All ICDP Publications with Abstracts
From parent-sysfolder "Publications" + 2 folder-levels deep
444.
Building a natural earthquake laboratory at focal depth (DAFSAM-NELSAM project, South Africa)
Scientific Drilling,
1
(3)
30-33
2006
ISSN: 18168957443.
Anisotropy in the shallow crust observed around the San Andreas fault before and after the 2004 M 6.0 parkfield earthquake
Bulletin of the Seismological Society of America,
96
(4 B)
S364-S375
2006
ISSN: 00371106Keywords:▾
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
Abstract: ▾ 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.
442.
Analyses of shocked quartz at the global K-P boundary indicate an origin from a single, high-angle, oblique impact at Chicxulub
Earth and Planetary Science Letters,
251
(3-4)
264-279
2006
Abstract: ▾ 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.
441.
An effective medium inversion algorithm for gas hydrate quantification and its application to laboratory and borehole measurements of gas hydrate-bearing sediments
Geophysical Journal International,
166
(2)
543-552
2006
ISSN: 0956540X
Publisher: Oxford University Press
Keywords:▾
algorithm; borehole geophysics; elastic wave; gas hydrate; inverse problem; marine sediment; P-wave; S-wave; wave attenuation
Abstract: ▾ 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.
440.
Making continental crust through slab melting: Constraints from niobium-tantalum fractionation in UHP metamorphic rutile
Geochimica et Cosmochimica Acta,
70
(18)
4770 – 4782
2006
ISSN: 00167037Keywords:▾
continental crust; dehydration melting; eclogite; fractionation; geochemistry; metamorphism; niobium; rutile; slab; tantalum
Abstract: ▾ The formation of the continental crust (CC) is one of the most important processes in the evolution of the silicate Earth. Exactly how the CC formed is the subject of ongoing debate that focuses on its subchondritic Nb/Ta ratio. Nb and Ta are "geochemical identical twins," so they usually do not fractionate from each other. Here, we show that rutile grains from hydrous rutile-bearing eclogitic layers recovered from drillcores in the Dabie-Sulu ultrahigh pressure terrain have highly variable Nb/Ta values (ranging from 5.4 to 29.1, with an average of 9.8 ± 0.6), indicating major fractionation of Nb and Ta most likely occurred during blueschist to amphibole-eclogite transformation in the absence of rutile. It is suggested that the released fluids with subchondritic Nb/Ta were transported to, and retained by, hydrous rutile-bearing eclogite in colder regions, resulting in suprachondritic Nb/Ta ratios for drier eclogite in hotter regions. Further dehydration of hydrous rutile-bearing eclogites cannot transfer the fractionated Nb/Ta values to the CC due to the low solubility of Nb and Ta in fluids in the presence of rutile, while dehydration-melting results in a major component of the CC, the tonalite-trondhjemite-granodiorite (TTG) component, which is responsible for the low Nb/Ta of the CC. Consequently, residual eclogites have variable but overall suprachondritic Nb/Ta. © 2006 Elsevier Inc. All rights reserved.
439.
A reconstruction of the climate and vegetation of northeastern Siberia based on lake sediments
Paleontological Journal,
40
(SUPPL.5)
S622-S628
2006
ISSN: 00310301Abstract: ▾ 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.
438.
A multiscale study of the mechanisms controlling shear velocity anisotropy in the San Andreas Fault Observatory at Depth
Geophysics,
71
(5)
F131-F146
2006
ISSN: 00168033
Publisher: Society of Exploration Geophysicists
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
Abstract: ▾ 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.
437.
A field guide to the central, creeping section of the San Andreas fault and the San Andreas Fault Observatory at Depth
GSA Field Guides,
7237-272
2006
Keywords:▾
Creep; Curbs; Earthquakes; Infill drilling; Locks (fasteners); Observatories; Strike-slip faults, Creeping section; Fault structure; Hollister; Parkfield; SAFOD; San Andreas fault, Structural geology
Abstract: ▾ 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.
436.
14c in methane and dic in the deep terrestrial subsurface: implications for microbial methanogenesis
Geomicrobiology Journal,
23
(6)
453-462
2006
ISSN: 01490451Keywords:▾
carbon isotope; dissolved inorganic carbon; methane; methanogenesis; microbial activity; microbiology, Africa; South Africa; Southern Africa; Sub-Saharan Africa; Witwatersrand, Archaea
Abstract: ▾ 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.
435.
Drilling to the core of Japan's Unzen Volcano
Geodrilling International
(120)
37-39
2006
ISSN: 09693769Abstract: ▾ 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.
434.
Amplitude and frequency anomalies in regional 3D seismic data surrounding the Mallik 5L-38 research site, Mackenzie Delta, Northwest Territories, Canada
Geophysics,
71
(6)
B183-B191
2006
ISSN: 00168033
Publisher: Society of Exploration Geophysicists
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
Abstract: ▾ 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.
433.
Energy release and heat generation during the 1999 Ms7.6 Chi-Chi, Taiwan, earthquake
Journal of Geophysical Research: Solid Earth,
111
(11)
2006
ISSN: 21699313
Publisher: Blackwell Publishing Ltd
Keywords:▾
Chi-Chi earthquake 1999; earthquake magnitude; earthquake rupture; energy budget; faulting; seismicity; source parameters, Asia; Eurasia; Far East; Taiwan
Abstract: ▾ 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.
432.
ICDP Workshop on the Emerging Modern Aerobic Earth System
Scientific Drilling,
256--57
2006
431.
Establishing the link between the Chesapeake Bay impact structure and the North American tektite strewn field: The Sr-Nd isotopic evidence
Meteoritics and Planetary Science,
41
(5)
689-703
2006
ISSN: 10869379
Publisher: University of Arkansas
Keywords:▾
crater; Deep Sea Drilling Project; impact structure; isotopic composition; neodymium; strontium; tektite
Abstract: ▾ 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.
430.
Major and trace element compositions of melt particles and associated phases from the Yaxcopoil-1 drill core, Chicxulub impact structure, Mexico
Meteoritics and Planetary Science,
41
(9)
1361-1379
2006
Abstract: ▾ Melt particles found at various depths in impactites from the Yaxcopoil-1 borehole into the Chicxulub impact structure (Yucatán) have been analyzed for their major and trace element abundances. A total of 176 electron microprobe and 45 LA-ICP-MS analyses from eight different melt particles were investigated. The main purpose of this work was to constrain the compositions of precursor materials and secondary alteration characteristics of these melt particles. Individual melt particles are highly heterogeneous, which makes compositional categorization extremely difficult. Melt particles from the uppermost part of the impactite sequence are Ca- and Na-depleted and show negative Ce anomalies, which is likely a result of seawater interaction. Various compositional groupings of melt particles are determined with ternary and binary element ratio plots involving major and trace elements. This helps distinguish the degree of alteration versus primary heterogeneity of melt phases. Comparison of the trace element ratios Sc/Zr, Y/Zr, Ba/ Zr, Ba/Rb, and Sr/Rb with compositions of known target rocks provides some constraints on protolith compositions; however, the melt compositions analyzed exceed the known compositional diversity of possible target rocks. Normalized REE patterns are unique for each melt particle, likely reflecting precursor mineral or rock compositions. The various discrimination techniques indicate that the highly variable compositions are the products of melting of individual minerals or of mixtures of several minerals. Small, angular shards that are particularly abundant in units 2 and 3 represent rapidly quenched melts, whereas larger particles (>0.5 mm) that contain microlites and have fluidal, schlieric textures cooled over a protracted period. Angular, shard-like particles with microlites in unit 5 likely crystallized below the glass transition temperature or underwent fragmentation during or after deposition. © The Meteoritical Society, 2006.
429.
Magnetostratigraphy of the Cretaceous/Tertiary boundary and early Paleocene sedimentary sequence from the Chicxulub Impact Crater
earth, planets and space,
58
(10)
1309-1314
2006
Abstract: ▾ 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.
428.
Long-term sustainability of a high-energy, low-diweniff crystal-biome
Science,
314
(5798)
479-482
2006
ISSN: 00368075Keywords:▾
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
Abstract: ▾ 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.
427.
Leaky mode: A horizontal seismic attenuation mechanism in a gas hydrate-bearing sediment
SEG Technical Program Expanded Abstracts,
25
(1)
2304-2308
2006
ISSN: 10523812
Publisher: Society of Exploration Geophysicists
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
ISBN:
9781604236972
Abstract: ▾ 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.
426.
Late Quaternary climate-induced lake level variations in Lake Petén Itzá, Guatemala, inferred from seismic stratigraphic analysis
Palaeogeography, Palaeoclimatology, Palaeoecology,
230
(1-2)
52 – 69
2006
ISSN: 00310182Keywords:▾
Central America; Guatemala [Central America]; climate variation; lacustrine deposit; paleoclimate; Quaternary; seismic stratigraphy
Abstract: ▾ We used seismic images and sedimentary data from piston cores to conduct a sequence stratigraphic analysis of sediments in Lake Petén Itzá, 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 Petén Itzá (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 Petén Itzá 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 Petén Itzá 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.
425.
Lake Qinghai scientific drilling project
Scientific Drilling,
220-22
2006
424.
Investigating maar formation and the climate history of southern argentina-the potrok aike maar lake sediment archive drilling project (PASADO)
Scientific Drilling,
1
(3)
54 – 55
2006
423.
Intercorrelation of down-core variations of the high-resolution magnetic susceptibility for CON01-603 and CON01-605
422.
Inclination, declination and the reversal angle of the ChRM for CON01-603-2
421.
In situ measurement of the hydraulic diffusivity of the active Chelunepu Fault, Taiwan
Geophysical Research Letters,
33
(16)
2006
ISSN: 00948276
Publisher: American Geophysical Union
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
Abstract: ▾ [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.
420.
Magma mingling as indicated by texture and Sr / Ba ratios of plagioclase phenocrysts from Unzen volcano, SW Japan
Journal of Volcanology and Geothermal Research,
154
(1-2)
103-116
2006
ISSN: 03770273Keywords:▾
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
Abstract: ▾ 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.