The groundmass of the pale grey impact melt breccias consists of microcrystalline calcite, silicate impact melt glass, and anhydrite. Analytical data and microtextures indicate that these phases represent a series of impact-generated melts that were molten at the time of, and following, deposition. Impact melt glass clasts are present in approximately half of the samples studied. Consideration of the groundmass phases and impact glass clasts reveal that impactites of the crater interior contain shock-melted sedimentary material from depths of >920 to <1880 m in the pre-impact target sequence. Two principal impactites have been recognized in the near-surface crater rim region of Haughton. Pale yellow-brown allochthonous impact melt breccias and megablocks are overlain by pale grey allochthonous impact melt breccias. The former are derived from depths of >200 to <760 m and are interpreted as remnants of the continuous ejecta blanket. The pale grey impact melt breccias, although similar to the impact melt breccias of the crater interior, are more carbonate-rich and do not appear to have incorporated clasts from the crystalline basement. Thus, the spatial distribution of the crater-fill impactites at Haughton, the stratigraphic succession from target rocks to allochthonous impactites, the recognition of large volumes of impact melt breccias, and their probable original volume are all analogous to characteristics of coherent impact melt layers in comparatively sized structures formed in crystalline targets. © The Meteoritical Society, 2005.
cited By 51

Hypervelocity collisions into continental crust composed of sediments and an underlying crystalline basement: Comparing the Ries (∼24 km) and Chicxulub (∼180 km) impact craters - Kring, D.A. (Chemie der Erde, 2005)
The Chicxulub and Ries impact craters were excavated from layered continental terrains that were composed of carbonate-bearing sedimentary sequences and underlying crystalline silicate basement materials. The Chicxulub and Ries impact events were sufficiently large to produce complex peak-ring impact craters. The walls of transient craters and excavation cavities, with diameters of 12-16 km for the Ries and 90-100 km for Chicxulub, collapsed to form final crater diameters of ∼24 and ∼180 km, respectively. Debris from both the sedimentary and crystalline layers was ejected during crater formation, but the bulk of the melting occurred at depth, in the silicate basement. The volume of melt and proportion of melt among shock-metamorphosed debris was far larger at Chicxulub, producing a central melt sheet ∼3 km in depth. The central melt sheet was covered with melt-bearing polymict breccias and, at the Ries, similar breccias (crater suevites) filled the central cavity. Also at the Ries (and presumably at Chicxulub), large hill-size megablocks of crystalline basement material were deposited near the transient crater rim. Blocks and megablocks of sedimentary lithologies were ejected into the modification zone between the peak ring and final crater rim, while additional material was slumping inward during crater growth, and buried beneath a fallout deposit of melt-bearing polymict breccias. The melt and surviving clasts in the breccias are dominantly derived from the deeper, basement lithologies. At greater distances, however, the ejecta is dominated by near-surface sedimentary lithologies, large blocks of which landed with such high energy that they scoured and eroded the pre-existing surface. The excavation and ejecta pattern produced lithological and chemical variations with radial distance from the crater centers that evolve from basement components near the crater centers to sedimentary components far from the crater centers. In addition, carbonate (and anhydrite in the case of Chicxulub) was vaporized, producing environmentally active gases. The vaporized volume produced by the Ries impact event was too small to dramatically alter the evolution of life, but the vaporized volume produced by the Chicxulub impact event is probably a key factor in the Cretaceous-Tertiary boundary mass extinction event. © 2004 Elsevier GmbH. All rights reserved.
cited By 58

Hydrothermal activity associated with the Ries impact event, Germany - Osinski, G.R. (Geofluids, 2005)
Combined field studies, optical and scanning electron microscopy, and electron microprobe studies of impactites from the Ries impact structure, Germany, have allowed a clearer picture of the hydrothermal system associated with the Ries impact event to be made. Hydrothermal alteration is concentrated within impact-generated suevites in the interior of the crater (crater suevites) and around the periphery (surficial suevites), with minor alteration in the overlying sedimentary crater-fill deposits. The major heat source for the Ries hydrothermal system was the suevite units themselves. Hydrothermal alteration of crater-fill suevites is pervasive in nature and comprises several distinct alteration phases that vary with depth. An early phase of K-metasomatism accompanied by minor albitization of crystalline basement clasts and minor chloritization, was followed by pervasive intermediate argillic alteration (predominantly montmorillonite, saponite, and illite) and zeolitization (predominantly analcite, erionite, and clinoptilolite). Hydrothermal fluids were typically weakly alkaline during the main stage of alteration. In contrast to the crater-fill suevites, alteration within surficial suevites was typically restricted to montmorillonite and phillipsite deposition within cavities and fractures. The pervasive nature of the alteration within the crater-fill suevites was likely due to the presence of an overlying crater lake; whereas alteration within surficial suevites typically occurred under undersaturated conditions with the main source of water being from precipitation. There are exceptional outcrops of more pervasively altered surficial suevites, which can be explained as locations where water pooled for longer periods of time. Hydrothermal fluids were likely a combination of meteoric waters that percolated down from the overlying crater lake and groundwaters that flowed in from the surrounding country rocks. © 2005 Blackwell Publishing Ltd.
cited By 64

Holocene hydrologic variation at Lake Titicaca, Bolivia/Peru, and its relationship to North Atlantic climate variation - Baker, P.A.; Fritz, S.C.; Garland, J.; Ekdahl, E. (Journal of Quaternary Science, 2005)
A growing number of sites in the Northern Hemisphere show centennial- to millennial-scale climate variation that has been correlated with change in solar variability or with change in North Atlantic circulation. However, it is unclear how (or whether) these oscillations in the climate system are manifest in the Southern Hemisphere because of a lack of sites with suitably high sampling resolution. In this paper, we reconstruct the lake-level history of Lake Titicaca, using the carbon isotopic content of sedimentary organic matter, to evaluate centennial- to millennial-scale precipitation variation and its phasing relative to sites in the Northern Hemisphere. The pattern and timing of lake-level change in Lake Titicaca is similar to the ice-rafted debris record of Holocene Bond events, demonstrating a possible coupling between precipitation variation on the Altiplano and North Atlantic sea-surface temperatures (SSTs). The cold periods of the Holocene Bond events correspond with periods of increased precipitation on the Altiplano. Holocene precipitation variability on the Altiplano is anti-phased with respect to precipitation in the Northern Hemisphere monsoon region. More generally, the tropical Andes underwent large changes in precipitation on centennial-to-millennial timescales during the Holocene. Copyright © 2005 John Wiley & Sons, Ltd.
Keywords: Lake Titicaca; South America; climate variation; Holocene; paleoclimate; paleohydrology; precipitation (climatology)

High-resolution magnetostratigraphy of late quaternary sediments from Lake Baikal, Siberia: Timing of intracontinental paleoclimatic responses - Demory, François; Nowaczyk, Norbert R.; Witt, Annette; Oberh{ansli, Hedi (Global and Planetary Change, 2005)
Sediment cores retrieved from 6 locations in Lake Baikal were subjected to a paleomagnetic study in order to establish detailed age models based on correlations of relative paleointensity records. Additional data were provided by calibrated accelerator mass spectrometry (AMS) 14C dating, as well as by documentation of geomagnetic excursions like Laschamp at ∼42 ka and Iceland Basin at ∼185 ka. Few intervals were affected by diagenetic features like selective reductive dissolution of magnetite and greigite mineralization (Demory et al., 2005-this issue), and those that were left out of paleointensity records. These records were tuned to the well-dated paleomagnetic record from ODP Site 984 Channell [Channell, J.E.T., 1999. Geomagnetic paleointensity and directional secular variation at Ocean Drilling Program (ODP) site 984 (Bjorn Drift) since 500 ka: comparisons with ODP site 983 (Gardar drift). J. Geophys. Res., B: [Solid Earth], 104 (10):22, 937-22, 951]. The complex shape of the resulting depth/age curves highlights the need for a high-resolution age model. We focused on the climatic boundary between marine isotopic stage (MIS) 7 and 6 where the Iceland Basin paleomagnetic excursion is clearly documented in the North Atlantic Channell et al.[Channell, J.E.T., Hodell, D.A., Lehman, B., 1997. Relative geomagnetic paleointensity and d18O at ODP Site 983 (Gardar Drift, North Atlantic) since 350 ka. Earth Planet. Sci. Lett., 153 (1-2), 103-118] and in Lake Baikal Oda et al. [Oda, H., Nakamura, K., Ikehara, K., Nakano, T., Nishimura, M., Khlystov, O., 2002. Paleomagnetic record from Academician Ridge, Lake Baikal: a reversal excursion at the base of marine oxygen isotope stage 6. Earth Planet. Sci. Lett., 202 (1), 117-132] ; present study). During this period, we provide evidence for a return to cold conditions in the Lake Baikal region simultaneous to the sea surface cooling, but earlier than the global ice volume change observed in North Atlantic planktonic and benthic δ18O records, respectively. The classical strategy of age model reconstruction, based on direct correlation of the climatic record from Lake Baikal sediments with the marine δ18O reference curves is shown here to be unreliable. Moreover, this strategy does not consider (i) the nonlinearity of the age model in Lake Baikal sediments and (ii) the time lags between the global ice volume change and sea surface cooling observed in δ18O marine records. Finally, the Baikal 200 compilation of the paleointensity records established in this study provides a 200-ka-long synthetic paleomagnetic record for Central Eurasia. © 2004 Elsevier B.V. All rights reserved.
Keywords: Eastern Hemisphere; Eurasia; Lake Baikal; Russian Federation; World; lacustrine deposit; magnetostratigraphy; paleoclimate; paleomagnetism; Quaternary

High-resolution geochemical record of Cretaceous-Tertiary boundary sections in Mexico: New constraints on the K/T and Chicxulub events - St{uben, D.; Kramar, U.; Harting, M.; Stinnesbeck, W.; Keller, G. (Geochimica et Cosmochimica Acta, 2005)
The investigation of eight Cretaceous-Tertiary (K/T) sections in Mexico, based on major and trace element, platinum group element (PGE), stable isotope, and multivariate statistical analysis, reveals a complex depositional history across the Chicxulub and K/T boundary events. At the biostratigraphically determined K/T boundary, a minor but significant Ir-dominated PGE anomaly (0.2-0.8 ng/g) is present in most sections. This Ir anomaly originated from an impact event and is always stratigraphically and geochemically decoupled from the underlying spherule-rich ejecta deposit related to the Chicxulub event. In all sections examined, one to three glass spherule ejecta layers and one or two chondrite-dominated PGE anomalies are separated by a bioturbated siliciclastic deposit and/or hemipelagic marl, which indicates the occurrence of at least two impact events separated by a considerable amount of time. In addition, bentonite layers and Pt and Pd-dominated PGE anomalies below and above the K/T boundary indicate volcanic activity. Above the K/T boundary, reduced bioproductivity is documented by a decrease in the biogenically bound fraction of nutrients and fluctuating ratios of immobile elements (e.g., Ti/Zr). Variations in detrital elements reflect changes in the depositional environment. Carbon and oxygen isotope and trace element distribution patterns indicate a gradually changing climate during the latest Maastrichtian, an abrupt change at the K/T boundary, and a slight recovery during the lowermost Paleocene. Copyright © 2005 Elsevier Ltd.
cited By 41

Heterogeneity effects in thermal borehole measurements in the Chicxulub impact crater - Wilhelm, H.; Popov, Y.; Burkhardt, H.; Šafanda, J.; Čerm'ak, V.; Heidinger, P.; Korobkov, D.; Romushkevich, R.; Mayr, S. (Journal of Geophysics and Engineering, 2005)
Boreholes drilled in impact structures are especially suited for investigations of the influence of heterogeneities on petrophysical properties and thermal field. In the scientific well Yaxcopoil-1 drilled within the frame of the International Continental Deep Drilling Program (ICDP) and as a part of the Chicxulub Scientific Drilling Project (CSDP) high resolution temperature measurements and a dense petrophysical profile measured on core samples at ∼2.2 m depth intervals were recorded. From the calculated vertical component of the thermal gradient and the thermal conductivity measured on the core samples a mean heat flow density of 70.5 ± 1.9 mW m-2 in the depth interval 400-1400 m was determined. On the basis of a simple purely conductive heterogeneous 2D thermal model the effect of the refraction of heat caused by heterogeneities is demonstrated. A statistical investigation shows that if the scales of the heterogeneities influencing the values of the measured thermal conductivity and the calculated thermal gradient are small compared to the length of the borehole the effect of the heterogeneities on the vertical heat flow density can be interpreted as thermal noise. © 2005 Nanjing Institute of Geophysical Prospecting.
cited By 9

Geophysical evaluation of the enigmatic Bedout basement high, offshore northwestern Australia - M{uller, R.D.; Goncharov, A.; Kritski, A. (Earth and Planetary Science Letters, 2005)
The Bedout High in the Roebuck Basin (formerly offshore Canning Basin) on the northwest shelf of Australia is an unusual structure, which has been controversially interpreted as an end-Permian impact structure similar in size to the K-T boundary Chicxulub Crater. We present a geophysical perspective of the associated debate, based on deep seismic reflection, refraction and well data. The basement and crust in the Roebuck Basin display a number of features that distinguish them from other basins along the northwest Australian margin, including major crustal thinning and the presence of a thick layer of interpreted magmatic underplating. The Bedout High consists of two separate highs separated by a Paleozoic fault, and is associated with a Moho uplift of 7-8 km, and is about 40-50 km wide. The normal fault separating the two highs trends NNW-SSE, roughly paralleling a Paleozoic fault system associated with rifting in the Canning Basin and terminating below the interpreted top-Permian reflection. There are no circular, symmetric fault zones bounding the proposed annular trough, and the distinct difference in seismic character normally associated with impact breccias versus layered sediments above are not expressed in deep multichannel seismic data. The end-Permian horizon exhibits little topography, with well-layered units both below and above. The area around the Bedout High stands out as an area of low velocity basement: 5400-5600 m/s compared to 5800-6000 m/s for other nearby basement areas located in a similar depth range, but known complex impact sites are not characterized by a unique seismic basement velocity signature. Both seismic velocity analysis, revealing a thick underplated layer in the lower crust, and thermal modelling based on data from well La Grange-1 and basalts drilled on top of the Bedout High, are consistent with rifting above anomalously hot mantle. The available geophysical and geological data are compatible with an interpretation of the Bedout structure as a basement high formed by two consecutive Paleozoic and Mesozoic episodes of rifting roughly orthogonal to each other, associated with basin formation east and west of the Bedout High, but fail nearly all unequivocal criteria for impact crater recognition. © 2005 Elsevier B.V. All rights reserved.
cited By 32

Impact structures: What does crater diameter mean? - Turtle, E.P.; Pierazzo, E.; Collins, G.S.; Osinski, G.R.; Melosh, H.J.; Morgan, J.V.; Reimold, W.U. (Special Paper of the Geological Society of America, 2005)
The diameter of an impact crater is one of the most basic and important parameters used in energy scaling and numerical modeling of the cratering process. However, within the impact and geological communities and literature, there is considerable confusion about crater sizes due to the occurrence of a variety of concentric features, any of which might be interpreted as defining a crater's diameter. The disparate types of data available for different craters make the use of consistent metrics difficult, especially when comparing terrestrial to extraterrestrial craters. Furthermore, assessment of the diameters of terrestrial craters can be greatly complicated due to post-impact modification by erosion and tectonic activity. We analyze the terminology used to describe crater geometry and size and attempt to clarify the confusion over what exactly the term crater diameter means, proposing a consistent terminology to help avert future ambiguities. We discuss several issues of crater-size in the context of four large terrestrial examples for which crater diameters have been disputed (Chicxulub, Sudbury, Vredefort, and Chesapeake Bay) with the aim of moving toward consistent application of terminology. © 2005 Geological Society of America.
cited By 106

Geochemical and petrographic characteristics of impactites and Cretaceous target rocks from the Yaxcopoil-1 borehole, Chicxulub impact structure, Mexico: Implications for target composition - Tuchscherer, M.G.; Reimold, W.U.; Koeberl, C.; Gibson, R.L. (Meteoritics and Planetary Science, 2005)
We present major and trace element data as well as petrographic observations for impactites (suevitic groundmass, bulk suevite, and melt rock particles) and target lithologies, including Cretaceous anhydrite, dolomite, argillaceous limestone, and oil shale, from the Yaxcopoil-1 borehole, Chixculub impact structure. The suevitic groundmass and bulk suevite have similar compositions, largely representing mixtures of carbonate and silicate components. The latter are dominated by melt rock particles. Trace element data indicate that dolomitic rocks represented a significant target component that became incorporated into the suevites; in contrast, major elements indicate a strong calcitic component in the impactites. The siliceous end-member requires a mafic component in order to explain the low SiO2 content. Multicomponent mixing of various target rocks, the high alteration state, and dilution by carbonate complicate the determination of primary melt particle compositions. However, two overlapping compositional groups can be discerned - a high-Ba, low-Ta group and a high-Fe, high-Zn, and high-Hf group. Cretaceous dolomitic rocks, argillaceous limestone, and shale are typically enriched in U, As, Br, and Sb, whereas anhydrite contains high Sr contents. The oil shale samples have abundances that are similar to the North American Shale Composite (NASC), but with a comparatively high U content. Clastic sedimentary rocks are characterized by relatively high Th, Hf, Zr, As, and Sb abundances. Petrographic observations indicate that the Cretaceous rocks in the Yaxcopoil-1 drill core likely register a multistage deformation history that spans the period from pre- to post-impact. Contrary to previous studies that claimed evidence for the presence of impact melt breccia injection veins, we have found no evidence in our samples from a depth of 1347-1348 m for the presence of melt breccia. We favor that clastic veinlets occur in a sheared and altered zone that underwent intense diagenetic overprint prior to the impact event. © The Meteoritical Society, 2005.
cited By 22

Earth science: Microseismicity data forecast rupture area - Schorlemmer, D.; Wiemer, S. (Nature, 2005)
cited By 154
Keywords: earthquake rupture; article; diagnostic accuracy; earthquake; forecasting; mathematical analysis; prediction; priority journal; United States

Drilling operation of mallik 2002 gas hydrate production research well program - Takahashi, H. (Nihon Enerugi Gakkaishi/Journal of the Japan Institute of Energy, 2005)
At Mallik of the Mackenzie Delta in the Arctic Canada, over a 79 day period, from December 25, 2001 to March 14, 2002, three (3) wells were drilled through the hydrate formation beneath permafrost on a line at 40m distance, where coring, logging, various science experiments and production testing were performed. This research project was organized and funded by participants from five (5) countries of Japan, Canada, US, Germany and India. Japan National Oil Corporation (JNOC) and Japan Petroleum Exploration Co. Ltd. (JAPEX) undertook its operation while the Geological Survey of Canada (GSC) coordinated the science program. This paper describes the drilling operation including its logistics of the Mallik 2002 Gas Hydrate Production Research Well Program.
cited By 0
Keywords: Crude petroleum; Logistics; Natural gas well drilling; Project management; Societies and institutions; Arctic operations; Geological Survey of Canada (GSC); Japan National Oil Corporation (JNOC); Remote operations; Gas hydrates

Fluid convection observed from temperature logs in the karst formation of the Yucat'an Peninsula, Mexico - Šafanda, J.; Heidinger, P.; Wilhelm, H.; Čerm'ak, V. (Journal of Geophysics and Engineering, 2005)
Temperature-depth profiles, obtained during three campaigns of temperature logging in boreholes of the Yucat'an Peninsula in the period 2002-2004, display the effects of thermal fluid convection. These effects are most pronounced in the uppermost part of the 1.5 km deep borehole Yaxcopoil-1 drilled within the Chicxulub impact crater. The convective zone is clearly discernible in all three profiles measured here in March 2002, May 2003 and February 2004. The loggings have revealed a gradual downward propagation of the convective features from the uppermost 145 m of the temperature profile to 230 m between the first and second loggings (with a propagation rate of 6 m/month) and to 265 m between the second and third loggings (4 m/month). A signature of the fluid convection is also evident in all other temperature logs in the area measured during the 2003 campaign, namely in four UNAM boreholes 2, 5, 7 and 8, three hydrogeological boreholes 1A, 1B and 1C, in a borehole at the meteorological observatory M'erida and in the cenote Ucil. The fresh/salt water interface is clearly visible in most of the logs as a zone of increased temperature gradient. The varying intensity of the convective features among the individual logs seems to be correlated with the borehole position relative to the impact structure. © 2005 Nanjing Institute of Geophysical Prospecting.
cited By 12

Drilling the Eger Rift in Central Europe - Špič'ak, A.; F{orster, A.; Horsfield, B. (Scientific Drilling, 2005)
cited By 6
Keywords: Energy resources; Mechanical engineering; Drilling

Drillbit seismic images fractures of San Andreas fault system - Taylor, S.T.; Stolte, C.; Haldorsen, J.B.U.; Coates, R.; Malin, P.; Shalev, E. (Oil and Gas Journal, 2005)
Scientists recently used the drillbit seismic technique to image fractures and shear zones associated with the San Andreas fault near Parkfield, Calif. Low-level energy produced by the drillbit served as a downhole seismic source, and the wave response was measured by an array of geophones at the surface. In the complex structural environment underlying Parkfield, numerous faults that comprise a flower structure are interpreted to cut the Cenozoic sedimentary cover and the Salinian block at the San Andreas Fault Observatory at Depth (SAFOD) project drillsite. Based on updated surface geologic maps by Rymer, these fractures and faults appear to trend subparallel to the San Andreas fault (SAF) and have been mapped up to 3 km to the southwest of the main trace of the SAF. Our study included the interpretation of a surface seismic profile and the drillbit seismic data. These data indicate that faulting is more abundant and shearing more pervasive as one approaches the main trace of the San Andreas fault. In addition, our interpretation of the locations and dips of faults in the PSINE profile correlates very well with locations and dips of several linear features imaged in the migrated drillbit seismic data.
cited By 0
Keywords: Fracture; Sedimentary rocks; Seismic waves; Seismology; Structural geology; Tectonics; Drillbit seismic technique; Geophones; Surface seismic profile; Seismic prospecting; fracture network; imaging method; San Andreas Fault; seismic method

Eastern rim of the Chesapeake Bay impact crater: Morphology, stratigraphy, and structure - Poag, C.W. (Special Paper of the Geological Society of America, 2005)
This study reexamines seven reprocessed (increased vertical exaggeration) seismic reflection profiles that cross the eastern rim of the Chesapeake Bay impact crater. The eastern rim is expressed as an arcuate ridge that borders the crater in a fashion typical of the raised rim documented in many well preserved complex impact craters. The inner boundary of the eastern rim (rim wall) is formed by a series of raterfacing, steep scarps, 15-60 m high. In combination, these rim-wall scarps represent the footwalls of a system of crater-encircling normal faults, which are downthrown toward the crater. Outboard of the rim wall are several additional normal-fault blocks, whose bounding faults trend approximately parallel to the rim wall. The tops of the outboard fault blocks form two distinct, parallel, flat or gently sloping, terraces. The innermost terrace (Terrace 1) can be identified on each profile, but Terrace 2 is only sporadically present. The terraced fault blocks are composed mainly of nonmarine, poorly to moderately consolidated, siliciclastic sediments, belonging to the Lower Cretaceous Potomac Formation. Though the ridge-forming geometry of the eastern rim gives the appearance of a raised compressional feature, no compelling evidence of compressive forces is evident in the profiles studied. The structural mode, instead, is that of extension, with the clear dominance of normal faulting as the extensional mechanism. © 2005 Geological Society of America.
cited By 8
Keywords: Meteor impacts; Morphology; Stratigraphy; Structure (composition); Chesapeake Bay; Compressive forces; Impact craters; Lower Cretaceous; Normal faulting; Seismic reflection profiles; Siliciclastic sediments; Structural modes; Fault slips; crater; Cretaceous; dominance; faulting; morphology; seismic reflection; stratigraphy; terrace; Chesapeake Bay; United States

Evidence for ascending upper mantle-derived melt beneath the Cheb basin, central Europe - Br{auer, Karin; K{ampf, Horst; Niedermann, Samuel; Strauch, Gerhard (Geophysical Research Letters, 2005)
We present new 3He/4He data of CO2-rich gas exhalations of three presently nonvolcanic areas in Europe. The 3He/4He ratios from the Cheb basin (Czech Republic) are clearly higher than data obtained ten years ago, whereas the 3He/4He ratios from the other areas (Laacher See/Germany and Mari'ansk'e L'azně/CR) have remained nearly constant. No other locality showing such high 3He/4He ratios in free gases supplied by continental mantle degassing is known in the European Cenozoic rift system. At the northeastern edge of the Cheb basin swarm earthquakes repeatedly occur. In the context of contemporaneous periods of seismicity the increased 3He/4He ratios are interpreted as the first geochemical evidence for ascending mantle-derived melt beneath the Cheb basin, which is related to the triggering of earthquake swarms. Copyright 2005 by the American Geophysical Union.
Keywords: Central Europe; Cheb Basin; Czech Republic; Eastern Hemisphere; Eurasia; Europe; World; Catchments; Degassing; Earthquakes; Helium; Europe; Mantle; Nonvolcanic areas; Rifts; mantle plume; Geophysics

Experimental petrology of the 1991-1995 Unzen dacite, Japan. Part I: Phase relations, phase composition and pre-eruptive conditions - Holtz, François; Sato, Hiroaki; Lewis, Jared; Behrens, Harald; Nakada, Setsuya (Journal of Petrology, 2005)
Crystallization experiments were conducted on dry glasses from the Unzen 1992 dacite at 100 300 MPa, 775-875°C, various water activities, and fO2 buffered by the Ni-NiO buffer. The compositions of the experimental products and natural phases are used to constrain the temperature and water contents of the low-temperature and high-temperature magmas prior to the magma mixing event leading to the 1991-1995 eruption. A temperature of 1050 ± 75°C is determined for the high-temperature magma based on two-pyroxene thermometry. The investigation of glass inclusions suggests that the water content of the rhyolitic low-temperature magma could be as high as 8 wt % H2O. The phase relations at 300 MPa and in the temperature range 870-900°C, which are conditions assumed to be representative of the main magma chamber after mixing, show that the main phenocrysts (orthopyroxene, plagioclase, hornblende) coexist only at reduced water activity; the water content of the post-mixing dacitic melt is estimated to be 6 ± 1 wt % H2O. Quartz and biotite, also present as phenocrysts in the dacite, are observed only at low temperature (below 800-775°C). It is concluded that the erupted dacitic magma resulted from the mixing of c. 35 wt % of an almost aphyric pyroxene-bearing andesitic magma (1050 ± 75°C; 4 ± 1 wt % H2O in the melt) with 65 wt % of a phenocryst-rich low-temperature magma (760-780°C) in which the melt phase was rhyolitic, containing up to 8 ± 1 wt % H2O. The proportions of rhyolitic melt and phenocrysts in the low-temperature magma are estimated to be 65% and 35%, respectively. It is emphasized that the strong variations of phenocryst compositions, especially plagioclase, can be explained only if there were variations of temperature and /or water activity (in time and/or space) in the low-temperature magma. © Oxford University Press; all rights reserved.
Keywords: Asia; Eastern Hemisphere; Eurasia; Far East; Japan; Kyushu; Nagasaki [Kyushu]; Unzen Volcano; World; dacite; magma chemistry; petrology; volcanic rock

Experimental petrology of the 1991-1995 Unzen dacite, Japan. Part II: Cl/OH partitioning between hornblende and melt and its implications for the origin of oscillatory zoning of hornblende phenocrysts - Sato, Hiroaki; Holtz, François; Behrens, Harald; Botcharnikov, Roman; Nakada, Setsuya (Journal of Petrology, 2005)
High-temperature-pressure experiments were carried out to determine the chlorine-hydroxyl exchange partition coefficient between hornblende and melt in the 1992 Unzen dacite. Cl in hornblende and melt was analyzed by electron microprobe, whereas OH in hornblende and melt was calculated assuming anion stoichiometry of hornblende and utilizing the dissociation reaction constant for H2O + O = 2(OH) in water-saturated melt, respectively. The partition coefficient strongly depends on the Mg/(Mg + Fe) ratio of hornblende, and is expressed as In K1 = (Cl/OH)hb/(Cl/OH)melt = 2· 37 - 4·6[Mg/(Mg + Fe)]hb at 2-3kbar and 800-850°C. The twofold variation in Cl content in the oscillatory zoned cores of hornblende phenocrysts in the 1991-1995 dacite cannot be explained by the dependence of the Cl/OH partition coefficient on the Mg/(Mg + Fe) hb ratio, and requires c. 80% variation of the Cl/OH ratio of the coexisting melt. Available experimental data at 200 MPa on Cl/OH fractionation between fluid and melt suggest that c. 1· 2-1·8 wt % degassing of water from the magma can explain the required 80% variation in the Cl/OH ratio of the melt. The negative correlation between Al content and Mg/(Mg + Fe) ratio in the oscillatory zoned cores of the hornblende phenocrysts is consistent with repeated influx and convective degassing of the fluid phase in the magma chamber. © Oxford University Press 2004; All rights reserved.
Keywords: Asia; Eastern Hemisphere; Eurasia; Far East; Japan; Kyushu; Nagasaki [Kyushu]; Unzen Volcano; World; dacite; magma chemistry; petrology; volcanic rock

Rapid helium isotopic variability in Mauna Kea shield lavas from the Hawaiian Scientific Drilling Project - Kurz, Mark D.; Curtice, Joshua; Lott III, Dempsey E.; Solow, Andy (Geochemistry, Geophysics, Geosystems, 2004)
This paper presents new magmatic helium isotopic compositions in a suite of lavas from phase II of the Hawaiian Scientific Drilling Project (HSDP2) core, which sampled Mauna Kea volcano to a maximum depth of 3098 m below sea level. Most of the measurements were performed by in vacuo crushing of olivine phenocrysts, but include submarine pillow glasses from the 2200 to 2500 meter depth interval, and orthopyroxene phenocrysts from an intrusive at 1880 m. The magmatic 3He/4He ratios range from 6 to 24.7 times atmospheric (Ra), which significantly extends the range of values for Mauna Kea volcano. The 3He/4He ratios are lowest (i.e., close to MORB values of ∼8 Ra) near the top of the Mauna Kea section and rise slowly, to 10-12 Ra, at 1000 m below sea level, consistent with results from the HSDP1 core. At depths greater than 1000 m in the core, primarily in the submarine lavas, there are brief periods when the 3He/4He ratios are higher than 14.5 Ra, always returning to a baseline value. Twelve such excursions were identified in the core; all but one are in the submarine section, and most (7) are in the deepest section, at depths of 1950 to 3070 m. The baseline 3He/4He value rises from 10-12 Ra near 1000 m depth to 12-14 Ra at 3000 m. The helium spikes are found only in lavas that are older than 380 Ka in age, based on an age model derived from Ar-Ar data (W. D. Sharp et al., manuscript in preparation, 2003). Excluding the excursions defined by single lava flows (3) and intrusive units (3), the average spike duration is approximately 15 (±9) Ka (n = 6). The high 3He/4He spikes are interpreted as pulses of magma from the center of the actively upwelling Hawaiian hot spot. The short duration of the high 3He/4He excursions suggests that Mauna Kea was never directly over high the 3He/4He component of the plume (during the HSDP2 eruptive period), presumed to be the plume center. Assuming that the Mauna Kea helium spikes result from melting of heterogeneities within the plume, their short duration implies that the length scales of heterogeneities in the solid upwelling mantle are between 60 m and 12 km (for upwelling rates of 2 to 40 cm/yr). The high 3He/4He are associated with high 208Pb/204Pb, and relatively low 143Nd/144Nd, Zr/Nb, and SiO2. The correlations with major elements, trace elements and isotopes demonstrate that helium is coupled to the other geochemical variations, and that the Mauna Kea isotopic variability is caused by heterogeneities within the upwelling plume. Copyright 2004 by the American Geophysical Union.

Overview of the mallik gas-hydrate production research well - Takahashi, H.; Yonezawa, T.; Fercho, E. (JPT, Journal of Petroleum Technology, 2004)
The first production test of methanehydrate layers was carried out in the Mackenzie Delta in the Canadian Arctic. Three wells were drilled through hydrate layers at a depth of approximately 900 to 1100 m beneath 640 m of permafrost on a line at 40-m spacing. Coring, logging, and other scientific experiments and production test were performed over a 79-day period. The Geologic Survey of Canada (GSC) was coordinator of the science program, and Japan Natl. Oil Corp. (JNOC) and Japan Petroleum Exploration Co. (JAPEX) were operators.
cited By 4
Keywords: Coring; Gas hydrate production; Hole enlargement; Openhole well logs; Core samples; Methane; Natural gas well drilling; Natural gas well logging; Natural gas well production; Permafrost; Gas hydrates

Pre-drill surface seismic in the vicinity of the AIG-10 well and post-drill VSP; [Sismique de surface avant forage du puits AIG-10, et VSP (Vertical Seismic Profiling) après forage] - Naville, Charles; Serbutoviez, Sylvain; Moretti, Isabelle; Daniel, Jean-Marc; Throo, Alexandre; Girard, François; Sotiriou, Andreas; Tselentis, Akis; Skarpzelos, Christos; Brunet, Christophe; Cornet, François (Comptes Rendus - Geoscience, 2004)
Pre-drill reflection surface seismic was carried out in the Aigion city area in February 2001. The poor quality of seismic reflections obtained from standard processing did not allow us to refine the location of the vertical well AIG-10 to be drilled. Subsequently, a brief study of refracted seismic arrivals was attempted in order to better define the well location, in conjunction with observations from a shallow well. In 2002, after drilling, an innovative 6-VSP position well seismic survey was successfully run, using a four-component downhole sensor tool, i.e., three orthogonal geophones and a hydrophone. Refracted seismic events and first arrivals from VSP indicated the throw of the Aigion Fault and the fault plane dip; a secondary accident striking about south-north was detected about 100 m west of the well. No P-P seismic reflection was evidenced from standard VSP processing. © 2004 Acad'emie des sciences. Published by Elsevier SAS. All rights reserved.
Keywords: Gulf of Corinth; Ionian Sea; Mediterranean Sea; borehole; fault; rift zone; seismic survey

Platinum group elements in impactites of the ICDP Chicxulub drill core Yaxcopoil-1: Are there traces of the projectile? - Tagle, R.; Erzinger, J.; Hecht, L.; Schmitt, R.T.; St{offler, D.; Claeys, P. (Meteoritics and Planetary Science, 2004)
This study presents results of platinum group element (PGE) analyses of impactites from the Yaxcopoil-1 (Yax- 1) and Yucat'an 6 drill cores of the 180 km-diameter Chicxulub crater. These are the main elements used for projectile identification. They were determined by nickel sulfide fire assay combined with inductively coupled plasma mass spectrometry. The concentration of PGE in the samples are low. The concentration patterns of the suevite samples resemble the pattern of the continental crust. We conclude that any meteoritic fraction in these samples is below 0.05%. A synand post-impact modification of the PGE pattern from meteoritic toward a continental crust pattern is very unlikely. The globally distributed fallout at the Cretaceous-Tertiary (K/T) boundary, however, has high PGE concentrations. Therefore, the lack of a significant meteoritic PGE signature in the crater is not an argument for a PGE-poor impactor. Taking the results of three-dimensional numerical simulations of the Chicxulub event into account, the following conclusions are drawn: 1) The main fraction of the impactor was ejected into and beyond the stratosphere, distributed globally, and deposited in the K/T boundary clay; and 2) the low amount of projectile contamination in the Yax-1 lithologies may reflect an oblique impact. However, the role of volatiles in the mixing process between projectile and target is not well-understood and may also have played a fundamental role.
cited By 25

Physical properties of rocks from the upper part of the Yaxcopoil-1 drill hole, Chicxulub crater - Popov, Y.; Romushkevich, R.; Bayuk, I.; Korobkov, D.; Mayr, S.; Burkhardt, H.; Wilhelm, H. (Meteoritics and Planetary Science, 2004)
Physical properties were determined in a first step on post-impact tertiary limestones from the depth interval of 404-666 m of the Yaxcopoil-1 (Yax-1) scientific well, drilled in the Chicxulub impact crater (Mexico). Thermal conductivity, thermal diffusivity, density, and porosity were measured on 120 dry and water-saturated rocks with a core sampling interval of 2-2.5 m. Non-destructive, non-contact optical scanning technology was used for thermal property measurements including thermal anisotropy and inhomogeneity. Supplementary petrophysical properties (acoustic velocities, formation resisitivity factor, internal surface, and hydraulic permeability) were determined on a selected subgroup of representative samples to derive correlations with the densely measured parameters, establishing estimated depth logs to provide calibration values for the interpretation of geophysical data. Significant short- and long-scale variations of porosity (1-37%) turned out to be the dominant factor influencing thermal, acoustic, and hydraulic properties of this post impact limestone formation. Correspondingly, large variations of thermal conductivity, thermal diffusivity, acoustic velocities, and hydraulic permeability were found. These variations of physical properties allow us to subdivide the formation into several zones. A combination of experimental data on thermal conductivity for dry and water-saturated rocks and a theoretical model of effective thermal conductivity for heterogeneous media have been used to calculate thermal conductivity of mineral skeleton and pore aspect ratio for every core under study. The results on thermal parameters are the necessary basis for the determination of heat flow density, demonstrating the necessity of dense sampling in the case of inhomogeneous rock formations. © Meteoritical Society, 2004.
cited By 23

Physical properties and rock physics models of sediment containing natural and laboratory-formed methane gas hydrate - Winters, W.J.; Pecher, I.A.; Waite, W.F.; Mason, D.H. (American Mineralogist, 2004)
This paper presents results of shear strength and acoustic velocity (p-wave) measurements performed on: (1) samples containing natural gas hydrate from the Mallik 2L-38 well, Mackenzie Delta, Northwest Territories; (2) reconstituted Ottawa sand samples containing methane gas hydrate formed in the laboratory; and (3) ice-bearing sands. These measurements show that hydrate increases shear strength and p-wave velocity in natural and reconstituted samples. The proportion of this increase depends on (1) the amount and distribution of hydrate present, (2) differences, in sediment properties, and (3) differences in test conditions. Stress-strain curves from the Mallik samples suggest that natural gas hydrate does not cement sediment grains. However, stress-strain curves from the Ottawa sand (containing laboratory-formed gas hydrate) do imply cementation is present. Acoustically, rock physics modeling shows that gas hydrate does not cement grains of natural Mackenzie Delta sediment. Natural gas hydrates are best modeled as part of the sediment frame. This finding is in contrast with direct observations and results of Ottawa sand containing laboratory-formed hydrate, which was found to cement grains (Waite et al. 2004). It therefore appears that the microscopic distribution of gas hydrates in sediment, and hence the effect of gas hydrate on sediment physical properties, differs between natural deposits and laboratory-formed samples. This difference may possibly be caused by the location of water molecules that are available to form hydrate. Models that use laboratory-derived properties to predict behavior of natural gas hydrate must account for these differences.
cited By 204
Keywords: Acoustic wave velocity; Cements; Gases; Hydration; Laboratories; Methane; Molecules; Natural gas; Natural gas wells; Physical properties; Sand; Sediments; Seismic waves; Shear flow; Stress-strain curves; Wave propagation; Derived properties; Direct observations; Methane gas hydrates; Microscopic distribution; P-wave velocity; Reconstituted sample; Rock physics model; Sediment properties; Gas hydrates; acoustic property; clathrate; gas hydrate; methane; sediment property; shear strength; water content

Phyllosilicate mineral assemblages of the SAFOD Pilot Hole and comparison with an exhumed segment of the San Andreas Fault System - Solum, J.G.; Pluijm, B.A. (Geophysical Research Letters, 2004)
This paper establishes a reference phyllosilicate data set from the SAFOD Pilot Hole for future SAFOD drilling and presents an application of these data for studies of exhumed fault segments. The chlorite assemblages in cuttings from two intervals of the SAFOD Pilot Hole are separated into two populations based on X-ray diffraction characteristics. The first population is found in granite in both the deeper and shallower interval, whereas the second population occurs only in clastic sedimentary rocks in the shallower interval. The characteristics of the first population match those for protolith and cataclasite of the exhumed Punchbowl Fault, whereas samples from intensely deformed ultracataclasite are most similar to the second. This supports previous findings that the mineral assemblages in the ultracataclasite formed after the cessation of motion along the fault, above a depth of ∼2 km, and that mineral assemblages in these exhumed fault rocks have been overprinted by post-faulting alteration. Copyright 2004 by the American Geophysical Union.
cited By 21
Keywords: Boreholes; Drilling; Geophysical prospecting; Granite; Sedimentary rocks; X ray diffraction analysis; Fault rocks; Mineral assemblages; Pilot holes; Silicate minerals; borehole geophysics; cataclasite; fault slip; mineralogy; phyllosilicate; San Andreas Fault

Paleomagnetic and rock magnetic study of the Yaxcopoil-1 impact breccia sequence, Chicxulub impact crater (Mexico) - Urrutia-Fucugauchi, J.; Soler-Arechalde, A.M.; Rebolledo-Vieyra, M.; Vera-Sanchez, P. (Meteoritics and Planetary Science, 2004)
Results of a detailed paleomagnetic and rock magnetic study of samples of the impact breccia sequence cored in the Yaxcopoil-1 (Yax-1) borehole between about 800 m and 896 m are presented. The Yax-1 breccia sequence occurs from 794.63 m to 894.94 m and consists of redeposited melt-rich, clast-size sorted, fine-grained suevites; melt-rich, no clast-size sorting, medium-grained suevites; coarse suevitic melt agglomerates; coarse melt-rich heterogeneous suevites; brecciated suevites; and coarse carbonate and silicate melt suevites. The low-field susceptibility ranges from -0.3 to 4018 × 10-6 SI, and the NRM intensity ranges from 0.02 mA/m up to 37510 mA/m. In general, the NRM intensity and magnetic susceptibility present wide ranges and are positively correlated, pointing to varying magnetic mineral contents and textures of the melt-rich breccia sequence. The vectorial composition and magnetic stability of NRM were investigated by both stepwise alternating field and thermal demagnetization. In most cases, characteristic single component magnetizations are observed. Both upward and downward inclinations are present through the sequence, and we interpret the reverse magnetization as the primary component in the breccias. Both the clasts and matrix forming the breccia appear to have been subjected to a wide range of temperature/pressure conditions and show distinct rock magnetic properties. An extended interval of remanence acquisition and secondary partial or total remagnetization may explain the paleomagnetic results. © Meteoritical Society, 2004.
cited By 16

Preliminary hydrogeological interpretation of the Aigion area from the AIG10 borehole data; [Interpr'etation hydrog'eologique pr'eliminaire de la zone d'Aigion, bas'ee sur les donn'ees obtenues sur le forage AIG10] - Giurgea, Vlad; Rettenmaier, Detlev; Pizzino, Luca; Unkel, Ingmar; H{otzl, Heinz; F{orster, Andrea; Quattrocchi, Fedora (Comptes Rendus - Geoscience, 2004)
The AIG10 borehole provided direct access to hydraulic conditions. The first results of two open-hole pumping tests and one artesian production test are presented. From the first pumping test in the conglomerates, a hydraulic conductivity of 10-5-10-4 ms-1 was derived and it was suggested that there is a closed hydraulic system bounded by the Aigion Fault zone and other faults farther north. A second pumping test, in the Olonos-Pindos limestones, showed artesian flow (water pressure of 0.5 MPa and flow of 1.9×10-4 m3s-1). The transmissivity is about 4×10-6 m2s-1. Below the fault zone, pressure and flux increased, suggesting karstic water-flow conditions. Water-pressure difference of more than 0.5 MPa between the hangingwall and the footwall provides evidence that the Aigion Fault zone acts as an impervious zone. An artesian production test for the interval 708-1001 m showed a fluid pressure of >1 MPa and natural flow of 1.5×10-2 m3s-1. A transmissivity of 2-3 ×10-4 m2s-1 was determined. A preliminary conceptual hydrogeological model, containing flow parameters and flow paths, is developed, based on data from AIG10 and other wells and springs. © 2004 Acad'emie des sciences. Published by Elsevier SAS. All rights reserved.
Keywords: Gulf of Corinth; Ionian Sea; Mediterranean Sea; borehole logging; fault; hydraulic property; hydrogeology; rift zone; transmissivity

Numerical studies of gas production from Class 2 and Class 3 hydrate accumulations at the Mallik site, Mackenzie Delta, Canada - Moridis, G.J. (SPE Reservoir Evaluation and Engineering, 2004)
The Mallik site represents an onshore permafrost-associated methane hydrate accumulation in the Mackenzie Delta, Northwest Territories, Canada. This study focuses on gas production at the Mallik site from hydrate deposits that are underlain by either a freewater zone (Class 2) or an impermeable boundary (Class 3). The production analysis was conducted with a numerical simulator that can model the nonisothermal CH4 release, phase behavior, and flow under conditions typical of CH4-hydrate deposits by solving the coupled equations of mass and heat balance. Accumulations with a CH4-hydrate saturation of at least 50% were studied. Dissociation was induced mainly by a combination of thermal stimulation and depressurization as hot fluids circulated between injection and production wells. The effects of salinity and of pressure changes at the wells were also accounted for. The production strategy resulted in a zero net water production. The simulation results indicated that the amount of CH4 released from the dissociating hydrate deposits is sensitive to the hydrate saturation, the initial temperature, the specific enthalpy, and the flow rate of the circulating fluids. © 2004 Society of Petroleum Engineers.
cited By 77
Keywords: Computer simulation; Enthalpy; Fluid dynamics; Hydrates; Methane; Pressurization; Saturation (materials composition); Thermal effects; Freewater zone; Gas production; Production wells; Water production; Gas fuel analysis; gas hydrate; hydrocarbon reserve; hydrocarbon technology; methane; numerical model; permafrost; Canada; Mackenzie Delta; North America; Northwest Territories

Osmium isotope constraints on the proportion of bolide component in Chicxulub impact melt rocks - Gelinas, A.; Kring, D.A.; Zurcher, L.; Urrutia-Fucugauchi, J.; Morton, O. (Meteoritics and Planetary Science, 2004)
The spatial distribution and amount of material transferred from the bolide involved in the Cretaceous/Tertiary (K/T) event to the target rocks at Chicxulub is still poorly constrained. In this study, Re-Os isotopic analyses of impact melt breccias and lithic clasts from the Yaxcopoil-1 (Yax- 1) borehole were used to determine the distribution and proportion of the bolide component in the target rocks. Because of the much greater concentration of Os in chondritic meteorites compared to the target rocks, little addition of the bolide component would be necessary to greatly perturb the Os concentration and isotopic composition of target rocks. Hence, this is a very sensitive means of examining bolide contributions to the target rocks. For the examined suite of samples, the initial 187OS/188Os ratios vary from 0.19 to 2.3. Conservative mixing calculations suggest that the bolide component comprised as much as approximately 0.1%, by mass, of some samples. Most samples, however, have negligible contributions from the bolide. No samples have Os that is dominated by the bolide component, so for this suite of samples, it is impossible to fingerprint the chemical nature of the bolide using relative abundances of siderophile elements. These results suggest that the bolide did not contribute a significant amount of material to the target rocks. This may, in turn, indicate that most of the bolide was vaporized upon impact or otherwise ejected without mixing with the melt from the target.
cited By 28

Origin and emplacement of the impact formations at Chicxulub, Mexico, as revealed by the ICDP deep drilling at Yaxcopoil-1 and by numerical modeling - St{offler, D.; Artemieva, N.A.; Ivanov, B.A.; Hecht, L.; Kenkmann, T.; Schmitt, R.T.; Tagle, R.A.; Wittmann, A. (Meteoritics and Planetary Science, 2004)
We present and interpret results of petrographic, mineralogical, and chemical analyses of the 1511 in deep ICDP Yaxcopoil-1 (Yax-1 ) drill core, with special emphasis on the impactite units. Using numerical model calculations of the formation, excavation, and dynamic modification of the Chicxulub crater, constrained by laboratory data, a model of the origin and emplacement of the impact formations of Yax-1 and of the impact structure as a whole is derived. The lower part of Yax-1 is formed by displaced Cretaceous target rocks (610 m thick), while the upper part comprises six suevite-type allochthonous breccia units (100 m thick). From the texture and composition of these lithological units and from numerical model calculations, we were able to link the seven distinct impact-induced units of Yax-1 to the corresponding successive phases of the crater formation and modification, which are as follows: 1) transient cavity formation including displacement and deposition of Cretaceous megablocks; 2) ground surging and mixing of impact melt and lithic clasts at the base of the ejecta curtain and deposition of the lower suevite right after the formation of the transient cavity; 3) deposition of a thin veneer of melt on top of the lower suevite and lateral transport and brecciation of this melt toward the end of the collapse of the transient cavity (brecciated impact melt rock); 4) collapse of the ejecta plume and deposition of fall-back material from the lower part of the ejecta plume to form the middle suevite near the end of the dynamic crater modification; 5) continued collapse of the ejecta plume and deposition of the upper suevite; 6) late phase of the collapse and deposition of the lower sorted suevite after interaction with the inward flowing atmosphere; 7) final phase of fall-back from the highest part of the ejecta plume and settling of melt and solid particles through the reestablished atmosphere to form the upper sorted suevite; and 8) return of the ocean into the crater after some time and minor reworking of the uppermost suevite under aquatic conditions. Our results are compatible with: a) 180 km and 100 km for the diameters of the final crater and the transient cavity of Chicxulub, respectively, as previously proposed by several authors, and b) the interpretation of Chicxulub as a peak-ring impact basin that is at transition to a multi-ring basin. © Meteoritical Society, 2004.
cited By 84

Numerical studies of gas production from several CH4 hydrate zones at the Mallik site, Mackenzie Delta, Canada - Moridis, G.J.; Collett, T.S.; Dallimore, S.R.; Satoh, T.; Hancock, S.; Weatherill, B. (Journal of Petroleum Science and Engineering, 2004)
The Mallik site represents an onshore permafrost-associated gas hydrate accumulation in the Mackenzie Delta, Northwest Territories, Canada. A gas hydrate research well was drilled at the site in 1998. The objective of this study is the analysis of various gas production scenarios from five methane hydrate-bearing zones at the Mallik site. In Zone #1, numerical simulations using the EOSHYDR2 model indicated that gas production from hydrates at the Mallik site was possible by depressurizing a thin free gas zone at the base of the hydrate stability field. Horizontal wells appeared to have a slight advantage over vertical wells, while multiwell systems involving a combination of depressurization and thermal stimulation offered superior performance, especially when a hot noncondensible gas was injected. Zone #2, which involved a gas hydrate layer with an underlying aquifer, could yield significant amounts of gas originating entirely from gas hydrates, the volumes of which increased with the production rate. However, large amounts of water were also produced. Zones #3, #4 and #5 were lithologically isolated gas hydrate-bearing deposits with no underlying zones of mobile gas or water. In these zones, thermal stimulation by circulating hot water in the well was used to induce dissociation. Sensitivity studies indicated that the methane release from the hydrate accumulations increased with the gas hydrate saturation, the initial formation temperature, the temperature of the circulating water in the well, and the formation thermal conductivity. Methane production appears to be less sensitive to the specific heat of the rock and of the hydrate, and to the permeability of the formation. © 2004 Published by Elsevier B.V.
cited By 204
Keywords: Depressurization; Thermal stimulations; Computer simulation; Hydrates; Lithology; Methane; Specific heat; Thermal conductivity; Thermal effects; Gas fuel manufacture; gas hydrate; gas production; numerical model; permafrost; Canada; Mackenzie Delta; North America; Northwest Territories

Non-double-couple microearthquakes at Long Valley caldera, California, provide evidence for hydraulic fracturing - Foulger, G.R.; Julian, B.R.; Hill, D.P.; Pitt, A.M.; Shalev, E. (Journal of Volcanology and Geothermal Research, 2004)
Most of 26 small (0.4≲ M ≲3.1) microearthquakes at Long Valley caldera in mid-1997, analyzed using data from a dense temporary network of 69 digital three-component seismometers, have significantly non-double-couple focal mechanisms, inconsistent with simple shear faulting. We determined their mechanisms by inverting P - and S -wave polarities and amplitude ratios using linear-programming methods, and tracing rays through a three-dimensional Earth model derived using tomography. More than 80% of the mechanisms have positive (volume increase) isotropic components and most have compensated linear-vector dipole components with outward-directed major dipoles. The simplest interpretation of these mechanisms is combined shear and extensional faulting with a volume-compensating process, such as rapid flow of water, steam, or CO2 into opening tensile cracks. Source orientations of earthquakes in the south moat suggest extensional faulting on ESE-striking subvertical planes, an orientation consistent with planes defined by earthquake hypocenters. The focal mechanisms show that clearly defined hypocentral planes in different locations result from different source processes. One such plane in the eastern south moat is consistent with extensional faulting, while one near Casa Diablo Hot Springs reflects en echelon right-lateral shear faulting. Source orientations at Mammoth Mountain vary systematically with location, indicating that the volcano influences the local stress field. Events in a 'spasmodic burst' at Mammoth Mountain have practically identical mechanisms that indicate nearly pure compensated tensile failure and high fluid mobility. Five earthquakes had mechanisms involving small volume decreases, but these may not be significant. No mechanisms have volumetric moment fractions larger than that of a force dipole, but the reason for this fact is unknown. Published by Elsevier B.V.
cited By 120
Keywords: Crack initiation; Elastic moduli; Flow of water; Hydraulic fracturing; Stream flow; Stress analysis; Shear faulting; Tensile failure; Earthquakes; earthquake mechanism; faulting; focal mechanism; hydraulic fracture; microearthquake; moment tensor; seismicity; volcanic earthquake; California; Long Valley Caldera; North America; United States

Natural Gas Technology/Monetization: Overview of the Mallik Gas-Hydrate Production Research Well (Journal of Petroleum Technology, 2004)
cited By 5

More evidence that the Chicxulub impact predates the K/T mass extinction - Keller, G.; Adatte, T.; Stinnesbeck, W.; St{uben, D.; Berner, Z.; Kramar, U.; Harting, M. (Meteoritics and Planetary Science, 2004)
Yaxcopoil-1 (Yax-1), drilled within the Chicxulub crater, was expected to yield the final proof that this impact occurred precisely 65 Myr ago and caused the mass extinction at the Cretaceous-Tertiary (K/T) boundary. Instead, contrary evidence was discovered based on five independent proxies (sedimentologic, biostratigraphic, magnetostratigraphic, stable isotopic, and iridium) that revealed that the Chicxulub impact predates the K/T boundary by about 300,000 years and could not have caused the mass extinction. This is demonstrated by the presence of five bioturbated glauconite layers and planktic foraminiferal assemblages of the latest Maastrichtian zone CF1 and is corroborated by magnetostratigraphic chron 29r and characteristic late Maastrichtian stable isotope signals. These results were first presented in Keller et al. (2004). In this study, we present more detailed evidence of the presence of late Maastrichtian planktic foraminifera, sedimentologic, and mineralogic analyses that demonstrate that the Chicxulub impact breccia predates the K/T boundary and that the sediments between the breccia and the K/T boundary were deposited in a normal marine environment during the last 300,000 years of the Cretaceous. © Meteoritical Society, 2004. Printed in USA.
cited By 47

Major and trace element characteristics of impactites from the Yaxcopoil-1 borehole, Chicxulub structure, Mexico - Tuchscherer, M.G.; Reimold, M.U.; Koeberl, C.; Gibson, R.L. (Meteoritics and Planetary Science, 2004)
Approximately 100 m of impactites were retrieved from the ICDP borehole Yaxcopoil-1 (Yax-1), located ∼60 km south-southwest from the center of the Chicxulub impact crater on the Yucat'an Peninsula of Mexico. Here, we characterize and discuss this impact breccia interval according to its geochemical characteristics. Chemical analysis of samples from all five recognized breccia units reveals that the impactites are of heterogeneous composition with regard to both major and trace elements at the single sample (8-16 cm3) scale. This is primarily due to a strong mixing relationship between carbonate and silicate fractions. However, averaged compositions for suevitic units 1 to 3 are similar, and the silicate fraction (after removal of the carbonate component) indicates thorough mixing and homogenization. Analysis of the green melt breccia horizon, unit 4, indicates that it contains a distinct mafic component. Large brown melt particles (in units 2, 3, and 4) represent a mixture of feldspathic and mafic components, with high CaO abundances. Unit 5 shows the greatest compositional diversity, with highly variable abundances of SiO2, CaO, and MgO. Inter-sample heterogeneity is the result of small sample size combined with inherent heterogeneous lithological compositions, highly variable particle size of melt and lithic components, and post-depositional alteration. In contrast to samples from the Y6 borehole from closer to the center of the structure, Yax-1 impactites have a strong carbonate component. Elevated loss on ignition, Rb, and Cs contents in the upper two impactite units indicate strong interaction with seawater. The contents of the siderophile elements, including Ni, Co, Ir, and Cr, do not indicate the presence of a significant extraterrestrial component in the Yax-1 impactites. © Meteoritical Society, 2004.
cited By 20

Magnetostratigraphy of the impact breccias and post-impact carbonates from borehole Yaxcopoil-1, Chicxulub impact crater, Yucat'an, Mexico - Rebolledo-Vieyra, M.; Urrutia-Fucugauchi, J. (Meteoritics and Planetary Science, 2004)
We report the magnetostratigraphy of the sedimentary sequence between the impact breccias and the post-impact carbonate sequence conducted on samples recovered by Yaxcopoil-1 (Yax-1). Samples of impact breccias show reverse polarities that span up to ∼56 cm into the post-impact carbonate lithologies. We correlate these breccias to those of PEMEX boreholes Yucat'an-6 and Chicxulub-1, from which we tied our magnetostratigraphy to the radiometric age from a melt sample from the Yucat'an-6 borehole. Thin section analyses of the carbonate samples showed a significant amount of dark minerals and glass shards that we identified as the magnetic carriers; therefore, we propose that the mechanism of magnetic acquisition within the carbonate rocks for the interval studied is detrital remanent magnetism (DRM). With these samples, we constructed the scale of geomagnetic polarities where we find two polarities within the sequence, a reverse polarity event within the impact breccias and the base of the post-impact carbonate sequence (up to 794.07 m), and a normal polarity event in the last ∼20 cm of the interval studied. The polarities recorded in the sequence analyzed are interpreted to span from chron 29r to 29n, and we propose that the reverse polarity event lies within the 29r chron. The magnetostratigraphy of the sequence studied shows that the horizon at 794.11 m deep, interpreted as the K/T boundary, lies within the geomagnetic chron 29r, which contains the K/T boundary. © Meteoritical Society, 2004.
cited By 30

Real-time mud gas logging during drilling of the SAFOD Pilot Hole in Parkfield, CA - Erzinger, J.; Wiersberg, T.; Dahms, E. (Geophysical Research Letters, 2004)
The real-time mud gas monitoring during drilling of the 2.2 km deep SAFOD Pilot Hole has proved successful. A nearly complete depth profile was obtained for methane, radon, helium, and, with limitations, for carbon dioxide. Within the sedimentary section (0-768 m) mud gas yielded the highest CH4, CO2, and 222Rn concentrations, whereas He was comparably low. Four major gas-rich zones were identified. Methane is a mixture of microbial and thermogenic origin. Mud gas concentrations of CH4 and Rn are much lower in the underlying granitic rocks and the average background level of He is higher. In addition to several gas-rich zones, two shear zones inferred from geophysical measurements are also indicated by increased mud gas concentrations. Gas results permit speculation that one major shear zone is hydraulically connected to the sedimentary section above. Up to 9% of the helium is of mantle origin, which is probably a lower limit. Copyright 2004 by the American Geophysical Union.
cited By 25
Keywords: Carbon dioxide; Drilling; Geophysical prospecting; Granite; Helium; Methane; Mud logging; Radon; Sedimentology; Geophysical measurements; Mantle; Mud gas concentrations; Pilot holes; Boreholes; borehole geophysics; borehole logging; gas; San Andreas Fault; source rock; California; North America; Parkfield; United States; Western Hemisphere; World

Magnetic mineralogy of the Yaxcopoil-1 core, Chicxulub - Pilkington, M.; Ames, D.E.; Hildebrand, A.R. (Meteoritics and Planetary Science, 2004)
Core from the Yaxcopoil-1 (Yax-1) hole, drilled as a result of the Chicxulub Scientific Drilling Project (CSDP), has been analyzed to investigate the relationship between opaque mineralogy and rock magnetic properties. Twenty one samples of suevite recovered from the depth range 818-894 m are generally paramagnetic, with an average susceptibility of 2000 × 10-6 SI and have weak remanent magnetization intensities (average 0.1 A/m). The predominant magnetic phase is secondary magnetite formed as a result of low temperature ( <15 °C alteration. It occurs in a variety of forms, including vesicle infillings associated with quartz and clay minerals and fine aggregates between plagioclase/diopside laths in the melt. Exceptional magnetic properties are found in a basement clast (metamorphosed quartz gabbro), which has a susceptibility of >45000 × 10-6 SI and a remanent magnetization of 77.5 A/m. Magnetic mafic basement clasts are a common component in the Yax-1 impactite sequence. The high susceptibility and remanence in the mafic basement clasts are caused by the replacement of amphiboles and pyroxenes by an assemblage with fine <1 μm magnetite, ilmenite, K-feldspar, and stilpnomelane. Replacement of the mafic minerals by the magnetic alteration assemblage occurred before impact. Similar alteration mechanisms, if operative within the melt sheet, could explain the presence of the high amplitude magnetic anomalies observed at Chicxulub. © Meteoritical Society, 2004.
cited By 14

Re-evaluation of heat flow data near Parkfield, CA: Evidence for a weak San Andreas Fault - Fulton, P.M.; Saffer, D.M.; Harris, R.N.; Bekins, B.A. (Geophysical Research Letters, 2004)
Improved interpretations of the strength of the San Andreas Fault near Parkfield, CA based on thermal data require quantification of processes causing significant scatter and uncertainty in existing heat flow data. These effects include topographic refraction, heat advection by topographically-driven groundwater flow, and uncertainty in thermal conductivity. Here, we re-evaluate the heat flow data in this area by correcting for full 3-D terrain effects. We then investigate the potential role of groundwater flow in redistributing fault-generated heat, using numerical models of coupled heat and fluid flow for a wide range of hydrologic scenarios. We find that a large degree of the scatter in the data can be accounted for by 3-D terrain effects, and that for plausible groundwater flow scenarios frictional heat generated along a strong fault is unlikely to be redistributed by topographically-driven groundwater flow in a manner consistent with the 3-D corrected data. Copyright 2004 by the American Geophysical Union.
cited By 31
Keywords: Chemical analysis; Data acquisition; Groundwater; Hydrology; Refraction; Topology; Parkfield; Thermal data; Heat transfer; borehole geophysics; heat flow; San Andreas Fault; strength; weak rock; California; North America; Parkfield; United States; Western Hemisphere; World

The AIG10 drilling project (Aigion, Greece): Interpretation of the litho-log in the context of regional geology and tectonics; [Corr'elation des analyses lithologiques et structurales du forage AIG10 avec l̀environnement r'egional tectono-g'eologique] - Rettenmaier, Detlev; Giurgea, Vlad; H{otzl, Heinz; F{orster, Andrea (Comptes Rendus - Geoscience, 2004)
In the frame of the EU Project Corinth Rift Laboratory (CRL), the AIG10 borehole was successfully drilled from July until September 2002 through the Aigion normal fault in the harbour of Aigion, northern Peloponnesus, Greece. The scientific objective focuses on the investigation of fault mechanics and the relationship with fluid flow and geochemistry, fluid pressure, stress- and strain fields and earthquakes. Recognition of stratification encountered in the AIG10 borehole is based on an online analysis of well cuttings (0-708.8 m and 787.4-1001 m), core descriptions (708.8-787.4 m), monitoring of drilling parameters, as well as a preliminary geophysical well-log interpretation (0-1001 m). Geologically, the area is part of the Olonos-Pindos tectonic nappe, which is overthrusted on the Tripolitza unit during the Alpine orogeny. The litho-log of the AIG10 borehole comprises at first syn-rift deposits (graben fill). At 496 m, the Olonos-Pindos tectonic unit was encountered, however, not as expected in the platy limestones, but in the Olonos-Pindos radiolarite. The borehole has crossed at least one thrust-fault zone and a major normal fault zone at 760 m. This normal fault zone separates well-fractured platy, micritic limestone in the hangingwall from highly fractured radiolarite in the footwall, both of the Olonos-Pindos tectonic unit. The observed succession of multiple imbrication is an indicator of Alpine tectonic activity, whereas normal faulting is of the Miocene-Quaternary extension of the Gulf of Corinth, confirming our expectations gained from geologic-tectonic fieldwork. © 2004 Acad'emie des sciences. Published by Elsevier SAS. All rights reserved.
Keywords: Gulf of Corinth; Ionian Sea; Mediterranean Sea; borehole geophysics; drilling; normal fault; rift zone; seismotectonics; stratigraphic correlation

Regional tectonic stress near the San Andreas fault in central and southern California - Zoback, M.D. (Geophysical Research Letters, 2004)
Throughout central and southern California, a uniform NNE-SSW direction of maximum horizontal compressive stress is observed that is remarkably consistent with the superposition of stresses arising from lateral variations in lithospheric buoyancy in the western United States, and farfield Pacific-North America plate interaction. In central California, the axis of maximum horizontal compressive stress lies at a high angle to the San Andreas fault (SAF). Despite relatively few observations near (±10 km) the fault, observations in the greater San Francisco Bay area indicate an angle of as much as 85°, implying extremely low fault strength. In southern California, observations of stress orientations near the SAF are rotated slightly counter-clockwise with respect to the regional field. Nevertheless, we observe an approximately constant angle between the SAF and the maximum horizontal stress direction of 68 ± 7° along ∼400 km of the fault, indicating that the SAF has moderately low frictional strength in southern California. Copyright 2004 by the American Geophysical Union.
cited By 211
Keywords: Buoyancy; Compressive stress; Geophysics; Lithography; Stress analysis; Lithospheric buoyancy; Stress orientations; Tectonic stress; Tectonics; in situ stress; San Andreas Fault; stress field; tectonic setting

The effect of hydrate content on seismic attenuation: A case study for Mallik 2L-38 well data, Mackenzie delta, Canada - Chand, S.; Minshull, T.A. (Geophysical Research Letters, 2004)
Observations of velocities in sediments containing gas hydrates show that the strength of sediments increases with hydrate saturation. Hence it is expected that the attenuation of these sediments will decrease with increasing hydrate saturation. However, sonic log measurements in the Mallik 2L-38 well and cross hole tomography measurements in the Mallik field have shown that attenuation increases with hydrate saturation. We studied a range of mechanisms by which increasing hydrate saturation could cause increased attenuation. We found that a difference in permeability between the host sediment and the newly formed hydrate can produce the observed effect. We modelled attenuation in terms of Biot and squirt flow mechanisms in composite media. We have used our model to predict observed attenuations in the Mallik 2L-38 well, Mackenzie Delta, Canada. Copyright 2004 by the American Geophysical Union.
cited By 51
Keywords: Attenuation; Gas hydrates; Mechanical permeability; Sediments; Seismic attenuation; Squirt flow mechanism; Seismic waves; borehole geophysics; gas hydrate; permeability; seismic attenuation; seismic wave; Canada; Mackenzie Delta; North America; Northwest Territories

Looking into a volcano: Drilling unzen - Nakada, Setsuya; Eichelberger, John (Geotimes, 2004)

Yaxcopoil-1 and the Chicxulub impact - Stinnesbeck, W.; Keller, G.; Adatte, T.; Harting, M.; St{uben, D.; Istrate, G.; Kramar, U. (International Journal of Earth Sciences, 2004)
CSDP core Yaxcopoil-1 was drilled to a depth of 1,511 m within the Chicxulub crater. An organic-rich marly limestone near the base of the hole (1,495 to 1,452 m) was deposited in an open marine shelf environment during the latest Cenomanian (uppermost Rotalipora cushmani zone). The overlying sequence of limestones, dolomites and anhydrites (1,495 to 894 m) indicates deposition in various carbonate platform environments (e.g., sabkhas, lagoons). A 100-m-thick suevite breccia (894-794 m) identifies the Chicxulub impact event. Above the suevite breccia is a dolomitic limestone with planktic foraminiferal assemblages indicative of Plummerita hantkeninoides zone CF1, which spans the last 300 ky of the Maastrichtian. An erosional surface 50 cm above the breccia/dolomite contact marks the K/T boundary and a hiatus. Limestones above this contact contain the first Tertiary planktic foraminifera indicative of an upper P. eugubina zone P1a(2) age. Another hiatus 7 cm upsection separates zone P1a(2) and hemipelagic limestones of planktic foraminiferal Zone P1c. Planktic foraminiferal assemblages of Zone Plc to P3b age are present from a depth of 794.04 up to 775 m. The Cretaceous carbonate sequence appears to be autochthonous, with a stratigraphic sequence comparable to late Cretaceous sediments known from outside the Chicxulub crater in northern and southern Yucatan, including the late Cenomanian organic-rich marly limestone. There is no evidence that these sediments represent crater infill due to megablocks sliding into the crater, such as major disruption of sediments, chaotic changes in lithology, overturned or deep dipping megablocks, major mechanical fragmentation, shock or thermal alteration, or ductile deformation. Breccia units that are intercalated in the carbonate platform sequence are intraformational in origin (e.g., dissolution of evaporites) and dykes are rare. Major disturbances of strata by the impact therefore appear to have been confined to within less than 60 km from the proposed impact center. Yaxcopoil-1 may be located outside the collapsed transient crater cavity, either on the upper end of an elevated and tilted horst of the terrace zone, or even outside the annular crater cavity. The Chicxulub site thus records a large impact that predates the K/T boundary impact and mass extinction. © Springer-Verlag 2004.
cited By 30

Volatiles in glasses from the HSDP2 drill core - Seaman, Caroline; Sherman, Sarah Bean; Garcia, Michael O.; Baker, Michael B.; Balta, Brian; Stolper, Edward (Geochemistry, Geophysics, Geosystems, 2004)
H2O, CO2, S, Cl, and F concentrations are reported for 556 glasses from the submarine section of the 1999 phase of HSDP drilling in Hilo, Hawaii, providing a high-resolution record of magmatic volatiles over ∼200 kyr of a Hawaiian volcano's lifetime. Glasses range from undegassed to having lost significant volatiles at near-atmospheric pressure. Nearly all hyaloclastite glasses are degassed, compatible with formation from subaerial lavas that fragmented on entering the ocean and were transported by gravity flows down the volcano flank. Most pillows are undegassed, indicating submarine eruption. The shallowest pillows and most massive lavas are degassed, suggesting formation by subaerial flows that penetrated the shoreline and flowed some distance under water. Some pillow rim glasses have H2O and S contents indicating degassing but elevated CO2 contents that correlate with depth in the core; these tend to be more fractionated and could have formed by mixing of degassed, fractionated magmas with undegassed magmas during magma chamber overturn or by resorption of rising CO2-rich bubbles by degassed magmas. Intrusive glasses are undegassed and have CO2 contents similar to adjacent pillows, indicating intrusion shallow in the volcanic edifice. Cl correlates weakly with H2O and S, suggesting loss during low-pressure degassing, although most samples appear contaminated by seawater-derived components. F behaves as an involatile incompatible element. Fractionation trends were modeled using MELTS. Degassed glasses require fractionation at PH2o ≈ 5-10 bars. Undegassed low-SiO2 glasses require fractionation at PH2O ≈ 50 bars. Undegassed and partially degassed high-SiO2 glasses can be modeled by coupled crystallization and degassing. Eruption depths of undegassed pillows can be calculated from their volatile contents assuming vapor saturation. The amount of subsidence can be determined from the difference between this depth and the sample's depth in the core. Assuming subsidence at 2.5 mm/y, the amount of subsidence suggests ages of ∼500 ka for samples from the lower 750 m of the core, consistent with radiometric ages. H2O contents of undegassed low-SiO2 HSDP2 glasses are systematically higher than those of high-SiO2 glasses, and their H2O/K2O and H 2O/Ce ratios are higher than typical tholeiitic pillow rim glasses from Hawaiian volcanoes. Copyright 2004 by the American Geophysical Union.

Verification of magnetostratigraphic scales of miocene core section from lake Baikal - Horiuchi, K.; Gol'dberg, E.L.; Matsuzaki, H.; Kobayashi, K.; Shibata, Y. (Geologiya i Geofizika, 2004)
The dynamics of cosmogenic 10Be contents in the BDP-96-1 (100 to 200 m) and BDP-98-2 (200 to 600 m) cores was used to test published magnetostratigraphic scales for the Miocene section and to time the core base. Only one of three alternative scales for the BDP-98 core tested against the 10Be decay of T1/2 = 1.5 myr showed a perfect consistency with the decay law. The basal age of the 600 m BDP-98 section was estimted at 8.4 myr.
Keywords: Eurasia; Lake Baikal; Russian Federation; cosmogenic radionuclide; magnetostratigraphy; Miocene

Tracing the protolith, UHP metamorphism, and exhumation ages of orthogneiss from the SW Sulu terrane (eastern China): SHRIMP U –Pb dating of mineral inclusion-bearing zircons - Liu, Fulai; Xu, Zhiqin; Xue, Huaimin (Lithos, 2004)
Orthogneisses are the major country rocks hosting eclogites in the Sulu UHP terrane, eastern China. All of the analyzed orthogneiss cores from the main drilling hole of the Chinese Continental Scientific Drilling Project (CCSD-MH) have similar major and trace element compositions and a granite protolith. These rocks have relatively high LREE/HREE ratios, strong negative Eu anomalies (Eu/Eu*=0.20 –0.39), and negative Ba anomalies (Ba/Ba*=0.25 –0.64). Coesite and coesite-bearing UHP mineral assemblages are common inclusions in zircons separated from orthogneiss, paragneiss, amphibolite, and (retrograded) eclogite of the CCSD-MH. This suggests that the eclogite, together with its country rocks, experienced in situ ultrahigh-pressure (UHP) metamorphism. Laser Raman spectroscopy and cathodoluminescence (CL) images show that zircons from the orthogneisses are zoned and that they have distinct mineral inclusions in the different zones. Most zircons retain early magmatic cores with abundant low-pressure mineral inclusions, which are mantled with metamorphic zircon-containing inclusions of coesite and other UHP minerals. The outermost rims on these grains contain low-pressure mineral inclusions, such as quartz and albite. SHRIMP U –Pb dating of the zoned zircons gives three discrete and meaningful groups of ages: Proterozoic ages for the protolith, 227±2 Ma for the coesite-bearing mantles, and 209±3 Ma for the amphibolite facies retrograde rims. The widespread occurrence of UHP mineral inclusions in zircons from the Sulu metamorphic belt dated at about 227 Ma suggests that voluminous continental crust experienced late Triassic subduction to depths of at least 120 km and perhaps more than 200 km. Eighteen million years later, the terrane was rapidly exhumed to midcrustal levels, and the UHP rocks were overprinted by amphibolite facies metamorphism. The exhumation rate deduced from the zircon age data and previously obtained metamorphic P –T data is estimated to be 5.6 –11.0 km/Ma. Such rapid exhumation of the Sulu UHP terrane may be due to the buoyancy forces produced by subduction of low-density continental material into the deep mantle.
Keywords: Orthogneiss; Mineral inclusion in zircon; UHP metamorphism; SHRIMP U & ndash; Pb dating; Chinese Continental Scientific Drilling Project

Through thick and thin - Ribe, N.M. (Nature, 2004)
The sea floor around the Hawaiian island chain is unusually shallow. New seismic evidence suggests that this up-raised 'swell' is partly due to heating and thinning of the lithosphere beneath.
cited By 11
Keywords: Earth atmosphere; Sea level; Seismology; Lithosphere; Oceanography; crustal thickness; seafloor; asthenospheric upwelling; earthquake; geology; gravity; heat transfer; priority journal; short survey; United States; Hawaiian Islands; Pacific islands; Pacific Ocean; Hawaiia

The SAFOD Pilot Hole seismic array: Wave propagation effects as a function of sensor depth and source location - Chavarria, J.A.; Shalev, E. (Geophysical Research Letters, 2004)
In July 2002 we installed a vertical array of seismometers in the San Andreas Fault Observatory at Depth (SAFOD) Pilot Hole (PH). The bottom of this 32 level, 1240 m long array of 3- components is located at a depth of ∼2100 m below ground. Surface-explosion and microearthquake seismograms recorded by the array give valuable insights into the structure of the SAFOD site. The ratios of P- and S-wave velocities.(Vp/Vs) along the array suggest the presence of two faults intersecting the PH. The Vp/Vs ratios also depend on source-location, with high values to the NW, and lower ones to the SE, correlating with high and low creep rates along the SAF, respectively. Since higher ratios can be produced by increasing fluid saturation, we suggest that this effect might account for both our observations and their correlation with the creep distribution. Copyright 2004 by the American Geophysical Union.
cited By 11
Keywords: Correlation methods; Creep; Earthquakes; Geophysics; Observatories; Site selection; Wave propagation; Creep; Shear waves; Strike-slip faults; Wave propagation; Fluid saturation; Pilot hole (PH); Seismic arrays; Seismograms; Seismology; Seismology; earthquake; San Andreas Fault; seismic wave; seismometry; wave propagation; Creep distribution; Fluid saturations; P- and S-wave velocities; Propagation effect; San Andreas fault; Source location; Surface explosions; Vertical arrays

The hazard of near-Earth asteroid impacts on earth - Chapman, C.R. (Earth and Planetary Science Letters, 2004)
Near-Earth asteroids (NEAs) have struck the Earth throughout its existence. During epochs when life was gaining a foothold ∼ 4 Ga, the impact rate was thousands of times what it is today. Even during the Phanerozoic, the numbers of NEAs guarantee that there were other impacts, possibly larger than the Chicxulub event, which was responsible for the Cretaceous-Tertiary extinctions. Astronomers have found over 2500 NEAs of all sizes, including well over half of the estimated 1100 NEAs >1 km diameter. NEAs are mostly collisional fragments from the inner half of the asteroid belt and range in composition from porous, carbonaceous- chondrite-like to metallic. Nearly one-fifth of them have satellites or are double bodies. When the international telescopic Spaceguard Survey, which has a goal of discovering 90% of NEAs >1 km diameter, is completed, perhaps as early as 2008, nearly half of the remaining impact hazard will be from land or ocean impacts by bodies 70-600 m diameter. (Comets are expected to contribute only about 1% of the total risk.) The consequences of impacts for civilization are potentially enormous, but impacts are so rare that worldwide mortality from impacts will have dropped to only about 150 per year (averaged over very long durations) after the Spaceguard goal has, presumably, ruled out near- term impacts by 90% of the most dangerous ones; that is, in the mid- range between very serious causes of death (disease, auto accidents) and minor but frightening ones (like shark attacks). Differences in perception concerning this rather newly recognized hazard dominate evaluation of its significance. The most likely type of impact events we face are hyped or misinterpreted predicted impacts or near-misses involving small NEAs. © 2004 Elsevier B.V. All rights reserved.
cited By 105

The Corinth Rift Laboratory - Cornet, Francois H.; Bernard, Pascal; Moretti, Isabelle (Comptes Rendus - Geoscience, 2004)
Keywords: Gulf of Corinth; Ionian Sea; Mediterranean Sea; rift zone; seismotectonics

Rejuvenation of the lithosphere by the Hawaiian plume - Li, X.; Kind, R.; Yuan, X.; W{olbern, I.; Hanka, W. (Nature, 2004)
The volcanism responsible for creating the chain of the Hawaiian islands and seamounts is believed to mark the passage of the oceanic lithosphere over a mantle plume. In this picture hot material rises from great depth within a fixed narrow conduit to the surface, penetrating the moving lithosphere. Although a number of models describe possible plume-lithosphere interactions, seismic imaging techniques have not had sufficient resolution to distinguish between them. Here we apply the S-wave 'receiver function' technique to data of three permanent seismic broadband stations on the Hawaiian islands, to map the thickness of the underlying lithosphere. We find that under Big Island the lithosphere is 100-110 km thick, as expected for an oceanic plate 90-100 million years old that is not modified by a plume. But the lithosphere thins gradually along the island chain to about 50-60 km below Kauai. The width of the thinning is about 300 km. In this zone, well within the larger-scale topographic swell, we infer that the rejuvenation model (where the plume thins the lithosphere) is operative; however, the larger-scale topographic swell is probably supported dynamically.
cited By 219
Keywords: Earth atmosphere; Imaging systems; Seismology; Surface topography; Lithosphere; Oceanography; crustal thickness; lithosphere; mantle plume; seismic tomography; volcanism; article; atmosphere; heating; imaging system; island (geological); lithosphere; mechanics; plume; priority journal; sea; signal detection; temperature measurement; thickness; topography; United States; volcano; Hawaiian Islands; Pacific islands; Pacific Ocean; Hawaiia

The Chicxulub Scientific Drilling Project (CSDP) - Urrutia-Fucugauchi, J.; Morgan, J.; St{offler, D.; Claeys, P. (Meteoritics and Planetary Science, 2004)
cited By 72

TEM study of meteorite impact glass at New Zealand Cretaceous-Tertiary sites: Evidence for multiple impacts or differentiation during global circulation? - Bauluz, B.; Peacor, D.R. (Earth and Planetary Science Letters, 2004)
Study by transmission electron microscopy of samples from the Cretaceous-Tertiary (K-T) boundary clay at Flaxbourne River and Woodside Creek, New Zealand, has revealed the occurrence of nanometer-sized meteorite impact-derived glass. The average glass composition is exceptionally Ca-rich and is distinct from other glass found on Earth, apart from glass inferred to be of impact origin at Mexican and Haitian K-T sites. The glass shards are partially altered to montmorillonite-like smectite, with the dominant interlayer cation, Ca, reflecting the composition of the parent glass. The data imply a heterogeneous global distribution in composition of K-T boundary impact glass: Si-rich and Ca-rich in Mexico and Haiti, Si-rich in Denmark, and Ca-rich in New Zealand. This heterogeneous distribution may relate to dispersal processes similar to those used to account for the asymmetric distribution of clastic debris from the Chicxulub impact site. However, recent discovery of an impact crater of K-T boundary age in Ukraine raises the possibility of impact clusters which produce material of heterogeneous composition. © 2004 Elsevier B.V. All rights reserved.
cited By 17

Structure and impact indicators of the Cretaceous sequence of the ICDP drill core Yaxcopoil-1, Chicxulub impact crater, Mexico - Kenkmann, T.; Wittmann, A.; Scherler, D. (Meteoritics and Planetary Science, 2004)
As part of the ICDP Chicxulub Scientific Drilling Project, the Yaxcopoil-1 (Yax-1) bore hole was drilled 60 km south-southwest of the center of the 180 km-diameter Chicxulub impact structure down to depth of 1511 m. A sequence of 615 m of deformed Cretaceous carbonates and sulfates was recovered below a 100 m-thick unit of suevitic breccias and 795 m of post-impact Tertiary rocks. The Cretaceous rocks are investigated with respect to deformation features and shock metamorphism to better constrain the deformational overprint and the kinematics of the cratering process. The sequence displays variable degrees of impact-induced brittle damage and post-impact brittle deformation. The degree of tilting and faulting of the Cretaceous sequence was analyzed using 360°-core scans and dip-meter log data. In accordance with lithological information, these data suggest that the sedimentary sequence represents a number of structural units that are tilted and moved with respect to each other. Three main units and nine sub-units were discriminated. Brittle deformation is most intense at the top of the sequence and at 1300-1400 m. Within these zones, suevitic dikes, polymict clastic dikes, and impact melt rock dikes occur and may locally act as decoupling horizons. The degree of brittle deformation depends on lithology; massive dolomites are affected by penetrative faulting, while stratified calcarenites and bituminous limestones display localized faulting. The deformation pattern is consistent with a collapse scenario of the Chicxulub transient crater cavity. It is believed that the Cretaceous sequence was originally located outside the transient crater cavity and eventually moved downward and toward the center to its present position between the peak ring and the crater rim, thereby separating into blocks. Whether or not the stack of deformed Cretaceous blocks was already displaced during the excavation process remains an open question. The analysis of the deformation microstructure indicates that a shock metamorphic overprint is restricted to dike injections with an exception of the so called paraconglomerate. Abundant organic matter in the Yax-1 core was present before the impact and was mobilized by impact-induced heating and suggests that >12 km3 of organic material was excavated during the cratering process. Meteoritical Society, 2004.
cited By 30

Stress-induced seismic velocity anisotropy and physical properties in the SAFOD pilot hole in Parkfield, CA. - Boness, N.L.; Zoback, M.D. (Gulf Rocks 2004 - 6th North America Rock Mechanics Symposium, NARMS 2004, 2004)
A comprehensive suite of geophysical logs was collected in the SAFOD Pilot Hole in Parkfield, CA. from a depth of 775 m to 2150 m in highly fractured Salinian granite. The Pilot Hole intersected numerous macroscopic fractures and faults with extremely varied orientations. Despite the highly variable orientation of the fractures and faults, the fast polarization direction of the shear waves is very consistent with the direction of maximum horizontal compression determined from wellbore breakouts and drilling induced tensile fractures. At least three major shear zones were intersected by the borehole that are characterized by anomalously low velocity and resistivity, anomalously high shear velocity anisotropy and an absence of stress-induced wellbore breakouts (which suggests anomalously low differential stress). We argue that the physical mechanism responsible for the seismic velocity anisotropy observed in the Pilot Hole is the preferential closure of fractures in response to an anisotropic stress state. Copyright 2004, ARMA, American Rock Mechanics Association.
cited By 9
Keywords: Anisotropy; Boreholes; Fracture; Oil field equipment; Rock mechanics; Rocks; Seismic waves; Seismology; Shear waves; Velocity; Anisotropic stress; Differential stress; Drilling-induced tensile fractures; Fractures and faults; Macroscopic fractures; Physical mechanism; Polarization direction; Variable orientation; Shear flow

Stress-induced seismic velocity anisotropy and physical properties in the SAFOD Pilot Hole in Parkfield, CA - Boness, N.L.; Zoback, M.D. (Geophysical Research Letters, 2004)
A comprehensive suite of geophysical logs was collected in the SAFOD Pilot Hole from a depth of 775 m to 2150 m in highly fractured Salinian granite. The Pilot Hole intersected numerous macroscopic fractures and faults with extremely varied orientations. Despite the highly variable orientation of the fractures and faults, the fast polarization direction of the shear waves is very consistent with the direction of maximum horizontal compression determined from wellbore breakouts and drilling induced tensile fractures. At least three major shear zones were intersected by the borehole that are characterized by anomalously low velocity and resistivity, anomalously high shear velocity anisotropy and an absence of stress-induced wellbore breakouts (which suggests anomalously low differential stress). We argue that the physical mechanism responsible for the seismic velocity anisotropy observed in the Pilot Hole is the preferential closure of fractures in response to an anisotropic stress state. Copyright 2004 by the American Geophysical Union.
cited By 125
Keywords: Drilling; Electromagnetic wave polarization; Fracture; Granite; Magnetic anisotropy; Seismic prospecting; Pilot holes; Shear velocity; Wellbores; Boreholes; borehole geophysics; in situ stress; physical property; San Andreas Fault; seismic anisotropy; seismic velocity; California; North America; Parkfield; United States; Western Hemisphere; World

Stress orientations and magnitudes in the SAFOD pilot hole - Hickman, S.; Zoback, M. (Geophysical Research Letters, 2004)
Borehole breakouts and drilling-induced tensile fractures in the 2.2-km-deep SAFOD pilot hole at Parkfield, CA, indicate significant local variations in the direction of the maximum horizontal compressive stress, SHmax, but show a generalized increase in the angle between SHmax and the San Andreas Fault with depth. This angle ranges from a minimum of 25 ± 10° at 1000-1150 m to a maximum of 69 ± 14° at 2050-2200 m. The simultaneous occurrence of tensile fractures and borehole breakouts indicates a transitional strike-slip to reverse faulting stress regime with high horizontal differential stress, although there is considerable uncertainty in our estimates of horizontal stress magnitudes. If stress observations near the bottom of the pilot hole are representative of stresses acting at greater depth, then they are consistent with regional stress field indicators and an anomalously weak San Andreas Fault in an otherwise strong crust. Copyright 2004 by the American Geophysical Union.
cited By 204
Keywords: Compressive stress; Drilling; Fracture; Geophysical prospecting; Stress analysis; Borehole breakouts; Pilot holes; Stress fields; Stress orientations; Boreholes; borehole geophysics; in situ stress; orientation; San Andreas Fault; stress measurement; California; North America; Parkfield; United States; Western Hemisphere; World

Stratigraphic and sedimentological observations from seismic data across the Chicxulub impact basin - Bell, C.; Morgan, J.V.; Hampson, G.J.; Trudgill, B. (Meteoritics and Planetary Science, 2004)
Seismic data across the offshore half of the Chicxulub impact crater reveal a 145 km-diameter post-impact basin to be a thickening of Tertiary sediment, which thickens by ∼0.7 sec from the basin margin to the basin center. The basin existed long after the impact and was gradually infilled to its current flat surface. A suite of seismic horizons within the impact basin have been picked on four reflection lines across the crater. They reveal that the western and northwestern parts of the impact basin were filled first. Subsequently, there was a dramatic change in the depositional environment, indicated by an unconformable surface that can be mapped across the entire basin..A prograding shelf sequence downlaps onto this unconformity in the eastern basin. The seismic stratigraphic relationships suggest a marine regression, with sedimentation becoming gradually more passive as sediments fill the eastern part of the impact basin. The central and northeastern parts of the basin are filled last. The onshore hole Yaxcopoil-1 (Yax-1), which was drilled on the flanks of the southern basin, has been projected onto the offshore seismic data to the west of the crater center. Using dates obtained from this onshore well and regional data, approximate ages have been placed on the most significant horizons in the offshore seismic data. Our preliminary interpretation is that the western and northwestern basins were almost entirely filled by 40 Ma and that the marine regression observed in the eastern basin is early Miocene in age. Offshore seismic stratigraphic analyses and onshore data within Yax-1 suggest that the early Paleocene is highly attenuated across the impact basin. The Mesozoic section appears to be €1 km thicker offshore than onshore. We calculate that, given this offshore thickening, the volume of Mesozoic rocks that have been excavated, melted, or vaporized during impact is around 15% larger than expected from calculations that assume the offshore thickness is equal to that onshore. This has significant consequences for any environmental calculations. The current offset between the K-T boundary outside and inside the crater is ∼700 m. However, infilling of basins with sediments is usually accompanied by subsidence, and immediately following the impact, the difference would have been smaller. We calculate the original topographic offset on the K-T boundary to have been between 450 and 700 m, which is in agreement with depth-diameter scaling laws for a mixed target. © Meteoritical Society, 2004.
cited By 33

Secondary alteration of the impactite and mineralization in the basal Tertiary sequence, Yaxcopoil-1, Chicxulub impact crater, Mexico - Ames, D.E.; Kjarsgaard, I.M.; Pope, K.O.; Dressler, B.; Pilkington, M. (Meteoritics and Planetary Science, 2004)
The 65 Ma Chicxulub impact crater formed in the shallow coastal marine shelf of the Yucat'an Platform in Mexico. Impacts into water-rich environments provide heat and geological structures that generate and focus sub-seafloor convective hydrothermal systems. Core from the Yaxcopoil-1 (Yax-1) hole, drilled by the Chicxulub Scientific Drilling Project (CSDP), allowed testing for the presence of an impact-induced hydrothermal system by: a) characterizing the secondary alteration of the 100 m-thick impactite sequence; and b) testing for a chemical input into the lower Tertiary sediments that would reflect aquagene hydrothermal plume deposition. Interaction of the Yax-1 impactites with seawater is evident through redeposition of the suevites (unit 1), secondary alteration mineral assemblages, and the subaqueous depositional environment for the lower Tertiary carbonates immediately overlying the impactites. The least-altered silicate melt composition intersected in Yax-1 is that of a calc-alkaline basaltic andesite with 53.4-56 wt% SiO2 (volatile-free). The primary mineralogy consists of fine microlites of diopside, plagioclase (mainly Ab 47), ternary feldspar (Ab 37 to 77), and trace apatite, titanite, and zircon. The overprinting alteration mineral assemblage is characterized by Mg-saponite, K-montmorillonite, celadonite, K-feldspar, albite, Fe-oxides, and late Ca and Mg carbonates. Mg and K metasomatism resulted from seawater interaction with the suevitic rocks producing smectite-K-feldspar assemblages in the absence of any mixed layer clay minerals, illite, or chlorite. Rare pyrite, sphalerite, galena, and chalcopyrite occur near t he base of the impactites. These secondary alteration minerals formed by low temperature (0-150 °C) oxidation and fixation of alkalis due to the interaction of glass-rich suevite with down-welling seawater in the outer annular trough intersected at Yax-1. The alteration represents a cold, Mg-K-rich seawater recharge zone, possibly recharging higher temperature hydrothermal activity proposed in the central impact basin. Hydrothermal metal input into the Tertiary ocean is shown by elevated Ni, Ag, Au, Bi, and Te concentrations in marcasite and Cd and Ga in sphalerite in the basal 25 m of the Tertiary carbonates in Yax-1. The lower Tertiary trace element signature reflects hydrothermal metal remobilization from a mafic source rock and is indicative of hydrothermal venting of evolved seawater into the Tertiary ocean from an impact-generated hydrothermal convective system. © Meteoritical Society, 2004. Printed in USA.
cited By 37

Scientific drilling reveals geochemical heterogeneity within the Ko'olau shield, Hawai'i - Haskins, E.H.; Garcia, M.O. (Contributions to Mineralogy and Petrology, 2004)
The Ko'olau Scientific Drilling Project (KSDP) was initiated to determine if the distinctive geochemistry of Ko'olau lavas is a near-surface feature. This project successfully deepened a recent, ∼351 m deep, tri-cone rotary-drilled water well by coring another ∼328 m. Three Ar-Ar plateau ages of 2.8 to 2.9 Ma from the drill core section of 103 flows confirm stratigraphic interpretations that core drilling recovered the deepest and oldest subaerially erupted lavas yet sampled from this volcano. The petrography and geochemistry of the core, and cuttings from this and another new Ko'olau water well (∼433 m deep) were determined. These analyses revealed that the geochemically distinct lavas of Ko'olau form a veneer only 175-250 in thick at the drill sites, covering flows with more typical Hawaiian tholeiite compositions. The compositional change occurred near the end of shield volcanism and is not abrupt. Thus, it is probably not related to a catastrophic event such as the collapse of the northeast flank of this volcano. The distinct geochemistry of surface Ko'olau lavas cannot be explained by melting pyroxenitic or combined pyroxenitic and peridotitic sources. Additional recycled oceanic crustal components, such as plagioclase-rich cumulates and sediment, were probably involved. As the Ko'olau volcano drifted off the Hawaiian hotspot and the overall degree of melting decreased, the proportion of melts from recycled oceanic crustal material increased relative to those from mantle peridotite. © Springer-Verlag 2004.
cited By 52
Keywords: geochemistry; lava; petrography; volcanic rock; Hawaii [United States]; North America; United States

Low-velocity damaged structure of the San Andreas Fault at Parkfield from fault zone trapped waves - Li, Y.-G.; Vidale, J.E.; Cochran, E.S. (Geophysical Research Letters, 2004)
We used dense linear seismic arrays across and along the San Andreas Fault (SAF) at Parkfield, California to record fault zone trapped waves generated by explosions and microearthquakes in 2002. Prominent trapped waves appeared at stations close to the SAF main fault trace while some energy was trapped in the north strand at the array site. Observations and 3-D finite-difference simulations of trapped waves at 2-5 Hz show evidence of a damaged core zone on the main SAF. The zone from the surface to seismogenic depths is marked by a low-velocity waveguide ∼150 m wide, in which Q is 10-50 and shear velocities are reduced by 30-40% from wall-rock velocities, with the greatest velocity reduction at shallow depth. We interpret that this distinct low-velocity zone on the main SAF is a remanent of damage due to past large earthquakes on the principal fault plane at Parkfield. A less-developed low-velocity zone may be evident on the north strand that experienced minor breaks in the 1966 M6 event. Copyright 2004 by the American Geophysical Union.
cited By 100
Keywords: Computer simulation; Earthquakes; Finite difference method; Structural analysis; Velocity; Waveguides; Strike-slip faults; Velocity; Water waves; Fault zones; Seismic arrays; Trapped waves; Velocity reduction; Seismology; Shear flow; low velocity zone; San Andreas Fault; seismic velocity; trapped wave; California; North America; Parkfield; United States; Damaged structures; Fault-zone trapped waves; Finite difference simulations; Large earthquakes; Linear seismic array; Low velocity zones; Micro-earthquakes; San Andreas fault

The nature of the alteration clays and element mobilization in Chicxulub Yaxcopoil-1 drill hole - Nelson, M.J.; Newsom, H.E.; Shearer, C.K.; Rietmeijer, F.J.M. (Meteoritics and Planetary Science, 2004)
cited By 1

Joint inversion of gravity and arrival time data from Parkfield: New constraints on structure and hypocenter locations near the SAFOD drill site - Roecker, S.; Thurber, C.; McPhee, D. (Geophysical Research Letters, 2004)
Taking advantage of large datasets of both gravity and elastic wave arrival time observations available for the Parkfield, California region, we generated an image consistent with both types of data. Among a variety of strategies, the best result was obtained from a simultaneous inversion with a stability requirement that encouraged the perturbed model to remain close to a starting model consisting of a best fit to the arrival time data. The preferred model looks essentially the same as the best-fit arrival time model in areas where ray coverage is dense, with differences being greatest at shallow depths and near the edges of the model where ray paths are few. Earthquake locations change by no more than about 100 m, the general effect being migration of the seismic zone to the northeast, closer to the surface trace of the San Andreas Fault. Copyright 2004 by the American Geophysical Union.
cited By 69
Keywords: Drilling; Earthquakes; Gravitational effects; Mathematical models; Perturbation techniques; Seismology; Site selection; Earthquakes; Strike-slip faults; Best-fit arrival time model; Datasets; Seismic zones; Elastic waves; Elastic waves; earthquake hypocenter; elastic wave; gravity survey; modeling; San Andreas Fault; seismic hazard; California; North America; Parkfield; United States; Arrival-time data; Earthquake location; Hypocenter location; Joint inversion; Large datasets; San Andreas fault; Shallow depths; Surface traces

Chicxulub: Testing for post-impact hydrothermal input into the Tertiary ocean - Rowe, A.J.; Wilkinson, J.J.; Coles, B.J.; Morgan, J.V. (Meteoritics and Planetary Science, 2004)
Studies of large terrestrial impact craters indicate that post-impact hydrothermal activity is a likely consequence of the crustal deformation and heating induced by such events. In the case of the Chicxulub basin, where marine conditions were re-established soon after the impact, significant fluxing of seawater-through the crust and hydrothermal venting into the water column might be anticipated. We have carried out geochemical analyses of Tertiary carbonate sediments within the Yaxcopoil-1 (Yax-1) drill hole to test for evidence of such post-impact hydrothermal circulation. Hydrothermal activity is most likely to be found close to thick layers of melt rock inside the collapsed transient cavity, and it is estimated that Yax-1 is located ∼20 km outside this cavity. Consequently, the most likely signature of hydrothermal venting into the water column would be geochemical anomalies attributable to fallout of suspended particulate matter from a submarine hydrothermal plume. Samples of Tertiary biomicrites from depths of 794.01 to 777.02 m have high concentrations of manganese, iron, phosphorous, titanium, and aluminium and low iron/manganese ratios relative to samples from higher in the stratigraphic succession. This geochemical anomaly decreases' fairly systematically between 793.13 m and 777.02 m, above which an abrupt change in geochemistry is observed. A mass balance calculation suggests that the anomaly is unlikely to be the result of a decreasing detrital input to the carbonate sediments and the nature of the element enrichments is consistent with expectations for fallout from a distal hydrothermal plume. We conclude that a post-impact hydrothermal system did develop at Chicxulub, which led to the expulsion of hydrothermal fluids into the Tertiary water column. Preliminary biostratigraphic and magnetostratigraphic dating on Yax-1 core suggest that this hydrothermal activity lasted for at least 300 ka. © Meteoritical Society, 2004. Printed in USA.
cited By 13

Drilling through the active Aigion Fault: The AIG10 well observatory; [Forage au travers de la faille active d'Aigion : L'observatoire constitu'e par le puits AIG10] - Cornet, François; Doan, Mai Linh; Moretti, Isabelle; Borm, G{unter (Comptes Rendus - Geoscience, 2004)
The 1000 m-deep AIG10 borehole intersects the Aigion Fault within the limestone of the Pindos nappe at 760 m. It has demonstrated that the fault is dipping 60° with respect to horizontal, an angle consistent with the strength characteristics of the fault material as determined in the laboratory. It does not seem to be listric, as suggested by the location of superficial microseismic events. The fault is about 7 m thick, with a 50 cm core of clay derived from smeared radiolarite formation. The fault offsets the basement rock by 150±20 m and constitutes a hydraulic barrier that sustains a 0.5 MPa differential pressure. Below the fault, a strongly karstified limestone has been encountered down to 1000 m. It is the site of a 0.9 MPa overpressure and exhibits no temperature gradient. Water geochemistry demonstrates the continental origin of this significant flow, which obliterates the regional heat flux. The present monitoring of downhole pressure yields data on tidal variations with a resolution of 1/500 as well as pressure variations induced by teleseisms. Given preliminary results from 14C dating, the age of the fault is about 50 kyr and the mean slip rate equal about 3.5 mm yr-1. © 2004 Acad'emie des sciences. Published by Elsevier SAS. All rights reserved.
Keywords: Gulf of Corinth; Ionian Sea; Mediterranean Sea; active fault; dip; fault plane; fluid flow; hydrogeochemistry; rift zone

Drilling the central crater of the chesapeake bay impact structure: A first look - Sanford, W.E.; Gohn, G.S.; Powars, D.S.; Horton Jr., J.W.; Edwards, L.E.; Jean, M.; Morin, R.H. (Eos, 2004)
cited By 23

Differential waveform tomography for time-lapse crosswell seismic data with application to gas hydrate production monitoring - Watanabe, T.; Shimizu, S.; Asakawa, E.; Matsuoka, T. (SEG Technical Program Expanded Abstracts, 2004)
To detect the change of physical properties in small areas, a series of high-resolution waveform inversions is applied to time-lapse seismic data. A procedure by use of differentiation between the time-lapse data and normalization using reference data is proposed in this study. The procedure is derived as a straight-forward extension of waveform inversion as the scatterer imaging. Through numerical tests, the proposed approach was found to be more accurate than the conventional approach in obtaining the velocity change in small areas. The method was applied to the time-lapse crosswell seismic data obtained during the Mallik 2002 gas production test. A small area showing a velocity decrease near the production zone were found using the proposed method, indicating the existence of dissociated methane gas in the sand layers. © 2004 Society of Exploration Geophysicists.
cited By 74
Keywords: Geophysical prospecting; Petroleum prospecting; Seismic response; Seismic waves; Waveform analysis; Conventional approach; Gas production test; Gas-hydrate production; Numerical tests; Production zones; Time-lapse seismic data; Velocity changes; Waveform inversion; Gas hydrates

Determining SAFOD area microearthquake locations solely with the Pilot Hole seismic array data - Oye, V.; Chavarria, J.A. (Geophysical Research Letters, 2004)
In August 2002, an array of 32 three-component geophones was installed in the San Andreas Fault Observatory at Depth (SAFOD) Pilot Hole (PH) at Parkfield, CA. As an independent test of surface-observation-based microearthquake locations, we have located such events using only data recorded on the PH array. We then compared these locations with locations from a combined set of PH and Parkfield High Resolution Seismic Network (HRSN) observations. We determined the uncertainties in the locations as they relate to errors in the travel time picks and the velocity model by the bootstrap method. Based on the PH and combined locations, we find that the C2 cluster to the northeast of the PH has the smallest location uncertainties. Events in this cluster also have the most similar waveforms and largest magnitudes. This confirms earlier suggestions that the C2 cluster is a promising target for the SAFOD Main Hole. Copyright 2004 by the American Geophysical Union.
cited By 17
Keywords: Data acquisition; Earthquakes; Error analysis; Mathematical models; Observatories; Waveform analysis; Seismology; Strike-slip faults; Bootstrap method; High resolution seismic network (HRSN); Microearthquakes; Pilot hole (PH); Geophysics; Location; earthquake hypocenter; microtremor; San Andreas Fault; seismic method; Bootstrap method; High resolution seismic; Location uncertainty; San Andreas fault; Seismic arrays; Surface observation; Three component; Velocity model

Detailed kinematics, structure and recurrence of micro-seismicity in the SAFOD target region - Nadeau, R.M.; Michelini, A.; Uhrhammer, R.A.; Dolenc, D.; McEvilly, T.V. (Geophysical Research Letters, 2004)
Large numbers of small earthquakes recorded over 2 decades and analyzed with advanced techniques are used to characterize the detailed kinematics, structure and recurrence interval scaling properties of micro-seismicity in a 4 × 4 km lateral and 6 km deep crustal volume encompassing the region of the SAFOD deep drilling experiment. The characterization reveals that the seismically active San Andreas fault in the vicinity of SAFOD's repeating magnitude 2 target earthquakes is composed of two sub-parallel fault strands that are creeping at comparable rates and that one of the strands lies between the SAFOD drilling platform and SAFOD's target events. In the region, ∼55% of the earthquakes are members of 52 characteristically repeating earthquake sequences. The recurrence intervals of the repeating target events are consistent with the interval scaling of the other sequences. However this. scaling is contrary to that expected from standard constant stress-drop theory. Copyright 2004 by the American Geophysical Union.
cited By 50
Keywords: Creep; Geophysics; Kinematics; Seismology; Stresses; Structural analysis; Theory; Drilling platforms; Faulting; Geophysics; Kinematics; Strike-slip faults; Tectonics; Crustal volume; Micro-seismicity; Scaling properties; Stress-drop theory; Earthquakes; Earthquakes; deep drilling; earthquake recurrence; kinematics; microtremor; San Andreas Fault; seismicity; California; North America; United States; Constant stress; Fault strands; Recurrence intervals; Repeating earthquake; San Andreas fault; Scaling properties; Small earthquakes; Target regions

Crustal structure across the San Andreas Fault at the SAFOD site from potential field and geologic studies - McPhee, D.K.; Jachens, R.C.; Wentworth, C.M. (Geophysical Research Letters, 2004)
We present newly compiled magnetic, gravity, and geologic datasets from the Parkfield region around the San Andreas Fault Observatory at Depth (SAFOD) pilot hole in order to help define the structure and geophysical setting of the San Andreas Fault (SAF). A 2-D cross section of the SAF zone at SAFOD, based on new, tightly spaced magnetic and gravity observations and surface geology, shows that as drilling proceeds NE toward the SAF, it is likely that at least 2 fault bounded magnetic slivers, possibly consisting of magnetic granitic rock, serpentinite, or unusually magnetic sandstone, will be encountered. The upper 2 km of the model is constrained by an order of magnitude increase in magnetic susceptibility at 1400 m depth observed in pilot hole measurements. NE of the SAF, a flat lying, tabular body of serpentinite at 2 km depth separates two masses of Franciscan rock and truncates against the SAF.
cited By 60
Keywords: Data acquisition; Granite; Gravitational effects; Magnetic materials; Magnetic susceptibility; Mathematical models; Rock drilling; Rocks; Sandstone; Serpentine; Magnetic susceptibility; Magnetism; Rock drilling; Strike-slip faults; Crustal structure; Magnetic granitic rock; San Andreas Fault (SAF); Surface geology; Geology; Structural geology; crustal structure; geological mapping; gravity survey; magnetic survey; San Andreas Fault; serpentinite; Crustal structure; Granitic rocks; Pilot holes; Potential field; San Andreas fault; Serpentinite

Coring the Chesapeake Bay impact crater - Poag, C.W. (Geotimes, 2004)
In July 1983, the shipboard scientists of Deep Sea Drilling Project Leg 95 found an unexpected bonus in a core taken 150 kilometers east of Atlantic City, N.J. At Site 612, the scientists recovered a 10-centimeter-thick layer of late Eocene debris ejected from an impact about 36 million years ago. Microfossils and argon isotope ratios from the same layer reveal that the ejecta were part of a broad North American impact debris field, previously known primarily from the Gulf of Mexico and Caribbean Sea. Since that serendipitous beginning, years of seismic reflection profiling, gravity measurements and core drilling have confirmed the source of that strewn field - the Chesapeake Bay impact crater, the largest structure of its kind in the United States, and the sixth-largest impact crater on Earth.
cited By 1
Keywords: crater; ejecta; Eocene; impact structure; sediment core; Chesapeake Bay; North America; United States

Composition of impact melt particles and the effects of post-impact alteration in suevitic rocks at the Yaxcopoil-1 drill core, Chicxulub crater, Mexico - Hecht, L.; Wittmann, A.; Schmitt, R.-T.; St{offler, D. (Meteoritics and Planetary Science, 2004)
Petrographical and chemical analysis of melt particles and alteration minerals of the about 100 m-thick suevitic sequence at the Chicxulub Yax-1 drill core was performed. The aim of this study is to determine the composition of the impact melt, the variation between different types of melt particles, and the effects of post-impact hydrothermal alteration. We demonstrate that the compositional variation between melt particles of the suevitic rocks is the result of both incomplete homogenization of the target lithologies during impact and subsequent post-impact hydrothermal alteration. Most melt particles are andesitic in composition. Clinopyroxene-rich melt particles possess lower SiO2 and higher CaO contents. These are interpreted by mixing of melts from the silicate basement with overlying carbonate rocks. Multi-stage post-impact hydrothermal alteration involved significant mass transfer of most major elements and caused further compositional heterogeneity between melt particles. Following backwash of seawater into the crater, palagonitization of glassy melt particles likely caused depletion of SiO2, A12O3, CaO, Na2O, and enrichment of K2O and FeOtot during an early alteration stage. Since glass is very susceptible to fluid-rock interaction, the state of primary crystallization of the melt particles had a significant influence on the intensity of the post-impact hydrothermal mass transfer and was more pronounced in glassy melt particles than in well-crystallized particles. In contrast to other occurrences of Chicxulub impactites, the Yax-1 suevitic rocks show strong potassium metasomatism with hydrothermal K-feldspar formation and whole rock K2O enrichment, especially in the lower unit of the suevitic sequence. A late stage of hydrothermal alteration is characterized by precipitation of silica, analcime, and Na-bearing Mg-rich smectite, among other minerals. This indicates a general evolution from a silica-undersaturated fluid at relatively high potassium activities at an early stage toward a silica-oversaturated fluid at relatively high sodium activities at later stages in the course of fluid rock interaction. © Meteoritical Society, 2004. Printed in USA.
cited By 59

Composition of basaltic lavas sampled by phase-2 of the Hawaii Scientific Drilling Project: Geochemical stratigraphy and magma types - Rhodes, J.M.; Vollinger, M.J. (Geochemistry, Geophysics, Geosystems, 2004)
This paper presents major and trace element compositions oflavas from the entire 3098 m stratigraphic section sampled by phase-2 of the Hawaii Scientific Drilling Project. The upper 245 m are lavas from Mauna Loa volcano, and the lower 2853 m are lavas and volcanoclastic rocks from Mauna Kea volcano. These intervals are inferred to represent about 100 ka and 400 ka respectively of the eruptive history of the two volcanoes. The Mauna Loa tholeiites tend to be higher in SiO2 and lower in total iron, TiO2, alkalis, and incompatible elements at a given MgO content than Mauna Kea lavas. The transition from Mauna Loa to Mauna Kea lavas is all the more pronounced because the Mauna Loa tholeiites overlie a thin sequence of postshield Mauna Kea alkalic to transitional tholeiitic lavas. The Mauna Loa tholeiites display well- developed coherent trends with MgO that are indistinguishable in most respects from modern lavas. With depth, however, there is a slight decline in incompatible element abundances, and small shifts to depleted isotopic ratios. These characteristics suggest small changes in melt production and source components over time, superimposed on shallow melt segregation. The Mauna Kea section is subdivided into a thin, upper 107 m sequence of postshield tholeiites, transitional tholeiites and alkali basalts of the Hamakua volcanics, overlying four tholeiitic magma types that are intercalated throughout the rest of the core. These four magma types are recognized on the basis of MgO-normalized SiO2 and Zr/Nb values. Type-1 lavas (high SiO 2 and Zr/Nb) are ubiquitous below the postshield lavas and are the dominant magma type on Mauna Kea. They are inter-layered with the other three lava types. Type-2 lavas (low SiO2 but high Zr/Nb) are found only in the upper core, and especially above 850 m. Type-3 lavas (low SiO2 and Zr/Nb) are very similar to tholeiites from Loihi volcano and are present only below 1974 m. There are only 3 discrete samples of type-4 lavas (high SiO2 and low Zr/Nb), which are present in the upper and lower core. The differences between these magma types are inferred to reflect changes in melt production, depth of melt segregation, and differences in plume source components over about 400 ka of Mauna Kea's eruptive history. At the start of this record, eruption rates were high, and two distinct tholeiitic magmas (type-1 and 3) were erupting concurrently. These two magmas require two distinct source components, one similar to that of modern Loihi tholeiites and the other close to that of Kilauea magmas. Subsequently, the Loihi- like source of the type-3 magmas was exhausted, and these lavas are absent from the remainder of the core. For the next 200 ka or so, the eruptive sequence consists of inter-layered type-1 and -2 lavas that are derived from a common Mauna Kea source, the major difference between the two being the depth at which the melts segregated from the source. At around 440 ka (corresponding with the transition in the core from submarine to subaerial lavas) eruption rates began to decline and low-MgO lavas are suddenly much more abundant in the record. Continuing gradual decline in melting and eruption rates was accompanied by a decline in normalized SiO2 content of the type-1 magmas, and the eventual onset of postshield magmatism. Copyright 2004 by the American Geophysical Union.

Chicxulub central crater structure: Initial results from physical property measurements and combined velocity and gravity modeling - Vermeesch, P.M.; Morgan, J.V. (Meteoritics and Planetary Science, 2004)
The Chicxulub crater in Mexico is a nearly pristine example of a large impact crater. Its slow burial has left the central impact basin intact, within which there is an apparently uneroded topographic peak ring. Its burial, however, means that we must rely on drill holes and geophysical data to interpret the crater form. Interpretations of crater structures using geophysical data are often guided by numerical modeling and observations at other large terrestrial craters. However, such endeavors are hindered by uncertainties in current numerical models and the lack of any obvious progressive change in structure with increasing crater size. For this reason, proposed structural models across Chicxulub remain divergent, particularly within the central crater region, where the deepest well is only ∼1.6 km deep. The shape and precise location of the stratigraphic uplift are disputed. The spatial extent and distribution of the allogenic impact breccias and melt rocks remain unknown, as do the lithological nature of the peak ring and the mechanism for its formation. The objective of our research is to provide a well-constrained 3D structural and lithological model across the central region of the Chicxulub crater that is consistent with combined geophysical data sets and drill core samples. With this in mind, we present initial physical property measurements made on 18 core samples from the Yaxcopoil-1 (Yax-1) drill hole between 400 and 1500 m deep and present a new density model that is in agreement with both the 3D velocity and gravity data. Future collation of petrophysical and geochemical data from Yax-1 core, as well as further seismic surveys and drilling, will allow us to calibrate our geophysical models-assigning a suite of physical properties to each lithology. An accurate 3D model of Chicxulub is critical to our understanding of large craters and to the constraining of the environmental effects of this impact. © Meteoritical Society, 2004. Printed in USA.
cited By 38

Elastic velocity models for gas-hydrate-bearing sediments - A comparison - Chand, S.; Minshull, T.A.; Gei, D.; Carcione, J.M. (Geophysical Journal International, 2004)
The presence of gas hydrate in oceanic sediments is mostly identified by bottom-simulating reflectors (BSRs), reflection events with reversed polarity following the trend of the seafloor. Attempts to quantify the amount of gas hydrate present in oceanic sediments have been based mainly on the presence or absence of a BSR and its relative amplitude. Recent studies have shown that a BSR is not a necessary criterion for the presence of gas hydrates, but rather its presence depends on the type of sediments and the in situ conditions. The influence of hydrate on the physical properties of sediments overlying the BSR is determined by the elastic properties of their constituents and on sediment microstructure. In this context several approaches have been developed to predict the physical properties of sediments, and thereby quantify the amount of gas/gas hydrate present from observed deviations of these properties from those predicted for sediments without gas hydrate. We tested four models: the empirical weighted equation (WE); the three-phase effective-medium theory (TPEM); the three-phase Biot theory (TPB) and the differential effective-medium theory (DEM). We compared these models for a range of variables (porosity and clay content) using standard values for physical parameters. The comparison shows that all the models predict sediment properties comparable to field values except for the WE model at lower porosities and the TPB model at higher porosities. The models differ in the variation of velocity with porosity and clay content. The variation of velocity with hydrate saturation is also different, although the range is similar. We have used these models to predict velocities for field data sets from sediment sections with and without gas hydrates. The first is from the Mallik 2L-38 well, Mackenzie Delta, Canada, and the second is from Ocean Drilling Program (ODP) Leg 164 on Blake Ridge. Both data sets have Vp and Vs information along with the composition and porosity of the matrix. Models are considered successful if predictions from both Vp and Vs match hydrate saturations inferred from other data. Three of the models predict consistent hydrate saturations of 60-80 per cent from both Vp and Vs from log and vertical seismic profiling data for the Mallik 2L-38 well data set, but the TPEM model predicts 20 per cent higher saturations, as does the DEM model with a clay-water starting medium. For the clay-rich sediments of Blake Ridge, the DEM, TPEM and WE models predict 10-20 per cent hydrate saturation from Vp data, comparable to that inferred from resistivity data. The hydrate saturation predicted by the TPB model from Vp is higher. Using Vs data, the DEM and TPEM models predict very low or zero hydrate saturation while the TPB and WE models predict hydrate saturation very much higher than those predicted from Vp data. Low hydrate saturations are observed to have little effect on Vs. The hydrate phase appears to be connected within the sediment microstructure even at low saturations. © 2004 RAS.
cited By 163
Keywords: comparative study; hydration; marine sediment; numerical model; P-wave; S-wave; saturation; seismic reflection; seismic survey; seismic velocity

Borehole geophysical techniques to define stratigraphy, alteration and aquifers in basalt - Helm-Clark, C.M.; Rodgers, D.W.; Smith, R.P. (Journal of Applied Geophysics, 2004)
This paper concerns the interpretation of borehole geophysical data from basalt sequences, especially continental basalt sequences that host aquifers. Based on modifications of the rules used for interpreting borehole data from sedimentary rocks, new rules are proposed to identify the internal stratigraphy, aquifer boundaries, and alteration features in continental basalts.The value of several wireline tools is critiqued. Natural gamma logs have limited utility in basalt sequences unless anomalously high-potassium or low-potassium basalt flows and/or sedimentary interbeds exist which can act as marker beds for stratigraphic correlations. Neutron logs can usually discriminate between individual flows, flow breaks and interbeds, even in unsaturated basalts. Neutron logs and temperature logs can also be used to map aquifer thickness in basalt. Gamma-gamma density logs are usually sensitive to the density contrasts between interbeds and basalt flows, and in combination with neutron and natural gamma logs are crucial for the correct interpretation of large void spaces in basalt such as collapsed lava tubes and formerly inflated pahoehoe lobes. Basalt porosity calculated from neutron, resistivity and/or gamma-gamma density logs is commonly overestimated due to the presence of hydrous alteration minerals. Velocity and resistivity logs are best at discriminating between flows in saturated conditions. Magnetic susceptibility logs may capture magnetic mineralogy variations at a finer scale than that of flows and flow breaks and therefore should always be interpreted in combination with other logs. Non-spectral neutron-gamma logs are not useful in basalt, though spectral neutron-gamma logs have been used successfully for stratigraphic correlation and to locate pollutants. Geochemical logs or the inclination of magnetic remanence provide the best data to discriminate individual flows with a basalt sequence, and thus establish an internal stratigraphy. Other tools used alone cannot provide reliable stratigraphic information, but a combination of tools may work. We recommend the combination of natural gamma, neutron, and gamma-gamma density logs in unsaturated rocks, and these logs plus velocity and resistivity logs in saturated rocks. © 2003 Published by Elsevier B.V.
cited By 40
Keywords: Aquifers; Basalt; Boreholes; Magnetic susceptibility; Remanence; Sedimentary rocks; Magnetic mineralogy; Stratigraphy; aquifer; basalt; borehole geophysics; groundwater exploration; mineral alteration; stratigraphy

Benthic foraminifera and environmental turnover across the Cretaceous/Paleogene boundary at Blake Nose (ODP Hole 1049C, Northwestern Atlantic) - Alegret, L.; Thomas, E. (Palaeogeography, Palaeoclimatology, Palaeoecology, 2004)
Sediments recovered at lower bathyal ODP Site 1049 on Blake Nose (Northwestern Atlantic) offer an opportunity to study environmental changes at the Cretaceous/Paleogene (K/P) boundary relatively close to the Chicxulub impact structure on the Yucatan peninsula, Mexico. In Hole 1049C, the boundary is located at the base of a 9-cm-thick layer with abundant spherules, considered to be impact ejecta. Uppermost Maastrichtian oozes below, and lowermost Danian pelagic oozes above the spherule-bed contain well-preserved bathyal benthic foraminifera. The spherule-bed itself, in contrast, contains a mixture of shallow (neritic) and deeper (bathyal) species, and specimens vary strongly in preservation. This assemblage was probably formed by reworking and down-slope transport triggered by the K/P impact. Across the spherule-bed (i.e., the K/P boundary) only ∼7% of benthic foraminiferal species became extinct, similar to the low extinction rates of benthic foraminifera worldwide. Quantitative analysis of benthic foraminiferal assemblages and morphogroups in the >63-μm size fraction indicates a relatively eutrophic, stable environment during the latest Maastrichtian, interrupted by a sudden decrease in the food supply to the benthos at the K/P boundary and a decrease in diversity of the faunas, followed by a stepped recovery during the earliest Danian. The recovery was probably linked to the gradual recovery of surface-dwelling primary producers. © 2004 Elsevier B.V. All rights reserved.
cited By 56

Azimuthal anisotropy using shear dipole sonic: Insights from the AIG 10 well, Corinth Rift Laboratory; [Anisotropie azimutale par diagraphie sonique des ondes de cisaillement : Donn'ees du forage AIG 10, laboratoire du rift de Corinthe] - Prioul, Romain; Plona, Thomas; Kane, Michael; Sinha, Bikash; Kaufman, Peter; Signer, Claude (Comptes Rendus - Geoscience, 2004)
Within the Corinth Rift Laboratory, dipole sonic data have been acquired in the AIG 10 well between depths 711 and 1004 m during three passes of a sonic logging tool at three different borehole fluid pressures. Specific sections of the well reveal clear characteristics of either moderate-to-large azimuthal anisotropy (9-25%) below the Aigion Fault, e.g., 779-784 and 809-816 m, with a fast-shear azimuth directed 105°, or homogeneous isotropic medium (i.e., 735-753 m) between the casing shoe and the fault. The presence of the fault is coincident with the identification of lower velocities over an interval of approximately 12-14 m. Analysis of the data reveals that the formation is not acoustically stress-sensitive to the 3 MPa differential pressure applied. Interpretation of the sonic data with complementary image logs suggests the anisotropy is due to intrinsic fractures and bedding, and the fast-shear direction 105° is consistent with the regional maximum horizontal stress. © 2004 Acad'emie des sciences. Published by Elsevier SAS. All rights reserved.
Keywords: Gulf of Corinth; Ionian Sea; Mediterranean Sea; acoustic logging; borehole geophysics; fault; structural geology

Analysis of silicon concentration periodicity for the past 2.4 Ma in sediments from Lake Baikal site BDP 96-2 - Dawson, R.; Wei, R.; Tao, S.; Ito, Y.; Yamanaka, K.; Haraguchi, H. (Climate Research, 2004)
The current study examines the potential cyclical nature of the elemental concentration record of silicon in sediment from Lake Baikal site BDP 96-2 in an attempt to clarify trends in periodicity observed for periods from 700 kyr BP to the present, 760 kyr BP to 1.43 Ma, and 1.47 to 2.4 Ma. The results indicate the presence of the 100 kyr cycle during the past 700 000 yr, as identified in other research. They also indicate the presence of a strong 74 kyr cycle from 760 kyr BP to 1.43 Md, with a weaker 44 kyr cycle. For the period from 1.47 to 2.4 Ma, we observed evidence of strong 465 and 155 kyr cycles, in addition to a moderate 93 kyr cycle and various lesser cycles. This new evidence represents one of the first findings of longer cycles, as predicted by classic insolation theory, and sheds some initial light on Imbrie's 400 kyr question. © Inter-Research 2004.
Keywords: Eurasia; Lake Baikal; Russian Federation; climate change; lacustrine deposit; Milankovitch cycle; orbital forcing; paleoenvironment; silicon

Analysis of eleven years of deformation measured by GPS in the Corinth Rift Laboratory area; [Analyse de onze ann'ees de mesures de deformations collect'ees par GPS dans la zone du laboratoire du rift de corinthe] - Avallone, Antonio; Briole, Pierre; Agatza-Balodimou, Amalia Maria; Billiris, Harilaos; Charade, Olivier; Mitsakaki, Christiana; Nercessian, Alexandre; Papazissi, Kalliopi; Paradissis, Dimitris; Veis, George (Comptes Rendus - Geoscience, 2004)
GPS (Global Positioning System) data collected in the Corinth Rift during eleven campaigns between 1990 and 2001 provide velocities of 57 points with ∼1.5 mm yr-1 accuracy. Peloponnesos moves at 30 mm yr-1 to the N215° E with respect to a fixed Europe. Extension across the rift is accommodated in a narrow band offshore. Its rate increases from east to west and is 16 mm yr-1 near Aigion. Both sides of the rift behave as clockwise rotating blocks with rates of 7±0.5° M yr-1 and 2.8±0.8° M yr-1, respectively for the northern and southern blocks. After removing block rotations, the northern block shows a north-south extension rate of 120±50 nstrain yr-1, whereas the southern block indicates the internal deformation is still inside the error bar (<20 nstrain yr-1). The strain accumulation across the major faults located along the southern coast of the Corinth Gulf is less than 1 mm yr-1. This implies long recurrence periods for large earthquakes on these faults. © 2004 Acad'emie des sciences. Published by Elsevier SAS. All rights reserved.
Keywords: crustal deformation; GPS; rift zone; seismotectonics

A mechanical model of the San Andreas fault and SAFOD Pilot Hole stress measurements - Ch'ery, J.; Zoback, M.D.; Hickman, S. (Geophysical Research Letters, 2004)
Stress measurements made in the SAFOD pilot hole provide an opportunity to study the relation between crustal stress outside the fault zone and the stress state within it using an integrated mechanical model of a transform fault loaded in transpression. The results of this modeling indicate that only a fault model in which the effective friction is very low (<0.1) through the seismogenic thickness of the crust is capable of matching stress measurements made in both the far field and in the SAFOD pilot hole. The stress rotation measured with depth in the SAFOD pilot hole (∼28°) appears to be a typical feature of a weak fault embedded in a strong crust and a weak upper mantle with laterally variable heat flow, although our best model predicts less rotation (15°) than observed. Stress magnitudes predicted by our model within the fault zone indicate low shear stress on planes parallel to the fault but a very anomalous mean stress, approximately twice the lithostatic stress. Copyright 2004 by the American Geophysical Union.
cited By 53
Keywords: Friction; Geophysical prospecting; Mathematical models; Seismology; Shear stress; Stress analysis; Fault zones; Pilot holes; Stress measurements; Transpression; Boreholes; borehole geophysics; in situ stress; model; San Andreas Fault; stress measurement; California; North America; Parkfield; United States; Western Hemisphere; World

A magnetic mineral record of Late Quaternary tropical climate variability from Lake Bosumtwi, Ghana - Peck, John A.; Green, Ryan R.; Shanahan, Tim; King, John W.; Overpeck, Jonathan T.; Scholz, Christopher A. (Palaeogeography, Palaeoclimatology, Palaeoecology, 2004)
We report magnetic hysteresis results from sediment cores obtained from Lake Bosumtwi, Ghana. As a hydrologically closed basin, the water budget of Lake Bosumtwi is extremely sensitive to changes in the precipitation/ evapotranspiration balance. Lake Bosumtwi lies in the path of the seasonal migration of the intertropical convergence zone (ITCZ); hence, the lake is ideally situated to study monsoon variability in West Africa. Five distinctive magnetic mineral zones (A-E) were identified in the 11-m-long sediment cores that span the last 26,000 calendar years. Prior to 12 calendar (cal) ka, low concentrations of multidomain, high-coercivity magnetic minerals are present. Three prominent shifts towards very high concentrations of high-coercivity iron sulfide (greigite) magnetic minerals are centered at 12,470, 17,290, and 22,600 calendar years during the last glacial period (magnetic zones D1-3). Between 12 and 3.2 cal ka, there is an abrupt shift to moderately high concentrations of mixed multidomain and single-domain, low-coercivity minerals and an organic-rich sapropel lithology. Since 3.2 cal ka, the magnetic mineral parameters reveal a shift to increased amounts of high-coercivity magnetic minerals. These magnetic mineral zones document tropical climate variability on a variety of temporal scales. Glacial age sediments have a high-coercivity magnetic mineralogy due to increased aeolian dust transport from the Sahel to Lake Bosumtwi as well as postdepositional reductive diagenesis. During the last glacial period, the increased strength of Harmattan and North African continental trade winds, the southward depression of the ITCZ, and weakened summer monsoon strength resulted in increased regional aridity and greater dust flux out of Sahel source regions. The greigite-bearing D magnetic zones correspond to brief lowstands in the level of Lake Bosumtwi and likely represent periods of intensified aridity in West Africa. The D magnetic zones closely resemble the timing and duration of Heinrich events and suggest a hemispheric-scale climatic coupling between the tropics and poles. The well-documented African humid period (AHP) is characterized by abrupt shifts in magnetic parameters between 12 and 3.2 cal ka. Dust flux to Lake Bosumtwi is inferred to be very low during this humid interval due to the strengthening of the summer monsoon. Since 3.2 cal ka, the magnetic mineral parameters suggest increased aridity as compared to the AHP. This work demonstrates that the magnetic properties of Lake Bosumtwi sediment are a sensitive recorder of abrupt climate change of global significance. © 2004 Elsevier B.V. All rights reserved.
Keywords: Africa; Ashanti; Eastern Hemisphere; Ghana; Lake Bosumtwi; Sub-Saharan Africa; West Africa; World; climate change; dust; greigite; magnetic mineral; monsoon; paleoclimate; paleolimnology; Quaternary; sedimentation; water budget

14C dating of soil samples from the Unzen volcano scientific drilling boreholes - Xu, S.; Hoshizumi, H.; Ochiai, Y.; Aoki, H.; Uto, K. (Nuclear Instruments and Methods in Physics Research, Section B: Beam Interactions with Materials and Atoms, 2004)
Soil samples were collected from the upper layers of two Unzen volcano Scientific Drilling Project boreholes and dated by radiocarbon using accelerator mass spectrometry (AMS). The humic soils underlying volcanic related lahar flow deposits at a depth of 56.42 m of the USDP-1 borehole gave a 14C age of 18.8 kaBP, suggesting that the accumulation of the lahar flow deposits was caused by the Kureishibaru pyroclastic eruption (19 kaBP) dated at the surface. The humic soils underlying a pyroclastic flow deposit at depths of 71.60-71.90 m of the USDP-2 borehole gave 14C ages of 27.5 and 29.3 kaBP at the top and bottom layers, respectively. This reveals that the initial activity of Fugendake volcano occurred around 27 kaBP and the collapse of Myokendake volcano around 29 kaBP. © 2004 Elsevier B.V. All rights reserved.
cited By 8
Keywords: Boreholes; Carbon; Deposits; Geochronology; Leveling (machinery); Mass spectrometry; Project management; Radioactive elements; Stratigraphy; Volcanoes; Accelerator mass spectrometry (AMS); Pyroclastic eruption; Soil carbon fraction; Unzen volcano; Soils

Is the transition impact to post-impact rock complete? Some remarks based on XRF scanning, electron microprobe, and thin section analyses of the Yaxcopoil-1 core in the Chicxulub crater - Smit, J.; Van Der Gaast, S.; Lustenhouwer, W. (Meteoritics and Planetary Science, 2004)
The transition from impact to post-impact rocks in the Yaxcopoil-1 (Yax-1) core is marked by a 2 cm-thick clay layer characterized by dissolution features. The clay overlies a 9 cm-thick hardground, overlying a 66 cm-thick crossbedded unit, consisting of dolomite sandstone alternating with thin micro-conglomerates layers with litho- and bioclasts and the altered remains of impact glass, now smectite. The micro-conglomerates mark erosion surfaces. Microprobe and backscatter SEM analysis of the dolomite rhombs show an early diagenetic, complex-zoned, idiomorphic overgrowth, with Mn-rich zones, possibly formed by hot fluids related to cooling melt sheet in the crater. The pore spaces are filled with several generations of coelestite, barite, K-feldpar, and sparry calcite. XRF core scanning analysis detected high Mn values in the crossbedded sediments but no anomalous enrichment of the siderophile elements Cr, Co, Fe, and Ni in the clay layer. Shocked quartz occurs in the crossbedded unit but is absent in the clay layer. The basal Paleocene marls are strongly dissolved and do not contain a basal Paleocene fauna. The presence of a hardground, the lack of siderophile elements, shocked quartz, or Ni-rich spinels in the clay layer, and the absence of basal Paleocene biozones PO and Pa all suggest that the top of the ejecta sequence and a significant part of the lower Paleocene is missing. Due to the high energy sedimentation infill, a hiatus at the top of the impactite is not unexpected, but there is nothing in the biostratigraphy, geochemistry, and petrology of the Yax-1 core that can be used to argue against the synchroneity of the end-Cretaceous mass-extinctions and the Chicxulub crater. © Meteoritical Society, 2004.
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