Scientific drilling of active faults: Past and future - Reches, Z.; Ito, H. (Continental Scientific Drilling: A Decade of Progress, and Challenges for the Future, 2007)
Drilling into active faults has become a major scientific endeavor during the last decade and it appears as a most promising approach to resolve long-standing questions in earthquake and faulting processes. The first boreholes were drilled into the Nojima Fault following the 1995 Kobe earthquake. Since then, drilling into active faults has begun or been planned in a wide range of tectonic settings, such as a strike-slip plate boundary (San Andreas Fault, California), a thrust zone in an active orogenic belt (Chelungpu Fault, Taiwan), a normal fault in an active rift zone (Aigion Fault, Greece), a reactivated Archean fault (Pretorius Fault, South Africa), and a major subduction thrust (Nankai Thrust, Japan). These projects have already revealed many details on in-situ stresses, fault-zone structure, fault-rock composition, mechanical properties, heat flow, and near-field seismicity. Furthermore, most of these projects will continue to serve as observatories for monitoring fault deformation, fluid pressure and near-field earthquake source processes for a decade or two. Future drilling projects will focus on near-field observations and long-term monitoring of time-dependent processes and in-situ experimentation in active fault zones. Collaboration with industry and government will address practical issues pertaining to petroleum and geothermal energy, radioactive waste disposal, and urban seismic hazards. The outcome of these international efforts in drilling active faults will revolutionize our understanding of the processes controlling faulting and earthquakes and lead to a stronger scientific basis for earthquake hazard mitigation. © 2007 Springer-Verlag Berlin Heidelberg.
cited By 13

Precipitation at Lake Qinghai, NE Qinghai-Tibet Plateau, and its relation to Asian summer monsoons on decadal/interdecadal scales during the past 500 years - Xu, H.; Hou, Z.H.; Ai, L.; Tan, L.C. (Palaeogeography, Palaeoclimatology, Palaeoecology, 2007)
Knowledge of the variability of precipitation at Lake Qinghai and its relation to the Asian summer monsoons is helpful in constraining global climatic dynamics. Based on the high-resolution precipitation indicators of δ13C of the organic matter (δ13Corg), C/N atomic ratio, and the detrended total organic carbon content (TOCdetrended), we found that the trend of precipitation at Lake Qinghai is inversely correlated to that of the Indian Summer Monsoon (ISM) inferred from layer thickness of a stalagmite (S3) in southern Oman on decadal/interdecadal scales. The Chinese Drought/Flood (D/F) indices, which can indicate the dryness/wetness over large geographic areas, are also used to indicate the intensity of the monsoon rainfall. The D/F index of Xining near Lake Qinghai is synchronous with those of the regions in northern China where the East Asian Summer Monsoon (EASM) dominates; while it is anti-phase with those of southwestern China where ISM prevails. These materials suggest that, during the past 500 years, the source of moisture to Lake Qinghai on decadal/interdecadal scales is controlled mainly by the EASM, but not by the ISM. It is also suggested that the intensity of EASM is inversely related to that of the ISM on decadal/interdecadal scales. The decadal/interdecadal variability of ENSO may be responsible for the inverse relationship between the intensity of EASM and that of ISM. © 2007 Elsevier B.V. All rights reserved.
Keywords: Asia; China; Eurasia; Far East; Qinghai; Qinghai Lake; climate variation; global climate; organic matter; precipitation assessment; stalagmite

Petrography and shock-related remagnetization of pyrrhotite in drill cores from the Bosumtwi Impact Crater Drilling Project, Ghana - Kontny, Agnes; Elbra, Tiiu; Just, Jana; Pesonen, Lauri J.; Schleicher, Anja M.; Zolk, Jochen (Meteoritics and Planetary Science, 2007)
Rock magnetic and magnetic mineralogy data are presented from the International Continental Scientific Drilling Program (ICDP) drill cores LB-07A and LB-08A of the Bosumtwi impact structure in order to understand the magnetic behavior of impact and target lithologies and their impact-related remagnetization mechanism. Basic data for the interpretation of the magnetic anomaly patterns and the magnetic borehole measurements as well as for new magnetic modeling are provided. Magnetic susceptibility (150-500 × 10-6 SI) and natural remanent magnetization (10-3-10-1 A/m) are generally weak, but locally higher values up to 10.6 × 10-3 SI and 43 A/m occur. Sixty-three percent of the investigated rock specimens show Q values above 1 indicating that remanence clearly dominates over induced magnetization, which is a typical feature of impact structures. Ferrimagnetic pyrrhotite is the main magnetite phase, which occurs besides minor magnetite and a magnetic phase with a Curie temperature between 330 and 350 °C, interpreted as anomalous pyrrhotite. Coercive forces are between 20 and 40 mT. Brecciation and fracturing of pyrrhotite is a common feature confirming its pre-impact origin. Grain sizes of pyrrhotite show a large variation but the numerous stress-induced nanostructures observable by transmission electron microscopy (TEM) are assumed to behave as single-domain grains. We suggest that the drilled rocks lost their pre-shock remanence memory during the shock event and acquired a new, stable remanence during shock-induced grain size reduction. The observed brittle microstructures indicate temperatures not higher than 250 °C, which is below the Curie temperature of ferrimagnetic pyrrhotite (310 °C). © The Meteoritical Society, 2007. Printed in USA.

Post-drilling destabilization of temperature profile in borehole Yaxcopoil-1, Mexico - Šafanda, J.; Heidinger, P.; Wilhelm, H.; Čerm'ak, V. (Hydrogeology Journal, 2007)
As part of the International Continental Scientific Drilling Program (ICDP), the 1.5-km-deep borehole Yaxcopoil-1, located in the Chixculub meteor impact structure in Mexico, has undergone further study after drilling operations ceased. Temperature logs were repeated ten times at intervals 0.3-0.8, 15, 24 and 34 months after borehole shut-in. The logs bear a distinct signature of transient heat transfer by groundwater flow manifested by a gradual distortion of the linear temperature profile when acoldwaveof0.8-1.6°C amplitude was detected propagating downward from 145 to 312 m at a rate of 4-6 m/month. To understand the nature of this moving anomaly, a 20-day monitoring of the cold wave was carried out at a depth of 307 m that showed further cooling of 0.6°C during the first 16 days of the passage followed by temperature stabilisation. As an explanation of this unusual phenomenon, a theory is proposed, whereby the drilling mud has accumulated within the overlying and cooler highly porous and permeable karstic rocks during the drilling and migrates downward. The observed migration rate suggests a permeability higher than 10-11 m2. This indicates a high vulnerability to contamination of the only freshwater aquifer in the Yucatan region. © Springer-Verlag 2006.
cited By 8

Petrography, geochemistry, and alteration of country rocks from the Bosumtwi impact structure, Ghana - Karikari, Forson; Ferrière, Ludovic; Koeberl, Christian; Reimold, Wolf Uwe; Mader, Dieter (Meteoritics and Planetary Science, 2007)
Samples of the country rocks that likely constituted the target rocks at the 1.07 Myr old Bosumtwi impact structure in Ghana, West Africa, collected outside of the crater rim in the northern and southern parts of the structure, were studied for their petrographic characteristics and analyzed for their major- and trace-element compositions. The country rocks, mainly meta-graywacke, shale, and phyllite of the Early Proterozoic Birimian Supergroup and some granites of similar age, are characterized by two generations of alteration. A pre-impact hydrothermal alteration, often along shear zones, is characterized by new growth of secondary minerals, such as chlorite, sericite, sulfides, and quartz, or replacement of some primary minerals, such as plagioclase and biotite, by secondary sericite and chlorite. A late, argillic alteration, mostly associated with the suevites, is characterized by alteration of the melt/glass clasts in the groundmass of suevites to phyllosilicates. Suevite, which occurs in restricted locations to the north and to the south-southwest of the crater rim, contains melt fragments, diaplectic quartz glass, ballen quartz, and clasts derived from the full variety of target rocks. No planar deformation features (PDFs) in quartz were found in the country rock samples, and only a few quartz grains in the suevite samples show PDFs, and in rare cases two sets of PDFs. Based on a total alkali element-silica (TAS) plot, the Bosumtwi granites have tonalitic to quartz-dioritic compositions. The Nb versus Y and Ta versus Yb discrimination plots show that these granites are of volcanic-arc tectonic provenance. Provenance studies of the metasedimentary rocks at the Bosumtwi crater have also indicated that the metasediments are volcanic-arc related. Compared to the average siderophile element contents of the upper continental crust, both country rocks and impact breccias of the Bosumtwi structure show elevated siderophile element contents. This, however, does not indicate the presence of an extraterrestrial component in Bosumtwi suevite, because the Birimian country rocks also have elevated siderophile element contents, which is thought to result from regional hydrothermal alteration that is also related to widespread sulfide and gold mineralization. © The Meteoritical Society, 2007. Printed in USA.

Sedimentary geochemistry of core PG1351 from Lake El'gygytgyn-a sensitive record of climate variability in the East Siberian Arctic during the past three glacial-interglacial cycles - Melles, M.; Brigham-Grette, J.; Glushkova, O.Yu.; Minyuk, P.S.; Nowaczyk, N.R.; Hubberten, H.-W. (Journal of Paleolimnology, 2007)
The ca. 13 m long sediment core PG1351, recovered in 1998 from the central part of Lake El'gygytgyn, NE Siberia, was investigated for lithostratigraphy, water content, dry bulk density (DBD), total organic carbon (TOC), total nitrogen (TN), total sulphur (TS) and biogenic silica (opal) contents, and for TOC stable isotope ratios (δ13CTOC). The event stratigraphy recorded in major differences in sediment composition match variations in regional summer insolation, thus confirming a new age model for this core, which suggests that it spans the last 250 ka BP. Four depositional units of contrasting lithological and biogeochemical composition have been distinguished, reflecting past environmental conditions associated with relatively warm, peak warm, cold and dry, and cold but more moist climate modes. A relatively warm climate, resulting in complete summer melt of the lake ice cover and seasonal mixing of the water column, prevailed during the Holocene and Marine Isotope Stages (MIS) 3, 5.1, 5.3, 6.1, 6.3, 6.5, 7.1-7.3, 7.5, 8.1 and 8.3. MIS 5.5 (Eemian) was characterized by significantly enhanced aquatic primary production and organic matter supply from the catchment, indicating peak warm conditions. During MIS 2, 5.2, 5.4, 6.2 and 6.4 the climate was cold and dry, leading to perennial lake ice cover, little regional snowfall, and a stagnant water body. A cold but more moist climate during MIS 4, 6.6, 7.4, 8.2 and 8.4 is thought to have produced more snow cover on the perennial ice, strongly reducing light penetration and biogenic primary production in the lake. While the cold-warm pattern during the past three glacial-interglacial cycles is probably controlled by changes in regional summer insolation, differences in the intensity of the warm phases and in the degree of aridity (changing snowfall) during cold phases likely were due to changes in atmospheric circulation patterns. © 2006 Springer Science+Business Media B.V.
cited By 98
Keywords: climate variation; geochemistry; glacial-interglacial cycle; Holocene; insolation; marine isotope stage; paleoclimate; paleolimnology; sediment core; Chukchi; Eurasia; Lake El' gygytgyn; Russian Federation

Numerical modeling of impact-induced hydrothermal activity at the Chicxulub crater - Abramov, O.; Kring, D.A. (Meteoritics and Planetary Science, 2007)
Large impact events like the one that formed the Chicxulub crater deliver significant amounts of heat that subsequently drive hydrothenmal activity. We report on numerical modeling of Chicxulub crater cooling with and without the presence of water. The model inputs are constrained by data from borehole samples and seismic, magnetic, and gravity surveys. Model results indicate that initial hydrothermal activity was concentrated beneath the annular trough as well as in the permeable breccias overlying the melt. As the system evolved, the melt gradually cooled and became permeable, shifting the bulk of the hydrothermal activity to the center of the crater. The temperatures and fluxes of fluid and vapor derived from the model are consistent with alteration patterns observed in the available borehole samples. The lifetime of the hydrothermal system ranges from 1.5 to 2.3 Myr depending on assumed permeability. The long lifetimes are due to conduction being the dominant mechanism of heat transport in most of the crater, and significant amounts of heat being delivered to the near-surface by hydrothermal upwellings. The long duration of the hydrothermal system at Chicxulub should have provided ample time for colonization by thermophiles and/or hyperthermophiles. Because habitable conditions should have persisted for longer time in the central regions of the crater than on the periphery, a search for prospective biomarkers is most likely to be fruitful in samples from that region. © The Meteoritical Society, 2007.
cited By 78

On the fate of carbonates and anhydrite in impact processes – evidence from the Chicxulub event - Deutsch, A.; Langenhorst, F. (GFF, 2007)
In context of the “K/T” Chicxulub cratering event, the amounts of impact-released CO2and SOxas well as the consequences of this gaseous input into the atmosphere are discussed. It has been assumed that degassing of the sediments is an abrupt and violent effect, only related to the amplitude of the post-shock temperature after pressure decay. Here we provide evidence for a different, slow and probably equally important process of devolatilization: degassing and dissociation of sedimentary clasts in impact breccias. The sulfate and carbonate clasts in suevites, melt breccias, and melt rocks from the Chicxulub drill cores Y-6, C-1, and YAX-1, underwent various thermal effects, ranging from solid state re-crystallization over reaction with silicate melt to form pyroxene (diopside), melting (and re-crystallization) to total decomposition with dissolution of the lime in the melt matrix. These features reflect high, yet different formation temperatures of the breccias, and to a smaller degree, the fragment size. © 1996 Scandinavian University Press.
cited By 18

Origin and temporal evolution of Ko'olau Volcano, Hawai'i: Inferences from isotope data on the Ko'olau Scientific Drilling Project (KSDP), the Honolulu Volcanics and ODP Site 843 - Fekiacova, Z.; Abouchami, W.; Galer, S.J.G.; Garcia, M.O.; Hofmann, A.W. (Earth and Planetary Science Letters, 2007)
The Ko'olau component of the Hawaiian mantle plume represents an extreme (EM1-type) end member of Hawaiian shield lavas in radiogenic isotope space, and was defined on the basis of the composition of subaerial lavas exposed in the Makapu'u section of Ko'olau Volcano. The 679 m-deep Ko'olau Scientific Drilling Project (KSDP) allows the long-term evolution of Ko'olau Volcano to be reconstructed and the longevity of the Ko'olau component in the Hawaiian plume to be tested. Here, we report triple spike Pb isotope and Sr and Nd isotope data on KSDP core samples, and rejuvenation stage Honolulu Volcanics (HV) (together spanning ∼ 2.8 m.y.), and from ∼ 110 Ma basalts from ODP Site 843, thought to be representative of the Pacific lithosphere under Hawai'i. Despite overlapping ranges in Pb isotope ratios, KSDP and HV lavas form two distinct linear arrays in 208Pb/204Pb-206Pb/204Pb isotope space. These arrays intersect at the radiogenic end indicating they share a common component. This Kalihi component has more radiogenic Pb, Nd, Hf, but less radiogenic Sr isotope ratios than the Makapu'u component. The mixing proportions of these two components in the lavas oscillated through time with a net increase in the Makapu'u component upsection. Thus, the Makapu'u enriched component is a long-lived feature of the Hawaiian plume, since it is present in the main shield-building stage KSDP lavas. We interpret the changes in mixing proportions of the Makapu'u and Kalihi components as related to changes in both the extent of melting as well as the lithology (eclogite vs. peridotite) of the material melting as the volcano moves away from the plume center. The long-term Nd isotope trend and short-term Pb isotope fluctuations seen in the KSDP record cannot be ascribed to a radial zonation of the Hawaiian plume: rather, they reflect the short length-scale heterogeneities in the Hawaiian mantle plume. Linear Pb isotope regressions through the HV, recent East Pacific Rise MORB and ODP Site 843 datasets are clearly distinct, implying that no simple genetic relationship exists between the HV and the Pacific lithosphere. This observation provides strong evidence against generation of HV as melts derived from the Pacific lithosphere, whether this be recent or old (100 Ma). The depleted component present in the HV is unlike any MORB-type mantle and most likely represents material thermally entrained by the upwelling Hawaiian plume and sampled only during the rejuvenated stage. The Kalihi component is predominant in the main shield building stage lavas but is also present in the rejuvenated HV. Thus this material is sampled throughout the evolution of the volcano as it moves from the center (main shield-building stage) to the periphery (rejuvenated stage) of the plume. The presence of a plume-derived material in the rejuvenated stage has significant implications for Hawaiian mantle plume melting models. © 2007 Elsevier B.V. All rights reserved.
cited By 71
Keywords: Lithosphere; Mantle plumes; Radial zonation; Isotopes; Lithology; Melting; Seismology; Volcanic rocks; Volcanoes; basalt; igneous geochemistry; isotopic composition; lava; lead isotope; mantle plume; neodymium isotope; Ocean Drilling Program; strontium isotope; volcanic rock; Hawaii [United States]; Hawaiian Islands; Koolau; North America; Oahu; Pacific islands; Pacific Ocean; United States

Orthogonal to oblique rifting: Effect of rift basin orientation in the evolution of the North basin, Malawi Rift, East Africa - Mortimer, E.; Paton, D.A.; Scholz, C.A.; Strecker, M.R.; Blisniuk, P. (Basin Research, 2007)
The East African Rift system has long been considered the best modern example of the initial stages of continental rifting. The Malawi Rift is characteristic of the western branch of the East African Rift system, composed of half-grabens of opposing asymmetry along its length. There are striking similarities between basins within the Malawi Rift, and others along the western branch. Each exhibits similar bathymetry, border-fault length, rift zone width and fault segment length. The North Basin of the Malawi Rift differs from others in the rift only in its orientation: trending NW-SE as opposed to N-S. Although there is general agreement as to the geometry of the Malawi Rift; debate as to the amount of strike-slip vs. dip-slip deformation and the influence of underlying Pan-African foliation remains. This study presents new data from a closely spaced shallow [2 s two-way travel time (TWT)] seismic reflection data set integrated with basin-scale deeper (6 s TWT) seismic reflection data that document the structural evolution of the border and intra-basin faults. These data reveal that the different trend of the North Basin, most likely to have been influenced by the underlying Pan-African foliation, has played an extremely important role in the structural style of basin evolution. The border-fault and intra-basin structures nucleated during extension that was initially orthogonal (ENE). During this time (>8.6 to ∼0.5-0.4 Ma) intra-basin faults synthetic to the west-dipping border-fault nucleated, whereas strain was localised on the segmented border-fault early on. A later rotation of extension orientation (to NW) led to these established faults orienting oblique to rifting. This generated an overall dextral strike-slip setting that led to the development of transfer faults adjacent to the border-fault, and the generation of flower structures and folds over the greater displacement intra-basin faults. © © 2007 Blackwell Publishing Ltd.
cited By 57
Keywords: basin evolution; displacement; rift zone; rifting; seismic data; strike-slip fault; Africa; East Africa; Malawi; Sub-Saharan Africa

Overview and significance of a 250 ka paleoclimate record from El'gygytgyn Crater Lake, NE Russia - Brigham-Grette, J.; Melles, M.; Minyuk, P. (Journal of Paleolimnology, 2007)
Sediment piston cores from Lake El'gygytgyn (67°N, 172°E), a 3.6 million year old meteorite impact crater in northeastern Siberia, have been analyzed to extract a multi-proxy millennial-scale climate record extending to nearly 250 ka, with distinct fluctuations in sedimentological, physical, biochemical, and paleoecological parameters. Five major themes emerge from this research. First the pilot cores and seismic data show that El'gygytygn Crater Lake contains what is expected to be the longest, most continuous terrestrial record of past climate change in the entire Arctic back to the time of impact. Second, processes operating in the El'gygytygn basin lead to changes in the limnogeology and the biogeochemistry that reflect robust changes in the regional climate and paleoecology over a large part of the western Arctic. Third, the magnetic susceptibility and other proxies record numerous rapid change events. The recovered lake sediment contains both the best-resolved record of the last interglacial and the longest terrestrial record of millennial scale climate change in the Arctic, yielding a high fidelity multi-proxy record extending nearly 150,000 years beyond what has been obtained from the Greenland Ice Sheet. Fourth, the potential for evaluating teleconnections under different mean climate states is high. Despite the heterogeneous nature of recent Arctic climate change, millennial scale climate events in the North Atlantic/Greenland region are recorded in the most distal regions of the Arctic under variable boundary conditions. Finally, deep drilling of the complete depositional record in Lake El'gygytgyn will offer new insights and, perhaps, surprises into the late Cenozoic evolution of Arctic climate. © 2006 Springer Science+Business Media B.V.
cited By 67
Keywords: arctic environment; climate change; core analysis; crater lake; paleoclimate; paleolimnology; proxy climate record; seismic data; Chukchi; Eurasia; Lake El' gygytgyn; Russian Federation

Petrological and seismic studies of the lithosphere in the earthquake swarm region Vogtland/NW Bohemia, central Europe - Geissler, W.H.; K{ampf, H.; Seifert, W.; Dulski, P. (Journal of Volcanology and Geothermal Research, 2007)
New petrological and geochemical data of upper mantle and lower crustal xenoliths from a Quaternary tephra deposit in M'ytina, Czech Republic, are discussed in the frame of previous geophysical results (receiver functions, reflection seismology) of the western Eger/Ohře Rift area. The Vogtland/NW Bohemia region is well known for intraplate earthquake swarms, which are usually associated with volcanic activity. As previously reported, 3He/4He data of CO2 emissions in mofettes and mineral-water springs point at ongoing magmatic processes in this area. Using teleseismic P receiver functions, an approximately 40-km-wide Moho updoming (from 31 to 27 km) and indications for a seismic discontinuity at 50 to 60 km depth were observed beneath the active CO2-degassing field. The studied xenolith suite probes a lithospheric profile within the structural and gas geochemical anomaly field of the western Eger Rift. With regard to texture, composition, p-T estimates and origin, five xenolith groups can be discriminated. Upper crustal xenoliths (quartzites, phyllites, mica schists) resemble crystalline country rocks at surface. One noritic xenolith (6 kbar, 800 °C) could represent a sample of the lower crust. Clinopyroxenites and hornblendites probably represent cumulates of the nephelinitic magma or fragments of magmatic veins. Porous wehrlites and one hornblende peridotite xenolith reflect a metasomatied upper mantle. Megacrysts of Ti-rich amphibole, olivine, clinopyroxene, and phlogopite could be fragments of pegmatitic veins or high-pressure phenocrysts. Most of the ultramafic nodules (xenoliths and megacrysts) formed at pressures between 6 and 11 kbar (22 to 38 km depth), at temperatures well above regional geotherms of the Bohemian Massif calculated from surface heat flow studies. Orthopyroxene-bearing spinel-lherzolite xenoliths were not observed. Our petrographical, geochemical, and thermobarometric results indicate a lithospheric mantle strongly altered by magmatic processes. This metasomatism can cause slower than typical uppermost-mantle seismic velocities in a greater area and might help to explain observed seismic anomalies. © 2006 Elsevier B.V. All rights reserved.
Keywords: Bohemia; Central Europe; Czech Republic; Eger; Eurasia; Europe; Germany; Heves; Hungary; Vogtland; Barometers; Earthquakes; Geothermal fields; Lithology; Petrology; Seismology; Thermometers; Volcanic rocks; earthquake swarm; lithosphere; mantle chemistry; Moho; petrology; Quaternary; seismic velocity; teleseismic wave; tephra; upper mantle; xenolith; Earthquake swarm region; Lithosphere; Quaternary volcanism; Seismic structure; Volcanoes

Possible reasons of shock melt deficiency in the Bosumtwi drill cores - Artemieva, N. (Meteoritics and Planetary Science, 2007)
Pre-drilling numerical modeling of the Bosumtwi impact event predicted a 200 m thick coherent melt layer, as well as abundant highly shocked target material within the central part of the crater structure. However, these predictions are in disagreement with data from drill core obtained in 2004-2005. Here I provide a brief overview of previous results and discuss possible reasons behind melt deficiency, such as specific impact scenarios (low impact velocity and/or low impact angle), and specific target properties (different composition, high porosity, high content of volatiles). I conclude that the most likely explanation is the dispersion of impactites due to the vaporization of pore water, which was not included in the original numerical model. © The Meteoritical Society, 2007. Printed in USA.
cited By 23

Petrophysical and paleomagnetic data of drill cores from the Bosumtwi impact structure, Ghana - Elbra, Tiiu; Kontny, Agnes; Pesonen, Lauri J.; Schleifer, Norbert; Schell, Christina (Meteoritics and Planetary Science, 2007)
Physical properties from rocks of the Bosumtwi impact structure, Ghana, Central Africa, are essential to understand the formation of the relatively young (1.07 Ma) and small (10.5 km) impact crater and to improve its geophysical modeling. Results of our petrophysical studies of deep drill cores LB-07A and LB-08A reveal distinct lithological patterns but no depth dependence. The most conspicuous difference between impactites and target lithologies are the lower bulk densities and significantly higher porosities of the suevite and lithic breccia units compared to meta-graywacke and metapelites of target lithologies. Magnetic susceptibility shows mostly paramagnetic values (200-500 × 10-6 SI) throughout the core, with an exception of a few metasediment samples, and correlates positively with natural remanent magnetization (NRM) and Q values. These data indicate that magnetic parameters are related to inhomogeneously distributed ferrimagnetic pyrrhotite. The paleomagnetic data reveals that the characteristic direction of NRM has shallow normal (in a few cases shallow reversed) polarity, which is in agreement with the Lower Jaramillo N-polarity chron direction, and is carried by ferrimagnetic pyrrhotite. However, our study has not revealed the expected high magnetization body required from previous magnetic modeling. Furthermore, the LB-07A and LB08-A drill cores did not show the predicted high content of melt in the rocks, requiring a new interpretation model for magnetic data. © The Meteoritical Society, 2007. Printed in USA.

Physical property measurements: ICDP boreholes LB-07A and LB-08A, Lake Bosumtwi impact structure, Ghana - Morris, William A.; Ugalde, Hernan; Clark, Christina (Meteoritics and Planetary Science, 2007)
Physical rock property measurements provide the primary constraints for any geological models hypothesized from geophysical observations. Previous geophysical models of the Bosumtwi impact structure hypothesized that a highly magnetic and dense impact-melt sheet might be the source of the observed magnetic anomalies. However, magnetic susceptibility and density measurements made on International Continental Scientific Drilling Program (ICDP) cores LB-07A and LB-08A from the interior of the Bosumtwi meteorite impact structure contain no evidence for that. Both density and magnetic susceptibility logs on both boreholes exhibit low-amplitude contrasts between the uppermost polymict lithic breccia and suevite, the intermediate monomict lithic breccia, and the lowermost bedrock. The depth extent of fracture-related density reduction is much greater at LB-08A than at LB-07A. A total magnetic intensity log from borehole LB-08A supports the suggestion that magnetic anomalies over Lake Bosumtwi are mainly sourced in undetected and/or covered bedrock intrusions, like the ones outcropping at the northeast and to the southwest of the lake. © The Meteoritical Society, 2007. Printed in USA.

Phytoplankton response to climate changes in Lake Baikal during the Holocene and Kazantsevo Interglacials assessed from sedimentary pigments - Fietz, Susanne; Nicklisch, Andreas; Oberh{ansli, Hedi (Journal of Paleolimnology, 2007)
Lake Baikal, an ancient pristine lake in Siberia, has accumulated sediment deposits that span 25 million years. These deposits have the potential to provide a long-term record of climate changes and their interaction with the ecology of the lake. In order to investigate whether sedimentary phytoplankton pigments could be used to reconstruct past changes in total phytoplankton abundance and productivity, we compared the spatial variability in sedimentary pigment distributions in Holocene cores from three separate regions of the lake; Vidrino in the south, Posolski on Selenga Delta and Continent Ridge in the north. Furthermore, we present the first continuous sedimentary pigment and organic carbon sequence of the Kazantsevo interglacial (roughly a time equivalent to the European Eemian, and Marine Isotopic Stage MIS5e) at a resolution of approximately 150 years. Results of the spatial study showed marked differences in the sediment pigment deposition. Lowest chlorophyll a plus its degradation products versus organic carbon ratios (Chlas/TOC) indicating lowest production, but highest variability with time (indicating strongest climatic oscillations) were found at Continent Ridge. The study of sedimentary pigments deposited during the last two interglacial periods at Continent Ridge showed Chlas/TOC ratios 50-1000 times higher during the Kazantsevo Interglacial compared to the glacial periods, whereas the TOC content was only five times higher, thus indicating the significance of the Chlas/TOC ratio for the reconstruction of the phytoplankton abundance and productivity. Strong oscillations occurred during the Kazantsevo Interglacial within centennial time scales. Chlorophyllb plus its degradation products provided additional information on the past development of Chlorophyceae. Highest Chlas/TOC ratios were found during the early Holocene at approximately 9 kyr BP. Indications of short phytoplankton production maxima were also found during the late Atlantic (6 kyr BP) and at the Subboreal/Subatlantic transition (3 kyr BP). From this we conclude that sedimentary chlorophyll a is a reliable indicator of phytoplanktonic response to climate changes and may serve for validation of future climate scenarios in continental regions. © 2006 Springer Science+Business Media B.V.
Keywords: Buryatia; Eurasia; Lake Baikal; Russian Federation; Selenga Delta; Bacillariophyta; Chlorophyceae; abundance; chlorophyll; climate change; Holocene; interglacial; lacustrine deposit; organic carbon; paleoecology; phytoplankton; pigment; pristine environment

Platinum group elements provide no indication of a meteoritic component in ICDP cores from the Bosumtwi crater, Ghana - Goderis, S.; Tagle, R.; Schmitt, R.T.; Erzinger, J.; Claeys, Ph. (Meteoritics and Planetary Science, 2007)
In an attempt to identify the type of projectile, 14 samples from the Bosumtwi crater in Ghana were analyzed for platinum group element (PGE) concentrations by nickel sulfide fire assay inductively coupled plasma-mass spectrometry (ICP-MS). The majority of the samples come from the impactite material recovered by cores LB-07A and LB-08A, which were drilled by the International Continental Scientific Drilling program (ICDP). One sample originates from the fallback material found at the contact between the impactite and the overlying lake sediment in core LB-05B. No clear signature of a meteoritic contamination was identified in the 13 impactite samples. The target rock apparently dominates the PGE contribution in the impactites. These results agree with the PGE concentrations reported for the suevites collected at the crater rim and in other parts of the Bosumtwi ICDP cores. However, based on Cr and Os isotopic signatures, a meteoritic component could be present in the sample of fallback material, supporting the reports of the existence of meteoritic material in the Ivory Coast tektites. Further analyses of the fallback material from the Bosumtwi drill cores should confirm (or not) this first result. © The Meteoritical Society, 2007. Printed in USA.

Platinum group elements provide no indication of a meteoritic component in ICDP cores from the Bosumtwi crater, Ghana - Goderis, S.; Tagle, R.; Schmitt, R.T.; Erzinger, J.; Claeys, Ph. (Meteoritics and Planetary Science, 2007)
In an attempt to identify the type of projectile, 14 samples from the Bosumtwi crater in Ghana were analyzed for platinum group element (PGE) concentrations by nickel sulfide fire assay inductively coupled plasma-mass spectrometry (ICP-MS). The majority of the samples come from the impactite material recovered by cores LB-07A and LB-08A, which were drilled by the International Continental Scientific Drilling program (ICDP). One sample originates from the fallback material found at the contact between the impactite and the overlying lake sediment in core LB-05B. No clear signature of a meteoritic contamination was identified in the 13 impactite samples. The target rock apparently dominates the PGE contribution in the impactites. These results agree with the PGE concentrations reported for the suevites collected at the crater rim and in other parts of the Bosumtwi ICDP cores. However, based on Cr and Os isotopic signatures, a meteoritic component could be present in the sample of fallback material, supporting the reports of the existence of meteoritic material in the Ivory Coast tektites. Further analyses of the fallback material from the Bosumtwi drill cores should confirm (or not) this first result. © The Meteoritical Society, 2007. Printed in USA.
cited By 17

Pliocene to Holocene geomorphic evolution and paleogeography of the El'gygytgyn Lake region, NE Russia - Glushkova, O.Yu.; Smirnov, V.N. (Journal of Paleolimnology, 2007)
Geomorphic, lithologhic, and stratigraphic field studies as well as pollen data and mineralogical study have been used to propose Pliocene and Pleistocene paleogeographic reconstructions of the El'gygytgyn meteorite crater area. The moment of impact is recorded above the early Pliocene hill denudation plain as a chaotic horizon consisting of fragments of impactite rocks. This chaotic horizon lies between layers of late Pliocene alluvial sediments. During the second half of the late Pliocene, the region was tectonically active, when the Anadyr lowland was uplifted causing alluvial sediments to accumulate in the basins to the south of the crater. Regional climatic cooling, which supported the spread of tundra and the formation of permafrost is characteristically to late Pliocene. The 35-40 m high terrace that roughly follows the 530 m contour interval along the Enmyvaam River formed during the middle Pleistocene. This terrace represents the maximum lake level. Erosion and incision of the upper Enmyvaam River increased due to another wave of uplift. Additionally, El'gygytgyn Lake discharge increased causing lake level to begin to drop in the Middle Pleistocene. Cooling continued, which led to the development of herb-dominated arctic tundra. middle and late Pleistocene glaciations did not reach the El'gygytgyn lake region. The 9-11 m high lacustrine terrace was formed around the lake during the late Pleistocene and the 2-3 m high lacustrine terrace formed later during the Holocene. During the last 5000 years, the lake level has continued to drop as the modern coastline developed. © 2006 Springer Science+Business Media B.V.
cited By 48
Keywords: chronology; crater lake; Holocene; lacustrine environment; lake evolution; lake level; paleogeography; Pleistocene; Pliocene; pollen; terrace; Chukchi; Eurasia; Lake El' gygytgyn; Russian Federation; Zanclea

Possible reasons of shock melt deficiency in the Bosumtwi drill cores - Artemieva, N. (Meteoritics and Planetary Science, 2007)
Pre-drilling numerical modeling of the Bosumtwi impact event predicted a 200 m thick coherent melt layer, as well as abundant highly shocked target material within the central part of the crater structure. However, these predictions are in disagreement with data from drill core obtained in 2004-2005. Here I provide a brief overview of previous results and discuss possible reasons behind melt deficiency, such as specific impact scenarios (low impact velocity and/or low impact angle), and specific target properties (different composition, high porosity, high content of volatiles). I conclude that the most likely explanation is the dispersion of impactites due to the vaporization of pore water, which was not included in the original numerical model. © The Meteoritical Society, 2007. Printed in USA.

Search for a meteoritic component in drill cores from the Bosumtwi impact structure, Ghana: Platinum group element contents and osmium isotopic characteristics - McDonald, Iain; Peucker-Ehrenbrink, Bernhard; Coney, Louise; Ferrière, Ludovic; Reimold, Wolf Uwe; Koeberl, Christian (Meteoritics and Planetary Science, 2007)
An attempt was made to detect a meteoritic component in both crater-fill (fallback) impact breccias and fallout suevites (outside the crater rim) at the Bosumtwi impact structure in Ghana. Thus far, the only clear indication for an extraterrestrial component related to this structure has been the discovery of a meteoritic signature in Ivory Coast tektites, which formed during the Bosumtwi impact event. Earlier work at Bosumtwi indicated unusually high levels of elements that are commonly used for the identification of meteoritic contamination (i.e., siderophile elements, including the platinum group elements [PGE]) in both target rocks and impact breccias from surface exposures around the crater structure, which does not allow unambiguous verification of an extraterrestrial signature. The present work, involving PGE abundance determinations and Os isotope measurements on drill core samples from inside and outside the crater rim, arrives at the same conclusion. Despite the potential of the Os isotope system to detect even small amounts of extraterrestrial contribution, the wide range in PGE concentrations and Os isotope composition observed in the target rocks makes the interpretation of unradiogenic, high-concentration samples as an impact signature ambiguous. © The Meteoritical Society, 2007. Printed in USA.

The pollen record from El'gygytgyn Lake: Implications for vegetation and climate histories of northern Chukotka since the late middle Pleistocene - Lozhkin, A.V.; Anderson, P.M.; Matrosova, T.V.; Minyuk, P.S. (Journal of Paleolimnology, 2007)
Three types of pollen assemblages (shrub-dominated, mixed herb- and shrub-dominated, and herb-dominated) characterize the ∼300,000 year palynological record from El'gygytgyn Lake. Despite major changes in global climatic forcings, all pollen spectra, with a few isolated exceptions, have strong to possible analogs in the modern plant communities of Northeast Siberia and Alaska. Paleoclimatic reconstructions based on squared chord-distance analog analyses indicate two periods (∼8600-10,700 14C year B.P. and OIS 5e) when summers were perhaps ∼2 to 4°C warmer than modern. January temperatures were also warmer than present, and both July and January were wetter than today. Palynological data remain inconclusive as to the establishment of forests near El'gygytgyn Lake at these times. The wettest Julys occurred during OIS 5 d. July temperatures were near modern, and Januarys were colder and drier than now. January temperatures, even into the Middle Pleistocene, generally show little variability, suggesting that the suppression of arboreal taxa during glaciations was likely caused by cool summers with low effective moisture and not by frigid winters. Because age schemes that correlate magnetic susceptibility to variations in summer insolation or ∂18O have cool plant taxa persisting in warm times (and vice versa), we propose an alternative age model based on the palynological data. © 2006 Springer Science+Business Media B.V.
cited By 108
Keywords: paleoclimate; paleoecology; palynology; Pleistocene; pollen; vegetation history; Chukchi; Eurasia; Lake El' gygytgyn; Russian Federation

Sedimentology, clay mineralogy and grain-size as indicators of 65 ka of climate change from El'gygytgyn Crater Lake, Northeastern Siberia - Asikainen, C.A.; Francus, P.; Brigham-Grette, J. (Journal of Paleolimnology, 2007)
El'gygytgyn Crater Lake, NE Siberia was investigated for sedimentological proxies for regional climate change with a focus on the past 65 ka. Sedimentological parameters assessed relative to magnetic susceptibility include stratigraphy, grain size, clay mineralogy and crystallinity. Earlier work suggests that intervals of high susceptibility in these sediments are coincident with warmer (interglacial-like) conditions and well-mixed oxygenated bottom waters. In contrast, low susceptibility intervals correlate with cold (glacial-like) conditions when perennial ice-cover resulted in anoxia and the dissolution of magnetic carrier minerals. The core stratigraphy contains both well-laminated to non-laminated sequences. Reduced oxygen and lack of water column mixing preserved laminated sequences in the core. A bioturbation index based upon these laminated and non- laminated sequences co-varies with total organic carbon (TOC) and magnetic susceptibility. Clay mineral assemblages include illite, highly inter-stratified illite/smectite, and chlorite. Under warm or hydrolyzing conditions on the landscape around the lake, chlorite weathers easily and illite/smectite abundance increase, which produces an inverse relationship in the relative abundance of these clays. Trends in relative abundance show distinct down-core changes that correlate with shifts in susceptibility. The mean grain-size (6.92 μm) is in the silt-size fraction, with few grains larger than 65 μm. Terrigenous input to the lake comes from over 50 streams that are filtered through storm berms, which limits clastic deposition into the lake system. The sedimentation rate and terrigenous input grain-size is reduced during glacial intervals. Measurements of particle-size distribution indicate that the magnetic susceptibility fluctuations are not related to grain size. Lake El'gygytgyn's magnetic susceptibility and clay mineralogy preserves regional shifts in climate including many globally recognized events like the Younger Dryas and Bolling/Allerod. The sedimentary deposits reflect the climatic transitions starting with MIS4 through the Holocene transition. This work represents the first extensive sedimentological study of limnic sediment proxies of this age from Chukotka (Fig. 1). © 2006 Springer Science+Business Media B.V.
cited By 57
Keywords: bioturbation; clay mineral; climate change; crater lake; grain size; lacustrine deposit; paleoclimate; proxy climate record; sedimentology; Chukchi; Eurasia; Lake El' gygytgyn; Russian Federation

Three-dimensional shear-wave splitting tomography in the Parkfield, California, region - Zhang, H.; Liu, Y.; Thurber, C.; Roecker, S. (Geophysical Research Letters, 2007)
We developed a three-dimensional (3D) shear-wave splitting tomography method to image the spatial anisotropy distribution by back projecting shear wave splitting delay times along ray paths derived from a 3D shear velocity model, assuming the delay times are accumulated along the ray paths. The local strength of the anisotropy is indicated by a parameter of anisotropy percentage, K. Using the shearwave splitting delay times for 575 earthquakes measured at PASO and HRSN stations, we imaged a detailed 3D anisotropy percentage model around the San Andreas Fault Observatory at Depth (SAFOD). The anisotropy percentage model shows strong heterogeneities, consistent with the strong spatial variations in both measured delay times and fast polarization directions. The San Andreas Fault (SAF) zone is highly anisotropic down to a depth of ∼4 km and then becomes less anisotropic at greater depths. Outside the fault zone, the highly anisotropic zone extends as deep as ∼7 km, consistent with the systematic depth dependence of the average time delays. To the southwest of the SAF, the Salinian granitic block shows relatively strong anisotropic: anomalies that are presumably caused by aligned microcracks consistent with the direction of the regional maximum compressive horizontal stress. To the northeast of the fault zone, a strong anisotropic anomaly between depths ∼2 and ∼4 km corresponds to a serpentinite body sandwiched between Franciscan rocks. Copyright 2007 by the American Geophysical Union.
cited By 39
Keywords: Anisotropy; Compressive stress; Mathematical models; Microcracks; Shear waves; Three dimensional; Tomography; Velocity; Shear wave splitting delay; Shear-wave splitting tomography; Spatial anisotropy distribution; Seismic waves; anisotropy; fault zone; imaging method; microcrack; polarization; ray tracing; S-wave; San Andreas Fault; seismic tomography; spatial variation; wave splitting; California; North America; Parkfield; United States

The Cretaceous Songliao Basin: Volcanogenic Succession, Sedimentary Sequence and Tectonic Evolution, NE China - Pujun, WANG; Xiao'an, XIE; FRANK, Mattern; Yanguang, REN; Defeng, ZHU; Xiaomeng, SUN (Acta Geologica Sinica - English Edition, 2007)
Abstract: The Songliao basin (SB) is a superposed basin with two different kinds of basin fills. The lower one is characterized by a fault-bounded volcanogenic succession comprising of intercalated volcanic, pyroclastic and epiclastic rocks. The volcanic rocks, dating from 110 Ma to 130 Ma, are of geochemically active continental margin type. Fast northward migration of the SB block occurred during the major episodes of the volcanism inferred from their paleomagnetic information. The upper one of the basin fill is dominated by non-marine sag-style sedimentary sequence of siliciclastics and minor carbonates. The basin center shifted westwards from the early to late Cretaceous revealed by the GGT seismic velocity structure suggesting dynamic change in the basin evolution. Thus, a superposed basin model is proposed. Evolution of the SB involves three periods including (1) Alptian and pre-Aptian: a retroarc basin and range system of Andes type related to Mongolia-Okhotsk collisional belt (MOCB); (2) Albian to Companian: a sag-like strike-slip basin under transtension related to oblique subduction of the Pacific plate along the eastern margin of the Eurasian plate; (3) since Maastrichtian: a tectonic inverse basin under compression related to normal subduction of the Pacific plate under the Eurasian plate, characterized by overthrust, westward migration of the depocenter and eastward uplifting of the basin margin.
Keywords: Cretaceous superposed Songliao basin; volcanic rocks; sedimentary sequence; tectonic evolution; Mongolia-Okhotsk collisional belt; Pacific and Eurasian plates; retroarc strike-slip tectonic-inverse basins

The ICDP Lake Bosumtwi impact crater scientific drilling project (Ghana): Core LB-08A litho-log, related ejecta, and shock recovery experiments - Deutsch, Alexander; Luetke, Sabine; Heinrich, Volker (Meteoritics and Planetary Science, 2007)
The 1.07 Myr old Lake Bosumtwi impact crater in Ghana was drilled within the framework of the International Continental Scientific Drilling Project (ICDP). Hole LB-08A, drilled into the outer flank of the central uplift and with a total depth of 451 m, yielded 215.71 m of impact-related rocks. This paper summarizes observations of the lithological logging on core LB-08A. Between a depth of 235.6 and ∼260 m, the section consists of a melt-bearing allochthonous, polymict, and mostly clast-supported impact breccia. Down to ∼418 m, the section comprises a rather uniform unit of meta-graywacke alternating with phyllite to slate (lower greenschist facies); few (par-) autochthonous impact breccia bodies and rare impact dike breccias are present. The lowermost part of the section contains several centimeter- to decimeter-thick melt-bearing breccia dikes in country rocks identical to those occurring above. Omnipresent fracturing was mapped in a qualitative manner. Most prominent shock effects in the uplifted target rocks comprise planar fractures and deformation elements in quartz and polysynthetic twinning in carbonate minerals; the maximum shock pressure as evidenced by quartz is below 26 GPa. The allochthonous breccias occasionally contain a few vol% of melt particles. Suevites occur outside the crater rim, carrying diaplectic crystals, coesite, and ballen quartz as well as true melt glasses and a variety of lithic clasts, among those spectacular staurolite-rich mica-schists. The recorded shock level in the uplifted target rocks is lower than expected and modeled. Shock recovery experiments with analogue carbonaceous graywackes at 34 and 39.5 GPa yielded nearly complete transformation of quartz into diaplectic glass. We therefore exclude a specific shock behavior of the soft, fluid-rich target material (carbonaceous graywackes, shales, slates) in core LB-08A as the prime or only reason for the melt deficit and the generally low shock levels recorded inside the Lake Bosumtwi impact crater. © The Meteoritical Society, 2007. Printed in USA.

The Lake Bosumtwi impact structure in Ghana: A brief environmental assessment and discussion of ecotourism potential - Boamah, Daniel; Koeberl, Christian (Meteoritics and Planetary Science, 2007)
Lake Bosumtwi is a natural inland freshwater lake that originated from a meteorite impact. The lake is becoming a popular tourist attraction in Ghana and has the potential to be developed as an ecotourism site in the future. However, there have been some unregulated human activities and unplanned infrastructure development, and there are increased levels of pollutants in the lake water. In order to make ecotourism at Lake Bosumtwi successful in the long term, the Lake Bosumtwi Development Committee has been formed to ensure that local people are empowered to mobilize their own capacities. It has been realized that an important criterion required to develop ecotourism in a socially responsible, economically efficient, and environmentally viable way is to foster a constructive dialogue between the local people and tourists about the needs of the indigenous people. © The Meteoritical Society, 2007. Printed in USA.

The Lake Bosumtwi meteorite impact structure, Ghana - A magnetic image from a third observational level - Ugalde, Hernan; Morris, William A.; Clark, Christina; Miles, Brett; Milkereit, Bernd (Meteoritics and Planetary Science, 2007)
The Bosumtwi impact structure in Ghana is the youngest and best-preserved medium-sized impact structure on Earth, and because of the vast amount of prior geophysical and geological data gathered in the area, it constitutes a great natural laboratory to try to develop new geophysical interpretation and modeling techniques. During the 2004 International Continental Scientific Drilling Program (ICDP) drilling campaign at Lake Bosumtwi, we made magnetic field observations at 162 stations around the lake. This study differs from all previous magnetic surveys at Bosumtwi, which only measured the scalar portion of the Earth's magnetic field, in that we measured the full magnetic vector at each station. Acquisition of the full magnetic vector was made possible by innovative use of a borehole deviation probe, which uses a magnetic sensor for absolute orientation reference. Estimates of the magnetic vector orientation and magnitude at each observation station were derived from a series of measurements collected at 50 cm spacing over a depth range of 25 m. In this study, we report a comparison between the scalar total field intensity derived from this new survey approach with the other two previously acquired marine and airborne magnetic data sets. The scalar total magnetic intensity (TMI) computed from the vector data set compares in close agreement with the other two data sets. Some discrepancies between the data sets can be explained by differences in the distances between the sensor and the magnetic sources for the various surveys. The highlight of this study is that we demonstrate that is possible to acquire at least partial vector data with readily available instrumentation. © The Meteoritical Society, 2007. Printed in USA.

The Lake Bosumtwi meteorite impact structure, Ghana - Where is the magnetic source? - Ugalde, Hernan; Morris, William A.; Pesonen, Lauri J.; Danuor, Sylvester K. (Meteoritics and Planetary Science, 2007)
The Bosumtwi impact structure (Ghana) is a young and well-preserved structure where a vast amount of information is available to constrain any geophysical model. Previous analysis of the airborne magnetic data and results of numerical simulation of impact predicted a strongly magnetic impact-melt body underneath the lake. Recent drilling through the structure did not penetrate such an expected impact-melt rock magnetic source. A new 3-D magnetic model for the structure was constructed based on a newly acquired higher-resolution marine magnetic data set, with consideration of the observed gravity data on the lake, previous seismic models, and the magnetic properties and lithology identified in the two International Continental Scientific Drilling Program (ICDP) deep boreholes. The new model contains highly magnetic bodies located in the northeast sector of the structure, not centered onto the drilling sites. As in previous models, higher magnetization than that measured in outcropping impactites had to be assigned to the unexposed source bodies. Integration of the new model with the borehole petrophysics and published geology indicates that these bodies likely correspond to an extension to the south of the Kumasi batholith, which outcrops to the northeast of the structure. The possibility that these source bodies are related to the seismically identified central uplift or to an unmapped impact-melt sheet predicted by previous models of the structure is not supported. Detailed magnetic scanning of the Kumasi batholith to the north, and the Bansu intrusion to the south, would provide a test for this interpretation. © The Meteoritical Society, 2007. Printed in USA.

The Role of Fault Zone Drilling - Zoback, M.D.; Hickman, S.; Ellsworth, W. (Treatise on Geophysics, 2007)
The objective of fault-zone drilling projects is to directly study the physical and chemical processes that control deformation and earthquake generation within active fault zones. An enormous amount of field, laboratory, and theoretical work has been directed toward the mechanical and hydrological behavior of faults over the past several decades. Nonetheless, it is currently impossible to differentiate between - or even adequately constrain - the numerous conceptual models of active faults proposed over the years. For this reason, the Earth science community is left in the untenable position of having no generally accepted paradigm for the mechanical behavior of faults at depth. One of the primary causes for this dilemma is the difficulty of either directly observing or inferring physical properties and deformation mechanisms along faults at depth, as well as the need to observe directly key parameters such as the state of stress acting on faults at depth, pore fluid pressure (and its possible variation in space and time), and processes associated with earthquake nucleation and rupture. Today, we know very little about the composition of active faults at depth, their constitutive properties, the state of in situ stress or pore pressure within fault zones, the origin of fault-zone pore fluids, or the nature and significance of time-dependent fault-zone processes. © 2007 Elsevier B.V. All rights reserved.
cited By 38

Ultrahigh-pressure mineral assemblages in zircons from the surface to 5158 m depth in cores of the main drill hole, Chinese Continental Scientific Drilling Project, Southwestern Sulu Belt, China - Liu, Fulai; Xu, Zhiqin; Liou, J.G.; Dong, Hailiang; Xue, Huaimin (International Geology Review, 2007)
The 5158 m deep main hole of the Chinese Continental Scientific Drilling Project (CCSD-MH) at Maobei, southwestern Sulu Belt, penetrated five major lithologic units: eclogite, amphibolite ± retrograde eclogite, ultramafic rock, paragneiss, and orthogneiss. All analyzed samples are overprinted to various extents by amphibolite-facies retrograde metamorphism. Ultrahigh-pressure (UHP) index minerals are preserved as inclusions in zircons separated from 137 core samples. These index assemblages include: Coe + Phe + Grt from Grt-Ep-Bt paragneiss; Coe + Jd + Grt + Ap, Coe + Jd + Phe + Ap and Coe + Grt + Jd + Phe from Grt-Ep-Hbl paragneiss; Coe + Phe, Coe + Ky + Ap, and Coe + Ky + Ttn from orthogneiss; and Coe + Grt + Omp, Coe + Grt + Phe, Coe + Omp + Rt, and Coe + Grt + Omp + Phe from Bt-Ep amphibolite and other retrograde eclogites. The common occurrences of these UHP assemblages throughout the section indicate that the drill hole did not penetrate through the UHP tectonic slice. All of the UHP mineral assemblages, whether from retrograde ecologites or from the surrounding gneisses, record similar metamorphic conditions of 773-843°C and 3.2-4.0 GPa. Zircons separated from surface samples and cores from other shallow holes in the area also contain similar coesite-bearing UHP mineral inclusions. This widespread occurrence of UHP assemblages in the southwestern Sulu area suggests that voluminous continental materials were subducted and metamorphosed at mantle depths > 100 km, and then were rapidly exhumed to crustal levels. Copyright © 2007 by V. H. Winston & Son, Inc. All rights reserved.
Keywords: Asia; China; Eurasia; Far East; Sulu Belt; amphibolite facies; crustal structure; eclogite; exhumation; fluid inclusion; Ocean Drilling Program; orthogneiss; retrograde metamorphism; subduction; ultrahigh pressure metamorphism; ultramafic rock; zircon

The COral-REef Front (COREF) Project - Iryu, Y.; Matsuda, H.; Machiyama, H.; Piller, W. E.; Quinn, T. M.; Mutti, M. (Scientific Drilling, 2007)

Upper mantle beneath the Eger Rift (Central Europe): Plume or asthenosphere upwelling? - Plomerova, Jaroslava; Achauer, Ulrich; Babuška, Vladislav; Vecsey, Ludečk (Geophysical Journal International, 2007)
We present the first results of a high-resolution teleseismic traveltime tomography and seismic anisotropy study of the lithosphere-asthenosphere system beneath the western Bohemian Massif. The initial high-resolution tomography down to a depth of 250 km did not image any columnar low-velocity anomaly which could be interpreted as a mantle plume anticipated beneath the Eger Rift, similar to recent findings of small plumes beneath the French Massif Central and the Eifel in Germany. Alternatively, we interpret the broad low-velocity anomaly beneath the Eger Rift by an upwelling of the lithosphere-asthenosphere transition. We also map lateral variations of seismic anisotropy of the mantle lithosphere from spatial variations of P-wave delay times and the shear wave splitting. Three major domains characterised by different orientations of seismic anisotropy correspond to the major tectonic units - Saxothuringian, Moldanubian and the Tepl'a-Barrandian - and their fabrics fit to those found in our previous studies of mantle anisotropy on large European scales. © 2007 The Authors Journal compilation © 2007 RAS.
Keywords: Bohemian Massif; Central Europe; Eurasia; Europe; asthenosphere; lithosphere; P-wave; rift zone; seismic anisotropy; seismic tomography; teleseismic wave; travel time; upper mantle

Uppermost impact fallback layer in the Bosumtwi crater (Ghana): Mineralogy, geochemistry, and comparison with Ivory Coast tektites - Koeberl, Christian; Brandst{atter, Franz; Glass, Billy P.; Hecht, Lutz; Mader, Dieter; Reimold, Wolf Uwe (Meteoritics and Planetary Science, 2007)
In 2004, an International Continental Scientific Drilling Program (ICDP) drilling project at the Bosumtwi impact crater, Ghana (10.5 km in diameter, 1.07 Myr old), was performed to study the sediments that fill the lake as well as the underlying impactites. In one (LB-05) of 16 cores drilled into the lake sediments, the zone between the impact breccias and the post-impact sediments was penetrated, preserving the final, fine-grained impact fallback layer. This ∼30 cm thick layer contains in the top 10 cm. accretionary lapilli, microtektite-like glass spherules, and shocked quartz grains. Glass particles - mostly of splash form less than 1 mm size - make up the bulk of the grains (∼70-78% by number) in the coarser size fraction (> 125 μm) of the top of the fallback layer. About one-third of all quartz grains in the uppermost part of the layer are shocked, with planar deformation features (PDFs); almost half of these grains are highly shocked, with 3 or more sets of PDFs. K-feldspar grains also occur and some show shock deformation. The abundance of shocked quartz grains and the average shock level as indicated by the number of sets of PDFs, for both quartz and K-feldspar, decrease with depth into the layer. The well-preserved glass spherules and fragments are chemically rather homogeneous within each particle, and also show relatively small variations between the various particles. On average, the composition of the fallback spherules from core LB-5B is very similar to the composition of Ivory Coast tektites and microtektites, with the exception of CaO contents, which are about 1.5 to 2 times higher in the fallback spherules. This is a rare case in which the uppermost fallback layer and the transition to the post-impact sediments has been preserved in an impact structure; its presence indicates that the impactite sequence at Bosumtwi is complete and that Bosumtwi is a very well-preserved impact crater. © The Meteoritical Society, 2007. Printed in USA.

Using drill cutting separates to estimate the strength of narrow shear zones at SAFOD - Morrow, C.; Solum, J.; Tembe, S.; Lockner, D.; Wong, T.-F. (Geophysical Research Letters, 2007)
A technique is presented for estimating frictional strength of narrow shear zones based on hand selection of drillhole cuttings separates. Tests were conducted on cuttings from the SAFOD scientific drillhole near Parkfield, California. Since cuttings are mixed with adjacent material as they travel up the drillhole, these fault-derived separates give a better representation of the frictional properties of narrow features than measurements from the bulk material alone. Cuttings from two shear zones (one an active trace of the San Andreas fault) contain a significant weight percent of clay-rich grains that exhibit deformation-induced slickensides. In addition, cuttings from the active SAF trace contain around 1% serpentine. Coefficients of friction for clay-rich and serpentine grains were 0.3-0.5 and 0.4-0.45, respectively. These values are around 0.12 lower than the friction coefficient of the corresponding bulk cuttings, providing an improved estimate of the frictional strength of the San Andreas fault. Copyright 2007 by the American Geophysical Union.
cited By 30
Keywords: Drilling; Friction; Shear strength; Friction coefficient; Frictional strength; Shear zone; Tectonics; deformation; estimation method; San Andreas Fault; serpentine; shear zone; strength; California; North America; Parkfield; United States

Vibration characteristic analysis and information transmission test of bottom drilling tool (in Chinese with English abstract);[松科1井南孔钻井取心技术] - Sun, Q.; Shen, H.; Yang, X.; Yuan, F. (Oil Drilling & Production Technology, 2007)

Zoned Zircon from Eclogite Lenses in Marbles from the Dabie-Sulu UHP Terrane, China: A Clear Record of Ultra-deep Subduction and Fast Exhumation - Fulai, LIU; Gerdes, A.; ROBINSON, P. T.; Huaimin, XUE; Jianguo, YE (Acta Geologica Sinica - English Edition, 2007)
Abstract: Eclogite lenses in marbles from the Dabie-Sulu ultrahigh-pressure (UHP) terrane are deeply subducted meta-sedimentary rocks. Zircons in these rocks have been used to constrain the ages of prograde and UHP metamorphism during subduction, and later retrograde metamorphism during exhumation. Inherited (detrital) and metamorphic zircons were distinguished on the basis of transmitted light microscopy, cathodoluminescence (CL) imaging, trace element contents and mineral inclusions. The distribution of mineral inclusions combined with CL imaging of the metamorphic zircon make it possible to relate zircon zones (domains) to different metamorphic stages. Domain 1 consists of rounded, oblong and spindly cores with dark-luminescent images, and contains quartz eclogite facies mineral inclusion assemblages, indicating formation under high-pressure (HP) metamorphic conditions of T = 571 –668°C and P = 1.7-2.02 GPa. Domain 2 always surrounds domain 1 or occurs as rounded and spindly cores with white-luminescent images. It contains coesite eclogite facies mineral inclusion assemblages, indicating formation under UHP metamorphic conditions of T = 782 –849°C and P > 5.5 GPa. Domain 3, with gray-luminescent images, always surrounds domain 2 and occurs as the outermost zircon rim. It is characterized by low-pressure mineral inclusion assemblages, which are related to regional amphibolite facies retrograde metamorphism of T = 600 –710°C and P = 0.7-1.2 GPa. The three metamorphic zircon domains have distinct ages; sample H1 from the Dabie terrane yielded SHRIMP ages of 245 ± 4 Ma for domain 1, 235 ± 3 Ma for domain 2 and 215 ± 6 Ma for domain 3, whereas sample H2 from the Sulu terrane yielded similar ages of 244 ± 4 Ma, 233 ± 4 Ma and 214 ± 5 Ma for Domains 1, 2 and 3, respectively. The mean ages of these zones suggest that subduction to UHP depths took place over 10 –11 Ma and exhumation of the rocks occurred over a period of 19 –20 Ma. Thus, subduction from ∼ 55 km to > 160 km deep mantle depth took place at rates of approximately 9.5 –10.5 km/Ma and exhumation from depths >160 km to the base of the crust at −30 km occurred at approximately 6.5 km/Ma. We propose a model for these rocks involving deep subduction of continental margin Iithosphere followed by ultrafast exhumation driven by buoyancy forces after break-off of the UHP slab deep within the mantle.
Keywords: zoned zircon; SHRIMP U-Pb dating; ultra-deep subduction; fast exhumation; eclogite; lenses in marble; Dabie-Sulu UHP belt

μ-Synchrotron radiation excited X-ray fluorescence microprobe trace element studies on spherules of the Cretaceous/Tertiary boundary transitions of NE-Mexico and Haiti samples - Kramar, U.; Harting, M.; Rickers, K.; St{uben, D. (Spectrochimica Acta - Part B Atomic Spectroscopy, 2007)
Synchrotron radiation, collimated to a μm scale was used for the determination of trace elements in micro-tektites and spherule material for the first time. The experimental set-up of the SXRF microprobe at beamline L at HASYLAB at DESY offers a suitable method for performing non-destructive in situ multi-element analysis focusing on spatial trace element distributions and mineral phases of the melted ejecta material from the Cretaceous/Tertiary boundary. The spatial distribution of trace elements was determined in melt inclusions as well as in phase transitions in selected parts of chlorite-smectite spherules and tektite glass material by using a beam with a diameter of 15 μm collimated with a glass capillary for line- and area scans as well as for single point measurements for elements with Z between 19 and 92. The analyzed spherules show alteration features but also zonation and carbonate inclusions, originating from the Chicxulub impact event. These initial results demonstrate the potential of μ-SXRF analysis for the discrimination of alteration and primary signals of the spherules and re-construction of their genetic evolution. It could be shown that the spherules represent a complex mixture of different materials from the subsurface at the Chicxulub impact site. © 2007 Elsevier B.V. All rights reserved.
cited By 3

Natural resources - Elders, W.A.; Dallimore, S.R. (Continental Scientific Drilling: A Decade of Progress, and Challenges for the Future, 2007)
The vast scale of drilling worldwide for economic goals dwarfs the number of scientific drilling projects. This alone creates a strong incentive for the ICDP to collaborate with industry. Given the current world energy situation, the growing amount of drilling for unconventional energy resources is increasing opportunities for such collaboration. Two examples are discussed here, the Mallik Gas Hydrate Production Research Program, and the ongoing Iceland Deep Drilling Project. The Mallik project was collaboration between industry and government agencies that investigated gas hydrates in permafrost in Arctic Canada. Gas hydrates are a new, and potentially large, source of energy. Past periods of rapid atmospheric warming in the geologic record may have been initiated, or significantly accelerated, by dissociation of such gas hydrates. The Iceland Deep Drilling Project is another collaboration between industry and government, but it is investigating unconventional geothermal resources in Iceland. This project will drill a series of 4 to 5 km deep wells in the search for supercritical hydrothermal fluids at temperatures of 450-500 °C. It is estimated that power output from wells producing supercritical fluid will be an order of magnitude greater than produced from conventional geothermal wells. Supercritical fluids play a fundamental role in coupling magmatic and hydrothermal systems on mid-ocean ridges. In the future, many other important scientific questions could be addressed by the ICDP participating in similar natural resource drilling projects. © 2007 Springer-Verlag Berlin Heidelberg.
cited By 3

The Corinth Rift laboratory or an in situ investigation on interactions between fluids and active faults - Cornet, François Henri (Scientific Drilling, 2007)

The Chicxulub impact event and its environmental consequences at the Cretaceous-Tertiary boundary - Kring, D.A. (Palaeogeography, Palaeoclimatology, Palaeoecology, 2007)
An impact-mass extinction hypothesis for the Cretaceous-Tertiary (K/T) boundary transition has been confirmed with multiple lines of evidence, beginning with the discovery of impact-derived Ir in K/T boundary sediments and culminating in the discovery of the Chicxulub impact crater. Likewise, a link between the Chicxulub impact crater and K/T boundary sediments has been confirmed with multiple lines of evidence, including stratigraphic, petrological, geochemical, and isotopic data. The environmental effects of the Chicxulub impact event were global in their extent, largely because of the interaction of ejected impact debris with the atmosphere. The environmental consequences of the Chicxulub impact event and their association with the K/T boundary mass extinction event indicate that impact cratering processes can affect both the geologic and biologic evolution of our planet. © 2007 Elsevier B.V. All rights reserved.
cited By 161

Seismic investigation of the El'gygytgyn impact crater lake (Central Chukotka, NE Siberia): Preliminary results - Niessen, F.; Gebhardt, A.C.; Kopsch, C.; Wagner, B. (Journal of Paleolimnology, 2007)
The 12 km wide and about 175 m deep El'gygytgyn crater lake in Central Chukotka, NE Siberia, is of special interest for investigation as it could provide the first undisturbed 3.6 Ma terrestrial record from the Arctic realm, reaching back a million years before the first major glaciation of the Northern Hemisphere. A single-channel seismic survey was carried out on an expedition to the lake in 2000, in which both high resolution and deep penetration data were acquired. Seismic data suggest an impact crater structure in Cretaceous volcanic bedrock, indicated by velocities of >5000 m s-1, whose upper 500-600 m is brecciated. The lake is filled with two units of sediments, the upper one well stratified and the lower one massive. In the center of the lake, the combined thickness of the two sedimentary units is estimated to be 320-350 m. The upper unit is draped over the location of an interpreted central peak and is locally intercalated with debris flows, mainly in the western part of the lake and at the lake margins. Most of the lower unit is obscured by multiples as a result of high reflection coefficients in the upper unit. As at least the upper unit appears to be undisturbed by glaciation, the lake should yields unique information on the paleoclimatic development of the East Siberian Arctic. © 2006 Springer Science+Business Media B.V.
cited By 36
Keywords: crater lake; impact structure; lacustrine deposit; paleoclimate; seismic data; seismic survey; Chukchi; Eurasia; Lake El' gygytgyn; Russian Federation

Stress orientation evaluated from strain localisation analysis in Aigion Fault - Sulem, Jean (Tectonophysics, 2007)
Within the frame of the 'CRL' (Corinth Rift Laboratory project) [Cornet, F., Bernard, P., Moretti, I., (2004a): The Corinth Rift Laboratory, Comptes Rendus Geosciences, 336, 4-5, 235-242.] centered on the south western sector of the Gulf of Corinth (http://www.corinth-rift-lab.org), fault zone cores from the active Aigion Fault have been collected continuously from depths between 708 and 782 m. Inside this clayey core, a clear shearing surface with marked slip lines is visible on a plane that makes a 68° angle with respect to the core axis. This failure surface was not induced by the decompression process but is indeed a slip plane as clear striation is observed at the interface. On the basis of an elasto-plastic constitutive model calibrated on triaxial tests on the clayey gouge, it is shown that shear band formation inside the clayey core is possible. The solution for the orientation of the shear band is compared to the orientation of an existing slip surface inside the clayey gouge and this result is used to deduce the orientation of the principal stresses. It is shown that as commonly observed in weak fault zones, the orientation of the principal stresses is locally almost parallel and perpendicular to the fault axis. © 2007 Elsevier B.V. All rights reserved.
Keywords: Gulf of Corinth; Ionian Sea; Mediterranean Sea; active fault; constitutive equation; elastoplasticity; fault zone; shear band; strain analysis; stress analysis; triaxial test

Seismically induced changes of the fluid signature detected by a multi-isotope approach (He, CO2, CH4, N2) at the Wettinquelle, Bad Brambach (central Europe) - Br{auer, Karin; K{ampf, Horst; Koch, Ulrich; Niedermann, Samuel; Strauch, Gerhard (Journal of Geophysical Research: Solid Earth, 2007)
We present a study considering the systematics of gas and isotope compositions (3He/4He, δ13 CCO2, δ13 CCH4, δ 15N) of a permanent magmatic CO2 flux in the hydrothermal system of the spring Wettinquelle in Bad Brambach and their relation to the seismic activity beneath the western Eger rift. The gas and isotope compositions were monitored for more than 3 years. The time series includes periods before, during, and after a 4-month-long seismically active period from the end of August to the end of December 2000. Shifts due to admixture of crustal components were found for all monitored isotope ratios during and after the seismically active period. In case of helium and nitrogen, isotopic anomalies occurred already about 6 weeks before the beginning of the seismically active period, contemporaneous with a water level anomaly of the well VL4 near the Wettinquelle. On the one hand, preseismic deformations may be responsible for the observed isotope anomalies; on the other hand, coseismic fracturing processes in the surroundings of the hypocenters may play a role. Both effects produce greater permeability and result in the release of crustal fluids. The migration and admixture of these crustal components to the permanent upper mantle-derived fluid flux result in geochemical anomalies that persist for more than 2 years. The results of the detailed isotope monitoring have proven to be an important contribution to understand the geodynamic processes that may presently be going on in the region Vogtland/NW Bohemia. Copyright 2007 by the American Geophysical Union.
Keywords: Central Europe; Eger; Eurasia; Europe; Heves; Hungary; coseismic process; fluid composition; geodynamics; hydrothermal system; isotopic composition; thermal spring

Seismology inside the fault zone: Applications to fault-zone properties and rupture dynamics - Ellsworth, W.L.; Imanishi, K.; Nadeau, R.; Oye, V.; Waldhauser, F.; Boness, N.L.; Hickman, S.H.; Zoback, M.D. (Scientific Drilling, 2007)
cited By 13

Shock-metamorphic petrography and microRaman spectroscopy of quartz in upper impactite interval, ICDP drill core LB-07A, Bosumtwi impact crater, Ghana - Morrow, Jared R. (Meteoritics and Planetary Science, 2007)
Standard and universal stage optical microscope and microRaman spectroscopic examination of quartz from the upper impactite interval of the International Continental Scientific Drilling Program (ICDP) Lake Bosumtwi crater drill core LB-07A demonstrates widespread but heterogeneous evidence of shock metamorphism. In the upper impactite, which comprises interbedded polymict lithic breccia and suevite from a drilling depth of 333.4-415.7 m, quartz occurs as a major component within metasedimentary lithic clasts and as abundant, isolated, single-crystal grains within matrix. The noted quartz shock-metamorphic features include phenomena related to a) deformation, such as abundant planar microstructures, grain mosaicism, and reduced birefringence; b) phase transformations, such as rare diaplectic quartz glass and very rare coesite; c) melting, such as isolated, colorless to dark, glassy and devitrified vesicular melt grains; and d) secondary, post-shock features such as abundant, variable decoration of planar microstructures and patchy grain toasting. Common to abundant planar deformation features (PDFs) in quartz are dominated by ω 1013-equivalent crystallographic planes, although significant percentages of π1012 and other higher index orientations also occur; notably, c(0001) planes are rare. Significantly, the quartz PDF orientations match most closely those reported elsewhere from strongly shocked, crystalline-target impactites. Barometry estimates based on quartz alteration in the upper impactite indicate that shock pressures in excess of 20 GPa were widely reached; pressures exceeding 40-45 GPa were more rare. The relatively high abundances of decorated planar microstructures and grain toasting in shocked quartz, together with the nature and distribution of melt within suevite, suggest a water- or volatile-rich target for the Bosumtwi impact event. © The Meteoritical Society, 2007. Printed in USA.

Simulating the response of a closed-basin lake to recent climate changes in tropical West Africa (Lake Bosumtwi, Ghana) - Shanahan, Timothy M.; Overpeck, Jonathan T.; Sharp, W.E.; Scholz, Christopher A.; Arko, Justice A. (Hydrological Processes, 2007)
Historical changes in the level of Lake Bosumtwi, Ghana, have been simulated using a catchment-scale hydrological model in order to assess the importance of changes in climate and land use on lake water balance on a monthly basis for the period 1939-2004. Several commonly used models for computing evaporation in data-sparse regions are compared, including the Penman, the energy budget, and the Priestley - Taylor methods. Based on a comparison with recorded lake level variations, the model with the energy-budget evaporation model subcomponent is most effective at reproducing observed lake level variations using regional climate records. A sensitivity analysis using this model indicates that Lake Bosumtwi is highly sensitive to changes in precipitation, cloudiness and temperature. However, the model is also sensitive to changes in runoff related to vegetation, and this factor needs to be considered in simulating lake level variations. Both interannual and longer-term changes in lake level over the last 65 years appear to have been caused primarily by changes in precipitation, though the model also suggests that the drop in lake level over the last few decades has been moderated by changes in cloudiness and temperature over that time. Based on its effectiveness at simulating the magnitude and rate of lake level response to changing climate over the historical record, this model offers a potential future opportunity to examine the palaeoclimatic factors causing past lake level fluctuations preserved in the geological record at Lake Bosumtwi. Copyright © 2006 John Wiley & Sons, Ltd.
Keywords: Africa; Ashanti; Ghana; Lake Bosumtwi; Sub-Saharan Africa; West Africa; Atmospheric temperature; Catchments; Climate change; Computer simulation; Evaporation; Hydrology; Land use; Mathematical models; Precipitation (meteorology); Sensitivity analysis; Atmospheric temperature; Catchments; Climate change; Computer simulation; Evaporation; Land use; Mathematical models; Precipitation (meteorology); Sensitivity analysis; catchment; climate change; energy budget; hydrological modeling; lake level; lake water; land use; water budget; Catchment-scale hydrological model; Energy-budget evaporation model; Hydrology

Stratigraphy of the Hawai'i Scientific Drilling Project core (HSDP2): Anatomy of a Hawaiian shield volcano - Garcia, Michael O.; Haskins, Eric H.; Stolper, Edward M.; Baker, Michael (Geochemistry, Geophysics, Geosystems, 2007)
The Hawai'i Scientific Drilling Project (HSDP2) successfully drilled ∼3.1 km into the island of Hawai'i. Drilling started on Mauna Loa volcano, drilling 247mof subaerial lavas before encountering 832m of subaerial Mauna Kea lavas, followed by 2019 m of submarine Mauna Kea volcanic and sedimentary units. The 2.85 km stratigraphic record of Mauna Kea volcano spans back to ∼650 ka. Mauna Kea subaerial lavas have high average olivine contents (13 vol.%) and low average vesicle abundances (10 vol.%). Most subaerial Mauna Kea flows are 'a'ā (∼63%), whereas the Mauna Loa section contains nearly equal amounts of pāhoehoe and 'a'ā (like its current surface). The submarine Mauna Kea section contains an upper, ∼900 m thick, hyaloclastite-rich section and a lower, ∼1100 m thick, pillow-lavadominated section. These results support a model that Hawaiian volcanoes are built on a pedestal of pillow lavas capped by rapidly quenched, fragmented lava debris. The HSDP2 section is compared here to a 1.7 km deep hole (SOH1) on Kilauea's lower east rift zone. Differences in the sections reflect the proximity to source vents and the lower magma supply to Kilauea's rift zone. Both drill core sections are cut by intrusions, but the higher abundance of intrusions in SOH1 reflects its location within a rift zone, causing more extensive alteration in the SOH1 core. The HSDP2 site recovered a relatively unaltered core well suited for geochemical analyses of the single deepest and most complete borehole ever drilled through a Hawaiian or any other oceanic island volcano. Copyright 2007 by the American Geophysical Union.

Stress orientations of Taiwan Chelungpu-Fault Drilling Project (TCDP) hole-A as observed from geophysical logs - Wu, H.-Y.; Ma, K.-F.; Zoback, M.; Boness, N.; Ito, H.; Hung, J.-H.; Hickman, S. (Geophysical Research Letters, 2007)
The Taiwan Chelungpu-fault Drilling Project (TCDP) drilled a 2-km-deep research borehole to investigate the structure and mechanics of the Chelungpu Fault that ruptured in the 1999 Mw 7.6 Chi-Chi earthquake. Geophysical logs of the TCDP were carried out over depths of 500-1900 in, including Dipole Sonic Imager (DSI) logs and Formation Micro Imager (FMI) logs in order to identify bedding planes, fractures and shear zones. From the continuous core obtained from the borehole, a shear zone at a depth of 1110 meters is interpreted to be the Chelungpu fault, located within the Chinshui Shale, which extends from 1013 to 1300 meters depth. Stress-induced borehole breakouts were observed over nearly the entire length of the wellbore. These data show an overall stress direction (∼N115°E) that is essentially parallel to the regional stress field and parallel to the convergence direction of the Philippine Sea plate with respect to the Eurasian plate. Variability in the average stress direction is seen at various depths. In particular there is a major stress orientation anomaly in the vicinity of the Chelungpu fault. Abrupt stress rotations at depths of 1000 in and 1310 in are close to the Chinshui Shale's upper and lower boundaries, suggesting the possibility that bedding plane slip occurred during the Chi-Chi earthquake. Copyright 2007 by the American Geophysical Union.
cited By 82
Keywords: Boreholes; Earthquakes; Image sensors; Mechanics; Structural geology; Tectonics; Dipole Sonic Imager (DSI); Formation Micro Imager (FMI); Geophysical logs; Geophysical prospecting; bedding plane; borehole; Chi-Chi earthquake 1999; earthquake rupture; Eurasian plate; Philippine Sea plate; shear zone; stress field; well logging; Asia; Eurasia; Far East; Taiwan

The Campi Flegrei deep drilling project - De Natale, Giuseppe; Troise, Claudia; Sacchii, Marco (Scientific Drilling, 2007)

Structure and morphology of the Bosumtwi impact structure from seismic reflection data - Scholz, Christopher A.; Karp, Tobias; Lyons, Robert P. (Meteoritics and Planetary Science, 2007)
The Lake Bosumtwi impact structure in West Africa offers unparalleled opportunities for the study cratering processes, as the structure is young (1.1 Myr) and minimally eroded. Because the center part of the structure is covered by Lake Bosumtwi, which is 8 km in diameter and ∼70 m deep, it is possible to use marine-type seismic reflection techniques to obtain high-fidelity images of the lake subsurface, including key elements of the impact structure. Eight profiles of multichannel seismic reflection (MCS) data were acquired in a radial pattern across the basin, as well as two other high-resolution seismic reflection surveys. The MCS data show a well-defined central uplift near the north-central part of the lake. Observed within the annular moat surrounding the buried central uplift is a section of post-impact lacustrine sediments more than 300 m thick. The central uplift structure has a diameter of 1.9 km and a maximum height of 130 m above the annular moat. The central uplift has an overall irregular upper surface with a small graben structure. We observe a series of normal faults that extend as much as 120 m into the sedimentary section above the central uplift. We interpret the normal faults to be a consequence of ongoing compaction of the high-porosity materials that comprise the central uplift. The interpreted impact structure surface, defined using seismic reflection data, was combined with regional topographic data from outside the lake in the form of a digital elevation model, which provides a useful perspective of overall impact structure morphology. © The Meteoritical Society, 2007. Printed in USA.

Structure and properties of the San Andreas fault in central California: Recent results from the SAFOD experiment - Hickman, S.; Zoback, M.; Ellsworth, W.; Boness, N.; Malin, P.; Roecker, S.; Thurber, C. (Scientific Drilling, 2007)
cited By 31

Structure of the California Coast Ranges and San Andreas Fault at SAFOD from seismic waveform inversion and reflection imaging - Bleibinhaus, F.; Hole, J.A.; Ryberg, T.; Fuis, G.S. (Journal of Geophysical Research: Solid Earth, 2007)
A seismic reflection and refraction survey across the San Andreas Fault (SAF) near Parkfield provides a detailed characterization of crustal structure across the location of the San Andreas Fault Observatory at Depth (SAFOD). Steep-dip prestack migration and frequency domain acoustic waveform tomography were applied to obtain highly resolved images of the upper 5 km of the crust for 15 km on either side of the SAF. The resulting velocity model constrains the top of the Salinian granite with great detail. Steep-dip reflection seismic images show several strong-amplitude vertical reflectors in the uppermost crust near SAFOD that define an ∼2-km-wide zone comprising the main SAF and two or more local faults. Another prominent subvertical reflector at 2-4 km depth ∼9 km to the northeast of the SAF marks the boundary between the Franciscan terrane and the Great Valley Sequence. A deep seismic section of low resolution shows several reflectors in the Salinian crust west of the SAF. Two horizontal reflectors around 10 km depth correlate with strains of seismicity observed along-strike of the SAF. They represent midcrustal shear zones partially decoupling the ductile lower crust from the brittle upper crust. The deepest reflections from ∼25 km depth are interpreted as crust-mantle boundary. Copyright 2007 by the American Geophysical Union.
cited By 106
Keywords: crust-mantle boundary; crustal structure; prestack migration; San Andreas Fault; seismic reflection; seismic refraction; seismic tomography; seismic wave; waveform analysis; California; North America; Parkfield; United States

Subsurface structure, fault zone characteristics, and, stress state in scientific drill holes of Taiwan Chelungpu fault drilling project - Hung, J.-H.; Ma, K.-F.; Wang, C.-Y.; Song, S.-R.; Ito, H.; Lin, W.; Yeh, E.-C. (Scientific Drilling, 2007)
cited By 2

Sulfur geochemistry in the HSDP2 drilling, Hawaii: Effects of lava degassing and sea-water interaction - Berner, Z.; Istrate, G.; St{uben, D. (Water-Rock Interaction - Proceedings of the 12th International Symposium on Water-Rock Interaction, WRI-12, 2007)
The study investigates the geochemistry of sulfur in the sequence of volcanic rocks recovered by a 3098m deep drilling executed on the flank of the Mauna Kea volcano. Though mineralogic investigations indicate the presence of sulfur bearing minerals which may have crystallized directly from the plume melt, contents and isotopic composition of sulfur suggest that the original composition of the melt was altered due to degassing and interaction with seawater derived fluids. © 2007 Taylor & Francis Group, London.
Keywords: Degassing; Geochemistry; Offshore oil wells; Sulfur; Volcanic rocks; Weathering; Deep drilling; Isotopic composition; Seawater

Supplementary material to B. Heim et al.(2008): Assembly and concept of a web-based GIS within the paleoclimate project CONTINENT (Lake Baikal, Siberia) - Heim, Birgit; Klump, J; Oberh{ansli, Hedi; Fagel, N (GFZ Data Services, 2007)

Talc-bearing serpentinite and the creeping section of the San Andreas fault - Moore, D.E.; Rymer, M.J. (Nature, 2007)
The section of the San Andreas fault located between Cholame Valley and San Juan Bautista in central California creeps at a rate as high as 28 mm yr -1 (ref. 1), and it is also the segment that yields the best evidence for being a weak fault embedded in a strong crust. Serpentinized ultramafic rocks have been associated with creeping faults in central and northern California, and serpentinite is commonly invoked as the cause of the creep and the low strength of this section of the San Andreas fault. However, the frictional strengths of serpentine minerals are too high to satisfy the limitations on fault strength, and these minerals also have the potential for unstable slip under some conditions. Here we report the discovery of talc in cuttings of serpentinite collected from the probable active trace of the San Andreas fault that was intersected during drilling of the San Andreas Fault Observatory at Depth (SAFOD) main hole in 2005. We infer that the talc is forming as a result of the reaction of serpentine minerals with silica-saturated hydrothermal fluids that migrate up the fault zone, and the talc commonly occurs in sheared serpentinite. This discovery is significant, as the frictional strength of talc at elevated temperatures is sufficiently low to meet the constraints on the shear strength of the fault, and its inherently stable sliding behaviour is consistent with fault creep. Talc may therefore provide the connection between serpentinite and creep in the San Andreas fault, if shear at depth can become localized along a talc-rich principal-slip surface within serpentinite entrained in the fault zone. ©2007 Nature Publishing Group.
cited By 325
Keywords: mineral; serpentine; talc; creep; fault slip; fault zone; hydrothermal fluid; San Andreas Fault; serpentinite; shear strength; sliding; talc; ultramafic rock; article; chemical reaction; friction; gravity; priority journal; rock; temperature; California; Cholame Valley; North America; San Juan Bautista; San Luis Obispo County; United States

Nondestructive continuous physical property measurements of core samples recovered from hole B, Taiwan Chelungpu-Fault Drilling Project - Hirono, T.; Yeh, E.-C.; Lin, W.; Sone, H.; Mishima, T.; Soh, W.; Hashimoto, Y.; Matsubayashi, O.; Aoike, K.; Ito, H.; Kinoshita, M.; Murayama, M.; et al. (Journal of Geophysical Research: Solid Earth, 2007)
The Taiwan Chelungpu-Fault Drilling Project was undertaken in 2002 to investigate the faulting mechanism of the 1999 Mw 7.6 Taiwan Chi-Chi earthquake. Hole B penetrated the Chelungpu fault, and core samples were recovered from between 948.42- and 1352.60-m depth. Three major zones, designated FZB1136 (fault zone at 1136-m depth in hole B), FZB1194, and FZB1243, were recognized in the core samples as active fault zones within the Chelungpu fault. Nondestructive continuous physical property measurements, conducted on all core samples, revealed that the three major fault zones were characterized by low gamma ray attenuation (GRA) densities and high magnetic susceptibilities. Extensive fracturing and cracks within the fault zones and/or loss of atoms with high atomic number, but not a measurement artifact, might have caused the low GRA densities, whereas the high magnetic susceptibility values might have resulted from the formation of magnetic minerals from paramagnetic minerals by frictional heating. Minor fault zones were characterized by low GRA densities and no change in magnetic susceptibility, and the latter may indicate that these minor zones experienced relatively low frictional heating. Magnetic susceptibility in a fault zone may be key to the determination that frictional heating occurred during an earthquake on the fault. Copyright 2007 by the American Geophysical Union.
cited By 42
Keywords: active fault; Chi-Chi earthquake 1999; fault zone; faulting; magnetic susceptibility; Asia; Eurasia; Far East; Taiwan

An asteroid breakup 160 Myr ago as the probable source of the K/T impactor - Bottke, W.F.; Vokrouhlick'y, D.; Nesvorn'y, D. (Nature, 2007)
The terrestrial and lunar cratering rate is often assumed to have been nearly constant over the past 3 Gyr. Different lines of evidence, however, suggest that the impact flux from kilometre-sized bodies increased by at least a factor of two over the long-term average during the past ∼100 Myr. Here we argue that this apparent surge was triggered by the catastrophic disruption of the parent body of the asteroid Baptistina, which we infer was a ∼170-km-diameter body (carbonaceous-chondrite-like) that broke up Myr ago in the inner main asteroid belt. Fragments produced by the collision were slowly delivered by dynamical processes to orbits where they could strike the terrestrial planets. We find that this asteroid shower is the most likely source (>90 per cent probability) of the Chicxulub impactor that produced the Cretaceous/Tertiary (K/T) mass extinction event 65 Myr ago. ©2007 Nature Publishing Group.
cited By 142

Monitoring microearthquakes with the San Andreas fault observatory at depth - Oye, V.; Ellsworth, W.L. (69th European Association of Geoscientists and Engineers Conference and Exhibition 2007: Securing The Future. Incorporating SPE EUROPEC 2007, 2007)
In 2005, the San Andreas Fault Observatory at Depth (SAFOD) was drilled through the San Andreas Fault zone at a depth of about 3.1 km. The borehole has subsequently been instrumented with high-frequency geophones in order to better constrain locations and source processes of nearby microearthquakes that will be targeted in the upcoming phase of SAFOD. The microseismic monitoring software MIMO, developed by NORSAR, has been installed at SAFOD to provide near-real time locations and magnitude estimates using the high sampling rate (4000 Hz) waveform data. To improve the detection and location accuracy, we incorporate data from the nearby, shallow borehole (∼250 m) seismometers of the High Resolution Seismic Network (HRSN). The event association algorithm of the MIMO software incorporates HRSN detections provided by the USGS real time earthworm software. The concept of the new event association is based on the generalized beam forming, primarily used in array seismology. The method requires the pre-computation of theoretical travel times in a 3D grid of potential microearthquake locations to the seismometers of the current station network. By minimizing the differences between theoretical and observed detection times an event is associated and the location accuracy is significantly improved.
cited By 2
Keywords: Buildings; Exhibitions; Location; MIM devices; Multiplexing; Observatories; Satellite observatories; Seismographs; 3D grids; Beam forming; Detection times; High frequencies; High resolution seismic; Location accuracies; Micro earthquakes; Microseismic monitoring; Real times; Sampling rates; San andreas faults; Shallow boreholes; Source processes; Station networks; Travel times; Waveform datums; Seismology

Continuous record of environmental changes in Chukotka during the last 350 thousand years - Lozhkin, A.V.; Anderson, P.M.; Matrosova, T.V.; Minyuk, P.S.; Brigham-Grette, J.; Melles, M. (Russian Journal of Pacific Geology, 2007)
The comprehensive study of the upper 1283 cm of sediment from Lake El'gygytgyn, which formed nearly 4 Ma ago following a meteorite impact in northern Chukotka, yielded the first continuous record of the extreme changes in the Beringian climate and vegetation from the middle Middle Pleistocene to recent time (equivalent of marine isotope stages of 1-7 and the upper part of isotope stage 8). During this period, the climate was warmer than at present between 8600 and 10 7000 14C years and during the Late Pleistocene (isotope substage 5e, 116-128 ka ago). In 2003, the German-Russian-USA expedition continued studying sediments of Lake El'gygytgyn to obtain new evidence of the change in the vegetation cover in the Middle Pleistocene and the first information on the Middle Pleistocene interglacial (isotope stage 9; 297-347 ka ago). Pollen spectra characterizing the Middle Pleistocene interglacial are similar to spectra of the early stage of the Early Pleistocene interglacial and the climatic optimum in the Pleistocene to Holocene transitional period. The climatic history of Lake El'gygytgyn is basic for stratigraphic interpretations and correlations in the eastern sector of the Arctic. These data also expand our understanding of climatic changes that are studied within the framework of the Pole-Equator-Pole Paleoclimate, Past Global Changes, and other international projects. © Pleiades Publishing, Ltd. 2007.
cited By 15

Clast fabric examination of impact-generated breccias, borehole LB-07A, Bosumtwi, Ghana - Morris, William A.; Ugalde, Hernan; Clark, Christina; Miles, Brett (Meteoritics and Planetary Science, 2007)
An impact event always creates a cloud of ejecta generated through excavation of the target. Subsequent in-filling of the void by crater-fill deposits provides a record of post-impact processes. Full-core digital photographic scans of core segments from borehole LB-07 in the Bosumtwi impact crater provide a complete record of the in-fill process. The shape, orientation, and size of clasts within the impact breccia were measured using a best-fit ellipsoid approach. Clast size and variance, together with clast orientation data, suggest the impact breccias at Bosumtwi can be divided into a simple two-fold subdivision that loosely agrees with the lithological zonation of a lower monomict breccia overlain by a polymict breccia. The lower unit is characterized by a uniform and finer-grained clast size together with a uniform flat-lying clast orientation. The boundary between the two zones is defined by a sharp increase in clast size. The upper zone shows an average increase in clast size with decreasing depth, but full grain size spectrum together with increased grain size deviation suggest that this is a result of mixing between two populations with different grain size distribution. The main population of clasts shows an incremental decrease of clast size with decreasing depth. The upper zone also contains weakly defined shallowly dipping clast fabrics, which may be suggestive of horizontal transport or deposition onto an inclined surface. © The Meteoritical Society, 2007. Printed in USA.

CONTINENT sampling positions and related projects - Heim, Birgit; Klump, J; Schulze, Alexander; Dachnowski, Gangolf; Oberh{ansli, Hedi (GFZ Data Services, 2007)

Continental drilling and the study of impact craters and processes - An ICDP perspective - Koeberl, Christian; Milkereit, Bernd (Continental Scientific Drilling: A Decade of Progress, and Challenges for the Future, 2007)
Currently about 170 impact craters are known on Earth; about one third of those structures are not exposed on the surface and can only be studied by geophysics or drilling. The impact origin of geological structures can only be confirmed by petrographic and geochemical studies; thus, it is of crucial importance to obtain samples of subsurface structures. In addition, structures that have surface exposures commonly require drilling and drill cores to obtain information of the subsurface structure, to provide ground-truth for geophysical studies, and to obtain samples of rock types not exposed at the surface. For many years, drilling of impact craters was rarely done in dedicated projects, mainly due to the high cost involved. Structures were most often drilled for reasons unrelated to their impact origin. In the former Soviet Union a number of impact structures were drilled for scientific reasons, but in most of these cases the curation and proper care of the cores was not guaranteed. More recently the International Continental Scientific Drilling Program (ICDP) has supported projects to study impact craters. The first ICDPsupported study of an impact structure was the drilling into the 200-kmdiameter, K-T boundary age, subsurface Chicxulub impact crater, Mexico, which occurred between December 2001 and February 2002. The core retrieved from the borehole Yaxcopoil-1, 60 km SSW from the center of the structure, reached a depth of 1511 m and intersected 100 m of impact melt breccia and suevite, which has been studied by an international team. From June to October 2004, the 10.5 km Bosumtwi crater, Ghana, was drilled within the framework of an ICDP project, to obtain a complete 1 million year paleoenvironmental record in an area for which only limited data exist, and to study the subsurface structure and crater fill of one of the best preserved large, young impact structures. From September to December 2005, the main part of another ICDP-funded drilling project was conducted, at the 85-km-diameter Chesapeake Bay impact structure, eastern USA, which involved drilling to a depth of 1.8 km. In 2008, it is likely that the El'ygytgyn structure (Arctic Russia) will be drilled as well. So far only few craters have been drilled - not enough to gain a broad understanding of impact crater formation processes and consequences. In this chapter we summarize the current status of scientific drilling at impact craters, and provide some guidance and suggestions about future drilling projects that are relevant for impact research. Points we cover include: what is the importance of studying impact craters and processes, why is it important to drill impact craters or impact crater lakes, which important questions can be answered by drilling, which craters would be good targets and why; is there anything about the impact process, or of impact relevance, that can be learned by drilling outside any craters; what goals should be set for the future; how important is collaboration between different scientific fields? In the following report, we first briefly discuss the importance of impact cratering, then summarize experience from past drilling projects (ICDP and others), and finally we try to look into the future of scientific drilling of impact structures. © 2007 Springer-Verlag Berlin Heidelberg.

Continental drilling and the study of impact craters and processes - An ICDP perspective - Koeberl, C.; Milkereit, B. (Continental Scientific Drilling: A Decade of Progress, and Challenges for the Future, 2007)
Currently about 170 impact craters are known on Earth; about one third of those structures are not exposed on the surface and can only be studied by geophysics or drilling. The impact origin of geological structures can only be confirmed by petrographic and geochemical studies; thus, it is of crucial importance to obtain samples of subsurface structures. In addition, structures that have surface exposures commonly require drilling and drill cores to obtain information of the subsurface structure, to provide ground-truth for geophysical studies, and to obtain samples of rock types not exposed at the surface. For many years, drilling of impact craters was rarely done in dedicated projects, mainly due to the high cost involved. Structures were most often drilled for reasons unrelated to their impact origin. In the former Soviet Union a number of impact structures were drilled for scientific reasons, but in most of these cases the curation and proper care of the cores was not guaranteed. More recently the International Continental Scientific Drilling Program (ICDP) has supported projects to study impact craters. The first ICDPsupported study of an impact structure was the drilling into the 200-kmdiameter, K-T boundary age, subsurface Chicxulub impact crater, Mexico, which occurred between December 2001 and February 2002. The core retrieved from the borehole Yaxcopoil-1, 60 km SSW from the center of the structure, reached a depth of 1511 m and intersected 100 m of impact melt breccia and suevite, which has been studied by an international team. From June to October 2004, the 10.5 km Bosumtwi crater, Ghana, was drilled within the framework of an ICDP project, to obtain a complete 1 million year paleoenvironmental record in an area for which only limited data exist, and to study the subsurface structure and crater fill of one of the best preserved large, young impact structures. From September to December 2005, the main part of another ICDP-funded drilling project was conducted, at the 85-km-diameter Chesapeake Bay impact structure, eastern USA, which involved drilling to a depth of 1.8 km. In 2008, it is likely that the El'ygytgyn structure (Arctic Russia) will be drilled as well. So far only few craters have been drilled - not enough to gain a broad understanding of impact crater formation processes and consequences. In this chapter we summarize the current status of scientific drilling at impact craters, and provide some guidance and suggestions about future drilling projects that are relevant for impact research. Points we cover include: what is the importance of studying impact craters and processes, why is it important to drill impact craters or impact crater lakes, which important questions can be answered by drilling, which craters would be good targets and why; is there anything about the impact process, or of impact relevance, that can be learned by drilling outside any craters; what goals should be set for the future; how important is collaboration between different scientific fields? In the following report, we first briefly discuss the importance of impact cratering, then summarize experience from past drilling projects (ICDP and others), and finally we try to look into the future of scientific drilling of impact structures. © 2007 Springer-Verlag Berlin Heidelberg.
cited By 11

Core description and characteristics of fault zones from Hole-A of the Taiwan Chelungpu-Fault Drilling Project - Yeh, E.-C.; Sone, H.; Nakaya, T.; Ian, K.-H.; Song, S.-R.; Hung, J.-H.; Lin, W.; Hirono, T.; Wang, C.-Y.; Ma, K.-F.; Soh, W.; Kinoshita, M. (Terrestrial, Atmospheric and Oceanic Sciences, 2007)
Taiwan Chelungpu-fault Drilling Project was conducted in drill site Dakeng, Taichung City of central western Taiwan during 2004 - 2005 principally to investigate the rupture mechanism in the northern segment of the Chi-Chi earthquake of 21 September 1999, and also to examine regional stratigraphy and tectonics. Core examination (500 - 1800 m) of Hole-A gave profound results aiding in illustrating the lithologic column, deformation structure, and architectural pattern of fault zones along the borehole. Lithology column of Hole-A was identified downward as the Cholan Formation (500 - 1027 m), Chinshui Shale (1027 - 1268 m), Kueichulin Formation (1268 - 1712 m), and back to the Cholan Formation (1712 - 2003 m) again. A dramatic change is observed regarding sedimentation age and deformation structure around 1712 m. Along the core, most bedding dips 30° toward N105°. Around 1785 m, bedding dip jumps up to 70° until the bottom of borehole. Five structure groups of different orientations (dip direction/dip) are observed throughout the core. Based on the orientation and sense of shear, they are categorized as thrust (105/30), left-lateral fault (015/30 - 80), right-lateral fault (195/30 - 80), normal fault (105/5 - 10), and backthrust (285/40 - 50). Ten fault zones have been recognized between 500 and 2003 m. We interpret the fault zone located at around 1111 m as being the most likely candidate for rupture deformation during Chi-Chi earthquake. The fault zone seated around 1712 m is recognized as the Sanyi fault zone which is 600 m beneath the Chelungpu fault zone. Ten fault zones including thrust faults, strike-slip faults and backthrust are classified as the Chelungpu Fault System (<1250 m) and the Sanyi Fault System (>1500 m). According to the deformation textures within fault zones, the fault zones can be categorized as three types of deformation: distinct fracture deformation, clayey-gouge deformation, and soft-rock deformation. Fracture deformation is dominant within the Chelungpu Fault System and abother two architectures prevail in the Sanyi Fault System. The fracture deformation pattern is asymmetric, which depended the shear sense of fault zone. From the core examination of TCDP Hole-A, the lithology plays an important role in controlling the location and deformation of fault zones.
cited By 56
Keywords: Chi-Chi earthquake 1999; core analysis; deformation mechanism; drilling; fault zone; lithology; normal fault; rupture; tectonic structure; Asia; Eurasia; Far East; Taichung; Taiwan

Chicxulub impact predates K-T boundary: New evidence from Brazos, Texas - Keller, G.; Adatte, T.; Berner, Z.; Harting, M.; Baum, G.; Prauss, M.; Tantawy, A.; Stueben, D. (Earth and Planetary Science Letters, 2007)
Multidisciplinary studies, including stratigraphy, sedimentology, mineralogy and geochemistry, of the new core Mullinax-1 and outcrops along the Brazos River and Cottonmouth Creek, Falls County, Texas, reveal the complex history of the Chicxulub impact, the event deposit and the K-T boundary event. The K-T boundary, as identified by the negative δ13C shift, first occurrence of Danian planktic foraminifera and palynomorphs occurs 80 cm above the event deposit in core Mullinax-1. The underlying 80 cm interval was deposited in a shallow low oxygen environment during the latest Maastrichtian, as indicated by high stress microfossil assemblages, small shells and burrows infilled with framboidal pyrite. The underlying event deposit, commonly interpreted as K-T impact tsunami, consists of a basal conglomerate with clasts containing Chicxulub impact spherules, repeated upward fining units of spherule-rich sands, followed by hummocky cross-bedded and laminated sands, which are burrowed by Thalassinoides, Planolites and Ophiomorpha and truncated by erosion. This suggests a series of temporally separated storm events with re-colonization of the ocean floor by invertebrates between storms, rather than a series of waning tsunami-generated waves. The lithified clasts with impact spherules at the base of the event deposit provide strong evidence that the Chicxulub impact ejecta layer predates the event deposit, but was eroded and re-deposited during the latest Maastrichtian sea level lowstand. The original Chicxulub ejecta layer was discovered in a 3 cm thick yellow clay layer interbedded in undisturbed late Maastrichtian clay- and mudstones 40 cm below the base of the event deposit and near the base of planktic foraminiferal zone CF1, which spans the last 300 kyr of the Maastrichtian. The yellow clay consists of cheto smectite derived from alteration of impact glass, as indicated by rare altered glass spherules with similar chemical compositions as reworked spherules from the event deposit and Chicxulub impact spherules from NE Mexico and Haiti. The Brazos sections thus provide strong evidence that the Chicxulub impact predates the K-T boundary by about 300 kyr, consistent with earlier observations in NE Mexico and the Chicxulub crater core Yaxcopoil-1. © 2007 Elsevier B.V. All rights reserved.
cited By 77

Core slabbing and nannofossil analysis on the Chelungpu fault zone, Taichung, Taiwan - Wu, J.-C.; Huang, S.-T.; Wang, M.-H.; Tsai, C.-C.; Mei, W.-W.; Hung, J.-H.; Lee, T.-Y.; Yang, K.-M.; Lee, K.-F. (Terrestrial, Atmospheric and Oceanic Sciences, 2007)
The results of this nannofossil analysis supply essential information for determining the formation boundaries in the upper Pliocene to Pleistocene. These results also verify the existence of a repetition fossil zone. The TCDP well-A was sunk through the soft fine-grain muddy sandstone and mudstone dominated formations of the Pliocene and Pleistocene in the Taichung area. This study determines methods for providing core preservation in wells at fault zones and establishes a nannofossil biostratigraphy for the integrated Taiwan Chelungpu-fault Drilling Project (TCDP). Good core fabrics are useful for core description and sampling. In this present study, over 400 meters of subsurface cores were covered in resin and slabbed. Digitized images were created for all the core fabrics. More than 150 rock samples were analyzed for nannofossils to give a detailed appraisal of the biostratigraphic column of TCDP well-A. A fossil zone at a depth interval of 431 - 869 m is a NN16 - 18 biozone. This zone is within the Cholan Formation, a lithologic stratigraphy in northern and central Taiwan. The depth interval 883-1226 m is NN15, and is within the Chinshui Shale. The Chelungpu fault is composed of five major shear zones. These are all found at depth within the marine Chinshui Shale. At a depth interval of 1293.37 - 1710 m is a NN12 - 14 biozone; this interval is within the Kueichulin Formation. Interestingly, both the interval beneath 1714 m and the nannofossil zone near the well bottom are NN16 - 18 (Cholan Formation), indicating a repeat of the Cholan Formation. The lowest fossil zone is also abundant in secondary reworked fossils in its assemblages. Hence, the repetition of the younger fossil zone, NN16 - 18, at the bottom of the well verifies the subsurface position of the Sanyi Fault and indicates that TCDP well-A must have passed through it.
cited By 6
Keywords: biostratigraphy; core analysis; lithology; mudstone; nanofossil; Pliocene-Pleistocene boundary; shear zone; slab; Asia; Eurasia; Far East; Taichung; Taiwan

Crustal structure due to collisional and escape tectonics in the Eastern Alps region based on profiles Alp01 and Alp02 from the ALP 2002 seismic experiment - Br{uckl, Ewald; Bleibinhaus, Florian; Gosar, Andrej; Grad, Marek; Guterch, Aleksander; Hrubcov'a, Pavla; Keller, G. Randy; Majda'nski, Mariusz; Šumanovac, Franjo; Tiira, Timo; Yliniemi, Jukka; Hegedus, Endre; et al. (Journal of Geophysical Research: Solid Earth, 2007)
Alp01 and Alp02 are the longest profiles recorded during ALP 2002, a large international seismic refraction and wide-angle reflection experiment undertaken in the Eastern Alps in 2002. Alp01 crosses the Alpine orogen from north to south, thus providing a cross section mainly affected by the collision between Europe and the Adriatic microplate. Alp02 extends from the Eastern Alps to the Pannonian basin, supplying evidence on the relation between Alpine crustal structure and tectonic escape to the Pannonian basin. During this experiment, 363 single-channel recorders were deployed along these profiles with an average spacing of 3.2 km. Recordings from 20 inline shots were used in this study. Two-dimensional forward modeling using interactive ray-tracing techniques produced detailed P wave velocity models that contain many features of tectonic significance. Along Alp01, the European Moho dips generally to the south and reaches a maximum depth of 47 km below the transition from the Eastern to the Southern Alps. The Adriatic Moho continues further south at a significantly shallower depth. Moho topography and a prominent south-dipping mantle reflector in the Alpine area support the idea of southward subduction of the European lithosphere below the Adriatic microplate. The most prominent tectonic feature on the Alp02 profile is a vertical step of the Moho at the transition between the Alpine and Pannonian domains, suggesting the existence of a separate Pannonian plate fragment. The development of the Pannonian fragment is interpreted to be a consequence of crustal thinning due to tectonic escape from the Alpine collision area to the Pannonian basin. Copyright 2007 by the American Geophysical Union.
Keywords: Alps; Eastern Alps; Eurasia; Europe; Pannonian Basin; crustal structure; forward modeling; Moho; ray tracing; seismic reflection; seismic refraction; seismic velocity; seismotectonics

Diatom stratigraphy of the last 250 ka at Lake El'gygytgyn, northeast Siberia - Cherapanova, M.V.; Snyder, J.A.; Brigham-Grette, J. (Journal of Paleolimnology, 2007)
Diatom species counts were conducted on 171 sediment samples from the 13-m-long core PG1351 from Lake El'gygytgyn, northeast Siberia. The planktonic Cyclotella ocellata-complex dominates the diatom assemblage through most of the core record, persisting through a variety of climate conditions. Periphytic diatoms, although less abundant, have greater diversity and greater down-core assemblage variation. During warm climate modes, longer summer ice-free conditions may have allowed more complex diatom communities to develop in shallow-water habitats, and enhanced circulation may have increased transport of these diatoms to deeper parts of the lake. Zones of low overall diatom abundance further support inferred intervals of low lake productivity during times of extended lake ice and snow cover. More data on the modern spatial and temporal distribution of diatom species in the Lake El'gygytgyn system will improve inferences from core records. © 2006 Springer Science+Business Media B.V.
cited By 39
Keywords: climate conditions; core analysis; diatom; paleoclimate; periphyton; plankton; Quaternary; stratigraphy; Chukchi; Eurasia; Lake El' gygytgyn; Russian Federation; Bacillariophyta; Cyclotella ocellata

Directional Coring Technology in Well Songke-1 (in Chinese with English abstract);[松科1井定向取心技术] - Sun, S.; Yang, L.; Li, X.; Wu, X.; Wang, Z. (Petroleum Drilling Techniques, 2007)

Drill core LB-08A, Bosumtwi impact structure, Ghana: Geochemistry of fallback breccia and basement samples from the central uplift - Ferrière, Ludovic; Koeberl, Christian; Reimold, Wolf Uwe; Mader, Dieter (Meteoritics and Planetary Science, 2007)
The 1.07 Myr old Bosumtwi impact structure in Ghana (West Africa), which measures 10.5 km in diameter and is largely filled by Lake Bosumtwi, is associated with one of four currently known tektite strewn fields. Two boreholes were drilled to acquire hard-rock samples of the deep crater moat and from the flank of the central uplift (LB-07A and LB-08A, respectively) during a recent ICDP-sponsored drilling project. Here we present results of major and trace element analysis of 112 samples from drill core LB-08A. This core, which was recovered between 235.6 and 451.33 m depth below lake level, contains polymict lithic breccia intercalated with suevite, which overlies fractured/brecciated metasediment. The basement is dominated by meta-graywacke (from fine-grained to gritty), but also includes some phyllite and slate, as well as suevite dikelets and a few units of a distinct light greenish gray, medium-grained meta-graywacke. Most of the variations of the major and trace element abundances in the different lithologies result from the initial compositional variations of the various target rock types, as well as from aqueous alteration processes, which have undeniably affected the different rocks. Suevite from core LB-08A (fallback suevite) and fallout suevite samples (from outside the northern crater rim) display some differences in major (mainly in MgO, CaO, and Na2O contents) and minor (mainly Cr and Ni) element abundances that could be related to the higher degree of alteration of fallback suevites, but also result from differences in the clast populations of the two suevite populations. For example, granite clasts are present in fallout suevite but not in fallback breccia, and calcite clasts are present in fallback breccia and not in fallout suevite. Chondrite-normalized rare earth element abundance patterns for polymict impact breccia and basement samples are very similar to each other. Siderophile element contents in the impact breccias are not significantly different from those of the metasediments, or compared to target rocks from outside the crater rim. So far, no evidence for a meteoritic component has been detected in polymict impact breccias during this study, in agreement with previous work. © The Meteoritical Society, 2007. Printed in USA.

Drill core LB-08A, Bosumtwi impact structure, Ghana: Petrographic and shock metamorphic studies of material from the central uplift - Ferrière, Ludovic; Koeberl, Christian; Reimold, Wolf Uwe (Meteoritics and Planetary Science, 2007)
During a recent drilling project sponsored by the International Continental Scientific Drilling Progam (ICDP), two boreholes (LB-07A and LB-08A) were drilled into the crater fill of the Bosumtwi impact structure and the underlying basement, into the deep crater moat and the outer flank of the central uplift, respectively. The Bosumtwi impact structure in Ghana (West Africa), which is 10.5 km in diameter and 1.07 Myr old, is largely filled by Lake Bosumtwi. Here we present the lithostratigraphy of drill core LB-08A (recovered between 235.6 and 451.33 m depth below lake level) as well as the first mineralogical and petrographic observations of samples from this core. This drill core consists of approximately 25 m of polymict, clast-supported lithic breccia intercalated with suevite, which overlies fractured/brecciated metasediment that displays a large variation in lithology and grain size. The lithologies present in the central uplift are metasediments composed dominantly of fine-grained to gritty meta-graywacke, phyllite, and slate, as well as suevite and polymict lithic impact breccia. The suevites, principally present between 235.6 and 240.5 m and between 257.6 and 262.2 m, display a fine-grained fragmental matrix (about 39 to 45 vol%) and a variety of lithic and mineral clasts that include meta-graywacke, phyllite, slate, quartzite, carbon-rich organic shale, and calcite, as well as melt particles, fractured quartz, unshocked quartz, unshocked feldspar, quartz with planar deformation features (PDFs), diaplectic quartz glass, mica, epidote, sphene, and opaque minerals). The crater-fill suevite contains calcite clasts but no granite clasts, in contrast to suevite from outside the northern crater rim. The presence of melt particles in suevite samples from the uppermost 25 meters of the core and in suevite dikelets in the basement is an indicator of shock pressures exceeding 45 GPa. Quartz grains present in suevite and polymict lithic impact breccia abundantly display 1 to (rarely) 4 sets of PDFs per grain. The shock pressures recorded by the PDFs in quartz grains in the polymict impact breccia range from 10 to ∼30 GPa. We also observed a decrease of the abundance of shocked quartz grains in the brecciated basement with increasing depth. Meta-graywacke samples from the basement are heterogeneously shocked, with shock pressures locally ranging up to 25-30 GPa. Suevites from this borehole show a lower proportion of melt particles and diaplectic quartz glass than suevites from outside the northern crater rim (fallback impact breccia), as well as a lack of ballen quartz, which is present in the external breccias. Similar variations of melt-particle abundance and shock-metamorphic grade between impact-breccia deposits within the crater and fallout impact breccia outside the crater have been observed at the Ries impact structure, Germany. © The Meteoritical Society, 2007. Printed in USA.

Chromium isotopic studies of terrestrial impact craters: Identification of meteoritic components at Bosumtwi, Clearwater East, Lappaj{arvi, and Rochechouart - Koeberl, Christian; Shukolyukov, Alex; Lugmair, G{unter W. (Earth and Planetary Science Letters, 2007)
Chromium isotopic compositions and elemental abundances in impact melt rock and impact glass samples from four terrestrial impact craters were measured to verify the presence of an extraterrestrial component and to identify the meteorite type of the impactor. All meteorite classes have Cr isotopic signatures that are different from those of terrestrial rocks; thus, precise measurements of Cr isotopic abundances can unequivocally distinguish terrestrial from extraterrestrial materials. For all four studied craters - Bosumtwi (Ghana), Clearwater East (Canada), Lappaj{arvi (Finland), and Rochechouart (France) we found positive 53Cr excesses that eliminate carbonaceous chondrite projectiles (because those would show apparent negative excesses) and enstatite chondrites (because of the magnitude of the excess). In all four cases, ordinary chondrites have been identified as the best fit for the data; in the case of Lappaj{arvi interelement correlations together with the Cr isotope data make an H-chondrite the most likely projectile, whereas in the case of Clearwater East both L or H chondrites are possible. For Bosumtwi and Rochechouart the high indigenous contents of the siderophile elements, and disturbances of the elemental abundances by weathering and hydrothermal alteration, respectively, do not allow further constraints to be placed on the type of ordinary chondrite involved in the impact. © 2007 Elsevier B.V. All rights reserved.
Keywords: Africa; Canada; Eurasia; Europe; Finland; France; Ghana; North America; Northern Europe; Scandinavia; Sub-Saharan Africa; West Africa; Western Europe; Chromium; Landforms; Meteorites; Rocks; Stratigraphy; Chondrites; Crater; Meteoritic component; chromium; crater; hydrothermal alteration; isotopic composition; meteorite; weathering; Geochemistry

Characterization of the log lithology of cores LB-07A and LB-08A of the Bosumtwi impact structure by using the anisotropy of magnetic susceptibility - Schell, Christina; Schleifer, Norbert; Elbra, Tiiu (Meteoritics and Planetary Science, 2007)
Petrophysical data are commonly used for the discrimination of different lithologies, as the variation in mineralogy, texture, and porosity is accompanied by varying physical properties. A special field of investigation is the analysis of the directional dependence (anisotropy) of the petrophysical properties, which can provide further information on the characteristics of the lithologies, due to the fact that this parameter is different in the various rock-forming and rock-changing processes, e.g., deformation or sedimentation. To characterize the rocks in drill cores LB-07A and LB-08A, which were drilled into the deep crater moat and central uplift of the Bosumtwi impact structure, Ghana, samples were taken for the study of petrophysical properties. In the present work the magnetic properties of these samples were determined in the laboratory. The results are discussed in relation to the various lithologies represented by this sample suite. The shape and degree of magnetic anisotropy, in combination with the magnetic susceptibility, proved useful in distinguishing between the different lithologies present in the drill cores (polymict lithic breccia, suevite, shale component, and meta-graywacke). It was possible to correlate layers of high (shale component), intermediate (graywacke, polymict lithic breccia), and low (suevite) anisotropy degree with the lithostratigraphic sequences determined for cores LB-07A and LB-08A. The shape of the anisotropy showed that foliation is most dominant within the shale component, whereas lineation is more pronounced in the meta-graywacke and polymict lithic breccia. An overall increase of the anisotropy degree was observed from core LB-07A towards core LB-08A. Thus magnetic anisotropy data provide a useful contribution towards an improved petrophysical characterization of the lithostratigraphic sequences in drillcores from the Bosumtwi impact structure. © The Meteoritical Society, 2007. Printed in USA.

East African megadroughts between 135 and 75 thousand years ago and bearing on early-modern human origins - Scholz, C.A.; Johnson, T.C.; Cohen, A.S.; King, J.W.; Peck, J.A.; Overpeck, J.T.; Talbot, M.R.; Brown, E.T.; Kalindekafe, L.; Amoako, P.Y.O.; Lyons, R.P.; Shanahan, T.M.; et al. (Proceedings of the National Academy of Sciences of the United States of America, 2007)
The environmental backdrop to the evolution and spread of early Homo sapiens in East Africa is known mainly from isolated outcrops and distant marine sediment cores. Here we present results from new scientific drill cores from Lake Malawi, the first long and continuous, high-fidelity records of tropical climate change from the continent itself. Our record shows periods of severe aridity between 135 and 75 thousand years (kyr) ago, when the lake's water volume was reduced by at least 95%. Surprisingly, these intervals of pronounced tropical African aridity in the early late-Pleistocene were much more severe than the Last Glacial Maximum (LGM), the period previously recognized as one of the most arid of the Quaternary. From these cores and from records from Lakes Tanganyika (East Africa) and Bosumtwi (West Africa), we document a major rise in water levels and a shift to more humid conditions over much of tropical Africa after ≈70 kyr ago. This transition to wetter, more stable conditions coincides with diminished orbital eccentricity, and a reduction in precession-dominated climatic extremes. The observed climate mode switch to decreased environmental variability is consistent with terrestrial and marine records from in and around tropical Africa, but our records provide evidence for dramatically wetter conditions after 70 kyr ago. Such climate change may have stimulated the expansion and migrations of early modern human populations. © 2007 by The National Academy of Sciences of the USA.
cited By 332
Keywords: Africa; article; climate change; drought; environment; evolution; human; migration; nonhuman; Pleistocene; priority journal; Africa; Eastern; Animals; Evolution; Hominidae; Humans; Natural Disasters; Paleontology; Population; Tropical Climate; Homo sapiens

An international and multidisciplinary drilling project into a young complex impact structure: The 2004 ICDP Bosumtwi Crater Drilling Project - An overview - Koeberl, Christian; Milkereit, Bernd; Overpeck, Jonathan T.; Scholz, Christopher A.; Amoako, Philip Y.O.; Boamah, Daniel; Danuor, Sylvester K.; Karp, Tobias; Kueck, Jochem; Hecky, Robert E.; King, John W.; Peack, John A. (Meteoritics and Planetary Science, 2007)
The Bosumtwi impact crater in Ghana, arguably the best-preserved complex young impact structure town on Earth, displays a pronounced rim and is almost completely filled by Lake Bosumtwi, a hydrologically closed basin. It is the source crater of the Ivory Coast tektites. The structure was excavated in 2.1-2.2 Gyr old metasediments and metavolcanics of the Birimian Supergroup. A drilling project was conceived that would combine two major scientific interests in this crater: 1) to obtain a complete paleoenvironmental record from the time of crater formation about one million years ago, at a near-equatorial location in Africa for which very few data are available so far, and 2) to obtain a complete record of impactites at the central uplift and in the crater moat, for ground truthing and comparison with other structures. Within the framework of an international and multidisciplinary drilling project led by the International Continental Scientific Drilling Program (ICDP), 16 drill cores were obtained from June to October 2004 at six locations within Lake Bosumtwi, which is 8.5 km in diameter. The 14 sediment cores are currently being investigated for paleoenvironmental indicators. The two impactite cores LB-07A and LB-08A were drilled into the deepest section of the annular moat (540 m) and the flank of the central uplift (450 m), respectively. They are the main subject of this special issue of Meteoritics & Planetary Science, which represents the first detailed presentations of results from the deep drilling into the Bosumtwi impactite sequence. Drilling progressed in both cases through the impact breccia layer into fractured bedrock. LB-07A comprises lithic (in the uppermost part) and suevitic impact breccias with appreciable amounts of impact melt fragments. The lithic clast content is dominated by graywacke, besides various metapelites, quartzite, and a carbonate target component. Shock deformation in the form of quartz grains with planar microdeformations is abundant. First chemical results indicate a number of suevite samples that are strongly enriched in siderophile elements and Mg, but the presence of a definite meteoritic component in these samples cannot be confirmed due to high indigenous values. Core LB-08A comprises suevitic breccia in the uppermost part, followed with depth by a thick sequence of graywacke-dominated metasediment with suevite and a few granitoid dike intercalations. It is assumed that the metasediment package represents bedrock intersected in the flank of the central uplift. Both 7A and 8A suevite intersections differ from suevites outside of the northern crater rim. Deep drilling results confirmed the gross structure of the crater as imaged by the pre-drilling seismic surveys. Borehole geophysical studies conducted in the two boreholes confirmed the low seismic velocities for the post-impact sediments (less than 1800 m/s) and the impactites (2600-3300 m/s). The impactites exhibit very high porosities (up to 30 vol%), which has important implications for mechanical rock stability. The statistical analysis of the velocities and densities reveals a seismically transparent impactite sequence (free of prominent internal reflections). Petrophysical core analyses provide no support for the presence of a homogeneous magnetic unit (= melt breccia) within the center of the structure. Borehole vector magnetic data point to a patchy distribution of highly magnetic rocks within the impactite sequence. The lack of a coherent melt sheet, or indeed of any significant amounts of melt rock in the crater fill, is in contrast to expectations from modeling and pre-drilling geophysics, and presents an interesting problem for comparative studies and requires re-evaluation of existing data from other terrestrial impact craters, as well as modeling parameters. © The Meteoritical Society, 2007. Printed in USA.

10.06 - Exogenic Dynamics, Cratering and Surface Ages - Ivanov, B.A.; Hartmann, W.K. (Treatise on Geophysics: Volume 1-10, 2007)
To use the information about impact craters on terrestrial planets for study of planetary geology and geophysics, one should combine a wide set of processes and parameters. The chapter presents a review of impact cratering processes, estimates of average impact velocities, and impact probability for terrestrial planets. Basics of the impact crater scaling is discussed to present to date level of confidence in problems, where one needs to correlate measured size of an impact crater and mass and size of a body, created the impact structure. Scaling laws for large impact crates are discussed in comparison with results of the direct numerical modeling of impact cratering. The accumulation rate for impact craters on terrestrial planets is believed to be constant (within a factor of 2) during the last ∼3 Ga of the solar system history. Measuring the number of accumulated craters of a given size in the area of interest, one can estimate relative age of the visible surface, provided the older surfaces accumulate larger number of craters. In connection with this technique measured size-frequency distribution of impact craters is discussed, including the now widely disputable topic of secondary cratering, preventing the simple interpretation of cratering record for small craters. © 2007 Elsevier B.V. All rights reserved.
cited By 0

A dynamic study of frictional and viscous effects on earthquake rupture: A case study of the 1999 Chi-Chi, Taiwan, earthquake - Wang, J.-H. (Bulletin of the Seismological Society of America, 2007)
Friction is commonly considered an important factor in controlling earthquake rupture. In this work, it is assumed that viscosity is also a significant factor. A strike-slip-type, two-body spring-slider model in the presence of both friction and viscosity is applied to approximate the rupture processes of an earthquake along the fault-striking direction. Results show that in addition to friction, viscosity is also an important factor in controlling rupture. The Ms 7.6 Chi-Chi earthquake which struck central Taiwan on 20 September 1999, ruptured a 100-km-long east-dipping transpressive fault (the Chelungpu fault). Measured and inferred results show that there are differences in physical properties between the northern and southern segments of the fault. Simulation results from a two-body model can explain the differences in displacement, velocity, acceleration, and predominant period between the two fault segments.
cited By 14
Keywords: Approximation theory; Computer simulation; Dynamics; Friction; Mathematical models; Physical properties; Viscosity; Earthquake rupture; Fault-striking direction; Transpressive fault; Earthquakes; Chi-Chi earthquake 1999; displacement; earthquake magnitude; earthquake rupture; friction; simulation; strike-slip fault; transpression; viscosity; Asia; Eurasia; Far East; Taiwan

A helium isotope cross-section study through the San Andreas Fault at seismogenic depths - Wiersberg, T.; Erzinger, J. (Geochemistry, Geophysics, Geosystems, 2007)
We have analyzed noble gas isotopes in 19 mud gas samples from 116-3943 m borehole depth of the San Andreas Fault Observatory at Depth (SAFOD) Main Hole in the context of origin and spatial variability of fluids occurring at seismogenic depths. The measured 3He/4He ratios range between 0.40 and 1.02 Ra (Ra is the atmospheric 3He/4He ratio of 1.39 × 10-6), with 4He/20Ne values between 0.33 and 4.92, revealing a mixture of three components to the total helium: (1) atmospheric helium, (2) helium with a crustal signature, and (3) mantle-derived helium. The air-corrected 3He/4He ratios fall between 0.2 Ra and 0.9 Ra. Samples from the 2117-3196 m depth show a relatively constant helium isotope composition (0.35-0.46 Ra), indicating that ∼5% of the helium in this section the Pacific Plate is derived from the mantle. The contribution of mantle-derived helium increases slightly in the transition from the Pacific Plate to the North American Plate and reaches maximal values of ∼12% on the North American Plate (below ∼3500 m borehole depth). On the basis of our observations, we suggest that the San Andreas Fault plays a role for fluid flux from greater depths, but higher amounts of mantle-derived fluids rise up through other, more permeable faults, situated on the North American Plate of the San Andreas Fault Zone (SAFZ). Lateral fluid dispersion at shallow depths through permeable country rock of the North American Plate may explain the observed increase in 3He/ 4He ratios with increasing distance to the SAF. Copyright 2007 by the American Geophysical Union.
cited By 46

A revised age model for core PG1351 from Lake El'gygytgyn, Chukotka, based on magnetic susceptibility variations tuned to northern hemisphere insolation variations - Nowaczyk, N.R.; Melles, M.; Minyuk, P. (Journal of Paleolimnology, 2007)
A combined analysis of magnetic susceptibility, total organic carbon (TOC), biogenic silica (opal), and TiO2 content of the 12.6 m long composite core PG1351 recovered from Lake El'gygytgyn, Chukotka Peninsula, indicate a clear response of the lacustrine sedimentary record to climate variations. The impact is not direct, but through variations in oxygenation of the bottom waters. Mixing of the water body is typical for warmer climates, whereas the development of a stratified water body associated with anoxic conditions at the lake floor appears during cold climates. Oxic conditions lead to a good magnetite preservation and thus to high magnetic susceptibilities, but also to a large-scale degradation of organic matter, as reflected by low TOC (total organic carbon) values. During anoxic conditions, magnetite is severely dissolved yielding very low susceptibility values, whereas organic matter is best preserved, reflected by high TOC values. Hence, in general, neither susceptibility reflects the lithogenic fraction, nor does TOC reflect bioproductivity in case of the studied El'gygytgyn sediments. Based on available infrared stimulated luminescence (IRSL) dating, the obtained susceptibility pattern of core PG1351 shows an obvious correlation to northern hemisphere insolation variations, with a dominating impact of the Earth's 18 and 23 kyr precessional cycles for the upper half of PG1351, that is, during the past 150 ka. Therefore, the whole susceptibility record, together with biogenic silica (as a proxy for bioproductivity), TOC (as an indicator for redox conditions), and TiO2 (as a proxy for lithogenic input), was systematically tuned to the northern hemisphere insolation yielding an age of about 250 ka for the base of the composite core. © 2006 Springer Science+Business Media B.V.
cited By 69
Keywords: anoxic conditions; climate variation; core analysis; lacustrine deposit; luminescence dating; magnetic susceptibility; Northern Hemisphere; opal; oxic conditions; total organic carbon; Chukchi; Eurasia; Lake El' gygytgyn; Russian Federation

Accurate relocation of İzmit earthquake (Mw = 7.4, 1999) aftershocks in Çιnarcιk basin using double difference method - Bulut, F.; Aktar, M. (Geophysical Research Letters, 2007)
We relocated part of the aftershock activity in Çιnarcιk Basin and surrounding areas that are associated with the 1999 İzmit earthquake Mw 7.4. Double difference relocation algorithm is used to relocate the aftershocks. The data set was obtained from a temporary seismic network deployed 10 days after the main shock by cooperation between Boǧaziçi University Kandilli Observatory and Earthquake Research Institute, LGIT (Grenoble), and IPGP (Paris). For a better station coverage, additional data set was obtained from a network operated by TUBITAK Marmara Research Center. Differential travel times were calculated using both arrival time readings and waveform cross correlation method. We relocated 1145 of the aftershocks and interpreted the results with emphasis on the Yalova and Tuzla cluster located within the Marmara Sea. The results show better focused seismicity patterns for the Yalova cluster, providing clear evidence for the proposed models. We present a revised location of the 1963 Çιnarcιk earthquake which took place in area of Yalova cluster. Finally we propose that the activity of Tuzla cluster represents a parallel subsidiary fault of the Main Marmara Fault. Copyright 2007 by the American Geophysical Union.
cited By 21
Keywords: Aftershocks; Double difference method; Double difference relocation algorithm; Seismic network; Seismicity; Algorithms; Correlation methods; Earthquake resistance; Mathematical models; Seismic response; Seismic waves; Waveform analysis; Earthquakes; aftershock; arrival time; Kocaeli earthquake 1999; seismicity; travel time; waveform analysis

Amplitude loss of sonic waveform due to source coupling to the medium - Lee, M.W.; Waite, W.F. (Geophysical Research Letters, 2007)
In contrast to hydrate-free sediments, sonic waveforms acquired in gas hydrate-bearing sediments indicate strong amplitude attenuation associated with a sonic velocity increase. The amplitude attenuation increase has been used to quantify pore-space hydrate content by attributing observed attenuation to the hydrate-bearing sediment's intrinsic attenuation. A second attenuation mechanism must be considered, however. Theoretically, energy radiation from sources inside fluid-filled boreholes strongly depends on the elastic parameters of materials surrounding the borehole. It is therefore plausible to interpret amplitude loss in terms of source coupling to the surrounding medium as well as to intrinsic attenuation. Analyses of sonic waveforms from the Mallik 5L-38 well, Northwest Territories, Canada, indicate a significant component of sonic waveform amplitude loss is due to source coupling. Accordingly, all sonic waveform amplitude analyses should include the effect of source coupling to accurately characterize a formation's intrinsic attenuation.
cited By 17
Keywords: Acoustic wave velocity; Boreholes; Gas hydrates; Sediments; Waveform analysis; Energy radiation; Sonic waveform; Petroleum prospecting; amplitude; pore space; seismic attenuation; sonic boom; wave velocity; waveform analysis; Canada; North America; Northwest Territories

Application of Coring with Formation Kept Techniques in SLCORE-I (in Chinese with English abstract);[保形取心技术及其在松科1井中的应用研究] - Lin, Z.; Yang, G.; Zhang, J. (Drilling Engineering, 2007)

Characterization of aftershock-fault plane orientations of the 1999 İzmit (Turkey) earthquake using high-resolution aftershock locations - Bulut, F.; Bohnhoff, M.; Aktar, M.; Dresen, G. (Geophysical Research Letters, 2007)
Joint inversion for hypocentral parameters and the velocity field is nowadays a state of the art tool to obtain high-resolution images of seismically active regions. In this study, we focus on the location accuracy of aftershocks of the 1999 Mw = 7.4 İzmit (NW Turkey) earthquake. We obtained a new velocity model for the region, and depicted its improvement on absolute locations in terms of uncertainty and misfit. Two well-developed aftershock clusters located in the Akyazi area and Karadere-D{uzce region, were analyzed in detail based on a waveform cross-correlation approach that allowed improving the location accuracy by a factor of 6. Relocation results reveal that hypocenters form narrow planes of activity that can be correlated with focal mechanisms of the larger aftershocks as well as nearby clouds of activity with no internal structure down to the resolved scale of ∼300 m. Copyright 2007 by the American Geophysical Union.
cited By 41
Keywords: Earthquake effects; Mathematical models; Seismic waves; Waveform analysis; Hypocenters; Seismically active regions; Turkey; Seismology; aftershock; earthquake event; earthquake hypocenter; fault plane; focal mechanism; seismic velocity; Eurasia; Izmit; Kocaeli [Turkey]; Turkey

Application of oriented Coring Techniques in the SLCORE-I (in Chinese with English abstract);[定向取心技术在松科1井中的应用] - Lin, Z.; Yang, G.; Zhang, J.; Jiang, B. (Drilling Engineering, 2007)

Mineralogic and textural analyses of drill cuttings from the San Andreas Fault Observatory at Depth (SAFOD) boreholes: Initial interpretations of fault zone composition and constraints on geologic models - Bradbury, K.K.; Barton, D.C.; Solum, J.G.; Draper, S.D.; Evans, J.P. (Geosphere, 2007)
We examine drill cuttings from the San Andreas Fault Observatory at Depth (SAFOD) boreholes to determine the lithology and deformational textures in the fault zones and host rocks. Cutting samples represent the lithologies from 1.7-km map distance and 3.2-km vertical depth adjacent to the San Andreas Fault. We analyzed two hundred and sixty-six grain-mount thin-sections at an average of 30-m-cuttings sample spacing from the vertical 2.2-km-deep Pilot Hole and the 3.99-km-long Main Hole. We identify Quaternary and Tertiary(?) sedimentary rocks in the upper 700 m of the holes; granitic rocks from 760-1920 m measured depth; arkosic and lithic arenites, interbedded with siltstone sequences, from 1920 to 3150 m measured depth; and interbedded siltstones, mudstones, and shales from 3150 m to 3987 m measured depth. We also infer the presence of at least five fault zones, which include regions of damage zone and fault core on the basis of percent of cataclasite abundances, presence of deformed grains, and presence of alteration phases at 1050, 1600-2000, 2200-2500, 2700-3000, 3050-3350, and 3500 m measured depth in the Main Hole. These zones are correlated with borehole geophysical signatures that are consistent with the presence of faults. If the deeper zones of cataclasite and alteration intensity connect to the surface trace of the San Andreas Fault, then this fault zone dips 80-85° southwest, and consists of multiple slip surfaces in a damage zone 250-300 m thick. This interpretation is supported by borehole geophysical studies, which show this area is a region of low seismic velocities, reduced resistivity, and variable porosity.
cited By 54
Keywords: borehole; cutting; deformation mechanism; drilling; fault zone; geophysical method; granite; mineralogy; numerical model; sedimentary rock; seismic velocity; slip; texture; California; San Andreas; United States

Basic hydrology, limnology, and meteorology of modern Lake El'gygytgyn, Siberia - Nolan, M.; Brigham-Grette, J. (Journal of Paleolimnology, 2007)
A survey of the modern physical setting of Lake El'gygytgyn, northeastern Siberia, is presented here to facilitate interpretation of a 250,000-year climate record derived from sediment cores from the lake bottom. The lake lies inside a meteorite impact crater that is approximately 18 km in diameter, with a total watershed area of 293 km2, 110 km2 of which is lake surface. The only surface water entering the lake comes from the approximately 50 streams draining from within the crater rim; a numbering system for these inlet streams is adopted to facilitate scientific discussion. We created a digital elevation model for the watershed and used it to create hypsometries, channel networks, and drainage area statistics for each of the inlet streams. Many of the streams enter shallow lagoons dammed by gravel berms at the lakeshore; these lagoons may play a significant role in the thermal and biological dynamics of the lake due to their higher water temperatures ( >6°C). The lake itself is approximately 12 km wide and 175 m deep, with a volume of 14.1 km3. Water temperature within a column of water near the center of this oligotrophic, monomictic lake never exceeded 4°C over a 2.5 year record, though the shallow shelves ( <10 m) surrounding the lake can reach 5°C in summer. Though thermally stratified in winter, the water appears completely mixed shortly after lake ice breakup in July. Mean annual air temperature measured about 200 m from the lake was -10.3°C in 2002, and an unshielded rain gage there recorded 70 mm of rain in summer of 2002. End of winter snow water equivalent on the lake was approximately 110 mm in May 2002. Analysis of NCEP reanalysis air temperatures (1948-2002) reveals that the 8 warmest years and 10 warmest winters have occurred since 1989, with the number of days below -30°C dropping from a pre-1989 mean of 35 to near 0 in recent years. The crater region is windy as well as cold, with hourly wind speeds exceeding 13.4 m s-1 (30 mph) typically at least once each month and 17.8 m s-1 (40 mph) in winter months, with only a few calm days per month; wind may also play an important role in controlling the modern shape of the lake. Numerous lines of evidence suggest that the physical hydrology and limnology of the lake has changed substantially over the past 3.6 million years, and some of the implications of these changes on paleoclimate reconstructions are discussed. © 2006 Springer Science+Business Media B.V.
cited By 83
Keywords: crater lake; digital elevation model; hydrology; lacustrine deposit; lacustrine environment; lake dynamics; limnology; meteorology; sediment core; Chukchi; Eurasia; Lake El' gygytgyn; Russian Federation

Broadside interferometric and reverse-time imaging of the San Andreas Fault - Vasconcelos, I.; Taylor, S.T.; Snieder, R.; Chavarria, J.A.; Sava, P.; Malin, P. (Society of Exploration Geophysicists - 77th SEG International Exposition and Annual Meeting, SEG 2007, 2007)
The San Andreas Fault Observatory at Depth provides the most comprehensive set of data on the structure and dynamics of the San Andreas fault. We use two independent experiments recorded by the seismometer arrays of the SAFOD Pilot and Main Holes to resolve the localized structure of the San Andreas fault zone and of an intermediate fault zone at depth. From Pilot Hole recordings of the drilling noise coming from the Main Hole, we reconstruct the waves that propagate between the Pilot Hole sensors and use them to image the fault zone structure. The use of correlated drilling noise leads to a high-resolution image of a major transform fault zone. Another independent image is generated from the detonation of a surface explosive charge recorded at a large 178-sensor array placed in the Main Hole. The images reveal the San Andreas fault as well as an active blind fault zone that could potentially rupture. This is confirmed by two independent methods. The structure and the activity of the imaged faults is of critical importance in understanding the current stress state and activity of the San Andreas fault system. © 2007 Society of Exploration Geophysicists. All rights reserved.
cited By 0
Keywords: Explosives; Geophysical prospecting; Infill drilling; Strike-slip faults; Transform faults; Current stress; Drilling noise; Explosive charges; Fault zone structures; High resolution image; Localized structures; San Andreas fault; Structure and dynamics; Structural geology

Broadside interferometric and reverse-time imaging of the San Andreas Fault at depth - Vasconcelos, I.; Taylor, S.T.; Snieder, R.; Chavarria, J.A.; Sava, P.; Malin, P. (SEG Technical Program Expanded Abstracts, 2007)
The San Andreas Fault Observatory at Depth provides the most comprehensive set of data on the structure and dynamics of the San Andreas fault. We use two independent experiments recorded by the seismometer arrays of the SAFOD Pilot and Main Holes to resolve the localized structure of the San Andreas fault zone and of an intermediate fault zone at depth. From Pilot Hole recordings of the drilling noise coming from the Main Hole, we reconstruct the waves that propagate between the Pilot Hole sensors and use them to image the fault zone structure. The use of correlated drilling noise leads to a high-resolution image of a major transform fault zone. Another independent image is generated from the detonation of a surface explosive charge recorded at a large 178-sensor array placed in the Main Hole. The images reveal the San Andreas fault as well as an active blind fault zone that could potentially rupture. This is confirmed by two independent methods. The structure and the activity of the imaged faults is of critical importance in understanding the current stress state and activity of the San Andreas fault system. © 2007 Society of Exploration Geophysicists.
cited By 3

Characterisation and engineering properties of methane hydrate soils - Soga, K.; Lee, S.L.; Ng, M.Y.A.; Klar, A. (Characterisation and Engineering Properties of Natural Soils, 2007)
Methane hydrate soil is a natural soil deposit that contains methane hydrate in its pores. Methane hydrate is a metastable solid material and it bonds the soil particles together. Methane hydrate soil can only develop and exist under a condition of high pressure and low temperature. Hence, natural methane hydrate soils are usually found under deep seabed or permafrost regions. This paper synthesises the available data of engineering properties of natural methane hydrate soil samples retrieved at four different sites; Nankai Trough, Mallik-Mackenzie Delta, Blake Ridge and Hydrate Ridge. The geotechnical data are obtained from index tests, oedometer tests, triaxial compression tests and wave velocity measurements. The effects of hydrate growth pattern and hydrate saturation on their mechanical properties are discussed. © 2007 Taylor & Francis Group, London.
cited By 97
Keywords: Blake Ridge; Deep sea-bed; Engineering properties; Geotechnical data; High pressure; Hydrate growth; Hydrate ridge; Hydrate saturation; Index tests; Low temperatures; Metastable solids; Methane hydrates; Natural methane; Natural soils; Oedometer tests; Permafrost region; Soil particles; Soil sample; Triaxial compression tests; Wave velocity; Compression testing; Hydration; Mechanical properties; Methane; Permafrost; Soil mechanics; Soil testing; Soils; Velocity measurement; Gas hydrates

Characteristics of the lithology, fault-related rocks and fault zone structures in TCDP hole-A - Song, S.-R.; Kuo, L.-W.; Yeh, E.-C.; Wang, C.-Y.; Hung, J.-H.; Ma, K.-F. (Terrestrial, Atmospheric and Oceanic Sciences, 2007)
The main objective of the Taiwan Chelungpu-fault Drilling Project (TCDP) was to conduct an in-depth probe into a fault zone of recent major activity so as to gain a better understanding of and more insight into the physical, mechanical and chemical properties involved. By the end of 2004, with the completion of the drilling of Hole-A, cuttings from 0 to 431.34 m and cores from a 431.34- to 2003.26-m depth had been obtained. Stratigraphically, the Pliocene to Pleistocene Cholan Formation is found from the surface to a 1029-m depth and is predominantly composed of sandstone and sandstone-siltstone alternations with weak to intense bioturbation. The Pliocene Chinshui Formation is observed from a depth of 1029- to 1303-m and predominantly consists of siltstone with weak bioturbation. From 1303- to 1712-m down there is the late Miocene to early Pliocene Kueichulin Formation which is predominantly composed of massive sandstone with minor siltstone. Below 1712 m, the Formation again resembles the younger Cholan Formation with mollusca-rich, thick, layered shale and heavy bioturbated sandstone. Four types of fault-related rocks are identified in the cores. They are the fault breccia, gouges, foliated and non-foliated cataclasites and pseudotachylytes. At least six major fault zones are found in the cores: FZ1111, FZ1153, FZ1220, FZ1580, FZ1712, and FZ1812. Among these, FZ1111 most probably corresponds to the slip surface of the Chi-Chi earthquake, the Chelungpu fault, while FZ1712 very likely represents the Sanyi fault.
cited By 51
Keywords: chemical property; fault zone; lithology; lithostratigraphy; mechanical property; Miocene; physical property; Pleistocene; Pliocene; Asia; Eurasia; Far East; Taiwan; Mollusca

Drilling of the chelungpu fault after the 1999 Chi-Chi, Taiwan earthquake (Mw7.6): Understanding physics of faulting - Ma, K.-F.; Tanaka, H. (Scientific Drilling, 2007)
cited By 1

Applying SAR-IRSL methodology for dating fine-grained sediments from Lake El'gygytgyn, north-eastern Siberia - Juschus, O.; Preusser, F.; Melles, M.; Radtke, U. (Quaternary Geochronology, 2007)
Lake El'gygytgyn is situated in a 3.6 Ma old impact crater in north-eastern Siberia and probably represents one of the most complete archives of Arctic climate change. Investigated here is the potential of infra-red stimulated luminescence (IRSL) using the single-aliquot regenerative-dose (SAR) approach for dating sediments from this lake. Independent age control is available from a published age model of a parallel core that is based on tuning sediment proxies with regional insolation and the results of previous multiple aliquot IRSL dating. Although the site is located within volcanic bedrock, anomalous fading seems to have little effect on the calculated ages. The modelled water content for the entire time of burial is seen as the most prominent uncertainty at this particular site. Despite these potential error sources, SAR-IRSL ages are in acceptable agreement with the given timeframe and clearly point to the possibility to establish independent chronologies at this site up to at least 400,000 years. © 2006.
cited By 37
Keywords: age determination; bedrock; climate change; crater; fine grained sediment; geochronology; insolation; lacustrine deposit; luminescence dating; Chukchi; Eurasia; Lake El' gygytgyn; Russian Federation; Siberia

East African megadroughts between 135 and 75 thousand years ago and bearing on early-modern human origins - Scholz, Christopher A.; Johnson, Thomas C.; Cohen, Andrew S.; King, John W.; Peck, John A.; Overpeck, Jonathan T.; Talbot, Michael R.; Brown, Erik T.; Kalindekafe, Leonard; Amoako, Philip Y. O.; Lyons, Robert P.; Shanahan, Timothy M.; et al. (Proceedings of the National Academy of Sciences of the United States of America, 2007)
The environmental backdrop to the evolution and spread of early Homo sapiens in East Africa is known mainly from isolated outcrops and distant marine sediment cores. Here we present results from new scientific drill cores from Lake Malawi, the first long and continuous, high-fidelity records of tropical climate change from the continent itself. Our record shows periods of severe aridity between 135 and 75 thousand years (kyr) ago, when the lake's water volume was reduced by at least 95%. Surprisingly, these intervals of pronounced tropical African aridity in the early late-Pleistocene were much more severe than the Last Glacial Maximum (LGM), the period previously recognized as one of the most arid of the Quaternary. From these cores and from records from Lakes Tanganyika (East Africa) and Bosumtwi (West Africa), we document a major rise in water levels and a shift to more humid conditions over much of tropical Africa after ≈70 kyr ago. This transition to wetter, more stable conditions coincides with diminished orbital eccentricity, and a reduction in precession-dominated climatic extremes. The observed climate mode switch to decreased environmental variability is consistent with terrestrial and marine records from in and around tropical Africa, but our records provide evidence for dramatically wetter conditions after 70 kyr ago. Such climate change may have stimulated the expansion and migrations of early modern human populations. © 2007 by The National Academy of Sciences of the USA.
Keywords: Africa; Eastern; Animals; Evolution; Hominidae; Humans; Natural Disasters; Paleontology; Population; Tropical Climate; Homo sapiens; Africa; article; climate change; drought; environment; evolution; human; migration; nonhuman; Pleistocene; priority journal

Lithostratigraphic and petrographic analysis of ICDP drill core LB-07A, Bosumtwi impact structure, Ghana - Coney, Louise; Gibson, Roger L.; Reimold, Wolf Uwe; Koeberl, Christian (Meteoritics and Planetary Science, 2007)
Lithostratigraphic and petrographic studies of drill core samples from the 545.08 m deep International Continental Scientific Drilling Program (ICDP) borehole LB-07A in the Bosumtwi impact structure revealed two sequences of impactites below the post-impact crater sediments and above coherent basement rock. The upper impactites (333.38-415.67 m depth) comprise an alternating sequence of suevite and lithic impact breccias. The lower impactite sequence (415.67-470.55 m depth) consists essentially of monomict impact breccia formed from meta-graywacke with minor shale, as well as two narrow injections of suevite, which differ from the suevites of the upper impactites in color and intensity of shock metamorphism of the clasts. The basement rock (470.55-545.08 m depth) is composed of lower greenschist-facies metapelites (shale, schist and minor phyllite), meta-graywacke, and minor meta-sandstone, as well as interlaminated quartzite and calcite layers. The basement also contains a number of suevite dikelets that are interpreted as injection veins, as well as a single occurrence of granophyric-textured rock, tentatively interpreted as a hydrothermally altered granitic intrusion likely related to the regional pre-impact granitoid complexes. Impact melt fragments are not as prevalent in LB-07A suevite as in the fallout suevite facies around the northern crater rim; on average, 3.6 vol% of melt fragments is seen in the upper suevites and up to 18 vol% in the lower suevite occurrences. Shock deformation features observed in the suevites and polymict lithic breccias include planar deformation features in quartz (1 to 3 sets), rare diaplectic quartz glass, and very rare diaplectic feldspar glass. Notably, no ballen quartz, which is abundant in the fallout suevites, has been found in the within-crater impact breccias. An overall slight increase in the degree of shock metamorphism occurs with depth in the impactites, but considerably lower shock degrees are seen in the suevites of the basement rocks, which show similar features to each other. The bulk of the suevite in LB-07A appears to have been derived from the <35 GPa shock zone of the transient crater. © The Meteoritical Society, 2007. Printed in USA.

Joint IODP-ICDP workshop examines challenges of fault zone drilling - Tobin, H.; Ito, H.; Behrmann, J.; Hickman, S.; Kimura, G. (Scientific Drilling, 2007)
cited By 6

Joint IODP/ICDP scientific drilling of the Chicxulub impact crater - Morgan, J.V.; Christeson, G.; Gulick, S.; Grieve, R.; Urrutia, J.; Barton, P.; Rebolledo, M.; Melosh, J. (Scientific Drilling, 2007)
cited By 2

Lake Van Drilling Project: A long continental record in eastern Turkey - Litt, T.; Krastel, S.; Orcen, S.; Karabiyikoglu, M. (Scientific Drilling, 2007)
cited By 6

Late Cenozoic climate changes in China's western interior: a review of research on Lake Qinghai and comparison with other records - Colman, Steven M.; Yu, Shi-Yong; An, ZhiSheng; Shen, Ji; Henderson, A.C.G. (Quaternary Science Reviews, 2007)
We review Late Cenozoic climate and environment changes in the western interior of China with an emphasis on lacustrine records from Lake Qinghai. Widespread deposition of red clay in the marginal basins of the Tibetan Plateau indicates that the Asian monsoon system was initially established by ∼8 Ma, when the plateau reached a threshold altitude. Subsequent strengthening of the winter monsoon, along with the establishment of the Northern Hemisphere ice sheets, reflects a long-term trend of global cooling. The few cores from the Tibetan Plateau that reach back a million years suggest that they record the mid-Pleistocene transition from glacial cycles dominated by 41 ka cycles to those dominated by 100 ka cycles. During Terminations I and II, strengthening of the summer monsoon in China's interior was delayed compared with sea level and insolation records, and it did not reach the western Tibetan Plateau and the Tarim Basin. Lacustrine carbonate δ18O records reveal no climatic anomaly during MIS3, so that high terraces interpreted as evidence for extremely high lake levels during MIS3 remain an enigma. Following the Last Glacial Maximum (LSM), several lines of evidence from Lake Qinghai and elsewhere point to an initial warming of regional climate about 14 500 cal yr BP, which was followed by a brief cold reversal, possibly corresponding to the Younger Dryas event in the North Atlantic region. Maximum warming occurred about 10 000 cal yr BP, accompanied by increased monsoon precipitation in the eastern Tibetan Plateau. Superimposed on this general pattern are small-amplitude, centennial-scale oscillations during the Holocene. Warmer than present climate conditions terminated about 4000 cal yr BP. Progressive lowering of the water level in Lake Qinghai during the last half century is mainly a result of negative precipitation-evaporation balance within the context of global warming. © 2007.
Keywords: Asia; Atlantic Ocean; Atlantic Ocean (North); China; Eurasia; Far East; Qinghai-Xizang Plateau; Qionghai Lake; Sichuan; Tarim Basin; Xinjiang Uygur; Carbonates; Glaciers; Global warming; Precipitation (meteorology); Sea level; carbonate; Cenozoic; climate change; climate conditions; comparative study; global warming; historical record; Holocene; ice sheet; Last Glacial Maximum; marginal basin; Northern Hemisphere; regional climate; Lake Qinghai; Tibetan Plateau; Climate change

Leaky mode: A mechanism of horizontal seismic attenuation in a gas-hydrate-bearing sediment - Zanoth, S.R.; Saenger, E.H.; Kr{uger, O.S.; Shapiro, S.A. (Geophysics, 2007)
The leaky mode is a possible attenuation mechanism of seismic waves propagating along lamination in gas-hydrate-bearing sediment layers. This horizontal propagation attenuation mechanism occurs when a high-velocity layer is embedded in a low-velocity zone. This is a typical situation for gas hydrate occurrences. To quantify this attenuation mechanism, a 2D digital rock model based on the crosswell data of the Mallik 2002 Gas Hydrate Production Research Well Program is used. For simplicity, our elastic simulations exclude attenuation mechanisms like scattering loss or intrinsic absorption. We demonstrate that the leaky mode is a significant horizontal attenuation mechanism that cannot be neglected. The effective attenuation of gas-hydrate-bearing sediments is a combination of intrinsic and scattering attenuation by small-scale heterogeneties and the leaky mode. © 2007 Society of Exploration Geophysicists.
cited By 7
Keywords: Absorption; Computer simulation; Elastic waves; Mathematical models; Seismic waves; Two dimensional; Wave propagation; Gas-hydrate-bearing sediment; Seismic attenuation; Gas hydrates; absorption; gas hydrate; seismic attenuation; seismic velocity; seismic wave; simulation; wave propagation

Lithological and structural characteristics of the Lake Bosumtwi impact crater, Ghana: Interpretation of acoustic televiewer images - Hunze, Sabine; Wonik, Thomas (Meteoritics and Planetary Science, 2007)
Bosumtwi is a very well-preserved 1.07 Myr old, complex terrestrial impact crater located in south-central Ghana, West Africa. The impact structure has a diameter of about 10.5 km and was formed in 2.1-2.2 Gyr Precambrian metasedimentary and metavolcanic rocks. Drilling and logging was carried out during the Lake Bosumtwi Drilling Project (BCDP) which was supported by the International Continental Scientific Drilling Program (ICDP). One of the aims of this project is to achieve detailed information on the subsurface structure and crater fill of one of the best preserved large young impact structures. We interpreted the wireline logs and televiewer images. The physical properties including shallow resistivity, p-wave velocity, magnetic susceptibility, and borehole diameter of the breccia differ significantly from those of the meta-graywackes and slate/phyllites. Fractures observed in the televiewer images are interpreted to determine their characteristic structural features. The fracture dip angles are steep (50-70°) and the two main dip directions are southeast and southwest. Most fractures observed in the borehole are open. The indicated main stress direction is north-south. © The Meteoritical Society, 2007. Printed in USA.

Low-velocity damage zone on the san andreas fault at depth near SAFOD site at parkfield delineated by fault-zone trapped waves - Li, Y.-G.; Vidal, J.E. (Scientific Drilling, 2007)
cited By 4

Integrated 3-D model from gravity and petrophysical data at the Bosumtwi impact structure, Ghana - Ugalde, Hernan; Danuor, Sylvester K.; Milkereit, Bernd (Meteoritics and Planetary Science, 2007)
The Bosumtwi impact structure of central Ghana was drilled in 2004 as part of the International Continental Scientific Drilling Program (ICDP). A vast amount of geoscience data is available from the pre-site surveys and the actual drilling phase. A 3-D gravity model was constructed and calibrated with the available data from the two ICDP boreholes, LB-07A and LB-08A. The 3-D gravity model results agree well with both the sediment thickness and size of the central uplift revealed by previously collected seismic data, and with the petrophysical data from the LB-08A and LB-07A core materials and the two borehole logs. Furthermore, the model exhibits lateral density variations across the structure and refines the results from previous 2.5-D modeling. An important new element of the 3-D model is that the thickness of the intervals comprising polymict lithic impact breccia and suevite, monomict lithic breccia and fractured basement is much smaller than that predicted by numerical modeling. © The Meteoritical Society, 2007. Printed in USA.

Luminescence geochronology for sediments from Lake El'gygytgyn, northeast Siberia, Russia: Constraining the timing of paleoenvironmental events for the past 200 ka - Forman, S.L.; Pierson, J.; G'omez, J.; Brigham-Grette, J.; Nowaczyk, N.R.; Melles, M. (Journal of Paleolimnology, 2007)
This study focused on the luminescence dating of sediments from Lake El'gygytgyn, a meteorite impact crater 100 km north of the Arctic Circle in northeast Siberia, formed 3.58 Ma ago. The sediment is principally eolian deposited in to a lake with nearly permanently ice. The fine-grained polymineral and quartz extracts taken from nine distinct levels from the upper 12.3 m of sediment core PG1351 were dated by infrared stimulated (IRSL) and green stimulated luminescence (GSL) using multiple aliquot additive dose procedures. The veracity of these ages is evaluated by comparing to an age model for the core derived from magnetic excursions and from correlation of variations of the magnetic susceptibility record to similar magnitude variations in δ 18O in the Greenland Ice core record. The IRSL ages from the upper 9 m of core correspond well with the independent age control for the past ca. 200 ka. However, sediments deeper in the core at 12.3 m with an inferred age of ca. 250 ka age yield a saturated IRSL response and therefore a non-finite OSL age. The youngest sediment dated from 0.70 m depth yielded the IRSL age of ca. 11.5 ka, older than the corresponding age of 9.3-8.8 ka, indicating a discrepancy in dating the youngest sediments in the upper 1 m of core. This study confirms the utility of IRSL by the multiple aliquot additive dose method to date sediments <200 ka old from eastern Siberia. © 2006 Springer Science+Business Media B.V.
cited By 30
Keywords: crater lake; eolian deposit; geochronology; lacustrine deposit; luminescence dating; paleoenvironment; sediment core; Chukchi; Eurasia; Lake El' gygytgyn; Russian Federation

Magnetic hysteresis parameters and Day plot analysis to characterize diagenetic alteration in gas hydrate-bearing sediments - Enkin, R.J.; Baker, J.; Nourgaliev, D.; Iassonov, P.; Hamilton, T.S. (Journal of Geophysical Research: Solid Earth, 2007)
The J meter coercivity spectrometer is a machine capable of rapid and simple measurement of magnetic hysteresis, isothermal remanence acquisition and magnetic viscosity of rocks and sediments. The J meter was used to study a suite of samples collected from strata in the gas hydrate-bearing JAPEX/JNOC/GSC Mallik 5L-38 well (69.5°N, 134.6°W) in the Mackenzie Delta of the northwestern Canadian Arctic. The Day plot of magnetic hysteresis ratios for these samples is exotic in that the points do not plot along a hyperbola as is usually observed. Rather, they plot as a scatter which is shown to contour into vertical slices using coercivity field (HC) or saturation magnetization (JS), and horizontal slices using the relative quantity of superparamagnetism (JSPM/JS). Optical microscopy reveals that the magnetic minerals are detrital magnetite and authigenic greigite. Greigite is dominant in sands which in situ had >70% gas hydrate saturation and in silts in which gas hydrate growth was blocked by insufficient porosity. We infer that the silts were the accumulation sites for solutes which had been excluded from the pore waters in neighboring coarser-grained sediments during the course of gas hydrate formation. Consequently, we conclude that magnetic properties are related to gas hydrate-related processes, and as such, may have potential as a method of remote sensing for gas hydrate deposits. Copyright 2007 by the American Geophysical Union.
cited By 29
Keywords: chemical alteration; detrital deposit; diagenesis; formation mechanism; gas hydrate; greigite; hysteresis; isotherm; magnetic field; magnetic property; magnetite; porosity; remanent magnetization; viscosity; Canada; Mackenzie Delta; North America; Northwest Territories

Measuring attenuation and velocity dispersion using vibrator sweeps - Sun, L.F.; Milkereit, B.; Schmitt, D. (SEG Technical Program Expanded Abstracts, 2007)
Heterogeneity of rocks, such as porosity, fractures, and fluids, causes attenuation and velocity dispersion of seismic waves, and induces waveform distortion. This distortion, once detected, offers an insight into the heterogeneous rock properties. In order to detect small velocity dispersion in the exploration seismic frequency band, a new seismic processing method has been developed for uncorrelated vibrator data. This method has been applied to the uncorrelated vibrator VSP data from Mallik gas hydrate research well, MacArthur River uranium mine area, and Outokumpu crystalline rock borehole. Different trends of Q and velocity dispersion have been detected in the above areas, which is a result of the heterogeneities in the rock volume surrounding the boreholes. © 2007 Society of Exploration Geophysicists.
cited By 1
Keywords: Boreholes; Crystalline rocks; Dispersion (waves); Gas hydrates; Petroleum prospecting; Rocks; Seismology; Velocity; Vibrators; Heterogeneous rocks; Seismic frequencies; Seismic processing; Velocity dispersion; Waveform distortions; Seismic prospecting

Mesoscopic structural observations of cores from the Chelungpu Fault System, Taiwan Chelungpu-fault Drilling Project Hole-A, Taiwan - Sone, H.; Yeh, E.-C.; Nakaya, T.; Hung, J.-H.; Ma, K.-F.; Wang, C.-Y.; Song, S.-R.; Shimamoto, T. (Terrestrial, Atmospheric and Oceanic Sciences, 2007)
Structural characteristics of fault rocks distributed within major fault zones provide basic information in understanding the physical aspects of faulting. Mesoscopic structural observations of the drilled cores from Taiwan Chelungpu-fault Drilling Project Hole-A are reported in this article to describe and reveal the distribution of fault rocks within the Chelungpu Fault System. The Chelungpu Fault System in Hole-A was encountered at a depth of between 1050 - 1250 m where deformation structures increased. Three major fault zone structures were found at approximate depths of 1111, 1153, and 1221 m. The presence of wide fault rock regions were mostly concentrated in these 3 fault zones. The fault zone at 1111 m mainly consists of a nearly brecciated fracture zone and a clayey fault gouge zone of about 1.05 m in thickness. Fault rocks from the fault zone at 1153 m are characterized by the presence of sand grains in the matrix content, consisting of a 1.1-m thick fault breccia zone and a 0.35-m thick fault gouge zone. The fault zone at 1221 m consists of fault breccia and fault gouge of 1.15 m in total thickness. These are relatively harder and darker in color than the previous 2 fault zones. Each of the 3 fault zones contains a few layers of dark colored rocks of approximately 5 - 80 mm in thickness within the fault breccia and fault gouge zones. These dark colored rocks were found distinctively within the fault rocks. However, there relation to the process of faulting is not clearly understood and shall be discussed in detail with the aid of microscopic observations.
cited By 33
Keywords: borehole; breccia; core analysis; deformation mechanism; drilling; fault gouge; fault zone; fracture zone; Asia; Eurasia; Far East; Taiwan

Microfacies of Deep-water Deposits and Forming Models of the Chinese Continental Scientific Drilling-SKII - Rihui, CHENG; Guodong, WANG; Pujun, WANG; Youfeng, GAO (Acta Geologica Sinica - English Edition, 2007)
Abstract: Extensive transgression of lake water occurred during the Cretaceous Qingshankou Stage and the Nengjiang Stage in the Songliao basin, forming widespread deep-water deposits. Eleven types of microfacies of deep-water deposits have been recognized in the continuous core rocks from the SKII, including mudstone of still water, marlite, dolostone, oil shale, volcanic ashes, turbidite, slump sediment, tempestite, seismite, ostracoda limestone and sparry carbonate, which are divided into two types: microfacies generated due to gradually changing environments (I) and microfacies generated due to geological events (II). Type I is composed of some special fine grain sediments such as marlite, dolomite stone and oil shale as well as mudstone and Type II is composed of some sediments related to geological events, such as volcanic ashes, turbiditie, slump sediment, tempestite, seismite, ostracoda limestone. The formation of sparry carbonate may be controlled by factors related to both environments and events. Generally, mudstone sediments of still water can be regarded as background sediments, and the rest sediments are all event sediments, which have unique forming models, which may reflect controlling effects of climatics and tectonics.
Keywords: Cretaceous; deep-water deposit; model of microfacies; Songliao basin; SKII

Migration-based location of the SAFOD target-earthquakes - Rentsch, S.; Buske, S.; Gutjahr, S.; Kummerow, J.; Shapiro, S.A. (69th European Association of Geoscientists and Engineers Conference and Exhibition 2007: Securing The Future. Incorporating SPE EUROPEC 2007, 2007)
In this work we present the determination of earthquake locations using an enhanced migration-based location method. We implemented the use of arrival time information into our migration-based location method in order to overcome acquisition geometry pitfalls of a data set from the San Andreas Fault Observatory at Depth (SAFOD). The major aim of this work was to locate the hypocentral region of a repeating earthquake cluster with high precision which is supposed to become the drilling target of the next SAFOD Main Hole drilling phase. Therefore we analysed high quality data from Paulsson Geophysical Services Inc. recorded with an 80 level borehole seismic receiver array deployed in the SAFOD Main Hole. We show step by step the analysis of the data and successful locations of target-earthquakes.
cited By 1
Keywords: Earthquakes; Exhibitions; Location; Seismology; Targets; Acquisition geometries; Arrival times; Data sets; Drilling targets; Earthquake clusters; Earthquake locations; High precisions; High quality datums; Hole drillings; Location methods; Receiver arrays; San andreas faults; Step by steps; Boreholes

Ejecta range: A simulation study of terrestrial impacts - Niem, D.; K{uhrt, E.; Motschmann, U. (Planetary and Space Science, 2007)
In this work the meteorite and target mass partition into high-speed ejecta during the formation of terrestrial impact craters is investigated. Multi-material hydrocode calculations are carried out through the entire excavation phase, and the mass of each material moving upwards with velocities inside a range of intervals is obtained. Impact of a 10 km diameter stony asteroid with 20 km s- 1 into the continental crust is compared for the cases of a single layer of granite, taken to be representative for the crust, and of a two-layer crust with a 3 km thick sedimentary cover of limestone on top of granite basement, more appropriate for the Chicxulub crater. The proportion of meteorite and crustal material in high-speed ejecta is found as a function of velocity and time, and maximum distances to the crater can be estimated. The resulting distal ( > 7000 km) ejecta mass for vertical impact is less than a percent of the impactor mass, assuming ballistic transport. Simulations of oceanic impact of a 1 km-sized stony asteroid into 5.5 km deep sea are also presented. Here, ejection of meteorite material initially is delayed, but finally it leaves the ocean in a cloud of steam and water. The velocities of meteorite material are much lower compared with the continental impact, insufficient to reach large distances on ballistic trajectories. © 2007 Elsevier Ltd. All rights reserved.
cited By 2

Integrated petrophysical and borehole seismic studies of Lake Bosumtwi impact crater, Ghana - Meillieux, Damien; Schmitt, Douglas; Milkereit, Bernd; Danour, Sylvester; Nkrumah, Kwame (Society of Exploration Geophysicists - 77th SEG International Exposition and Annual Meeting, SEG 2007, 2007)
We present here an integrated case study of the Lake Bosumtwi impact structure, Ghana, consisting of VSP, geophysical logging, and laboratory measurements on core samples. Bosumtwi crater is 1.07Ma old and has a diameter of 11km, the target rocks being metasediments of 2Ga. The seismic measurements show lower P and S-waves velocities than what is expected in those kinds of rocks. The data are altered by high energy reverberations originating from the air bubble in the water and the steel casing in the sediments. However we obtained velocities between 220 and 350m, going from 2500 to 3000 m/s for the P-wave and from 1000 to 1100 m/s for the S-wave. This indicates a high degree of fracturation and comminution in the hard rock. He-porosity values show a bimodal distribution, with porosities ranging from 1% to 9%, and from 15% to almost 40%. There is a strong linear relationship between porosity and envelope density, but nothing apparent between porosity and grain density or depth. First results with Hg-porosimetry show a bimodal distribution of pore sizes. More samples need to be analyzed in order to draw conclusions on pore structures, but first hypotheses suggest different sizes of crack or the presence of interstitial material depending on rock types and position within the crater. © 2007 Society of Exploration Geophysicists. All rights reserved.
Keywords: Lakes; Pore size; Rocks; Seismic waves; Seismology; Shear waves; Bimodal distribution; Different sizes; Geophysical logging; Impact structures; Interstitial materials; Laboratory measurements; Linear relationships; Seismic measurements; Seismic prospecting

Lake Baikal climatic record between 310 and 50 ky BP: Interplay between diatoms, watershed weathering and orbital forcing - Grygar, Tomas; Bl'ahov'a, Anna; Hradil, David; Bezdicka, Petr; Kadlec, Jaroslav; Schnabl, Petr; Swann, George; Oberh{ansli, Hedi (Palaeogeography, Palaeoclimatology, Palaeoecology, 2007)
The environmental record from Lake Baikal, Russia, from 310 to 50 ky BP (MIS 9a to MIS 3) was interpreted using rock magnetic, UV-Vis spectral, mineralogical, and diatom analyses. The age model was based on a correlation of the diatom and chemical weathering records and the summer insolation curve at 55°N and checked against an age model based on the proxy of relative palaeointensity of the Earth's magnetic field. Peaks in chemical weathering within the watershed, inferred from maximum concentration of magnetic and coloured minerals and mica, the lowest mean Fe oxidation state in silicates and highs in expandable clay minerals correlated with the Northern Hemisphere summer insolation minima at 55°N. Reconstructed changes in weathering intensity are better correlated to insolation patterns than to global ice volume records. We propose a scheme of yet missing palaeoenvironmental interpretation of the diatom assemblage, including also some extinct species. Aulacoseira baicalensis and Aulacoseira skvortzowii were abundant in the early stages of lake flora recovery immediately after deglaciation and during MIS 7e and MIS 5e; periods of more pronounced continental climate and peak chemical weathering. Stephanodiscus formosus var. minor, Cyclotella minuta and Cyclotella ornata dominated in intervals of decreased seasonality and decreased humidity at the end of most interglacial/interstadial diatom zones. Stephanodiscus grandis, Stephanodiscus carconeiformis and Stephanodiscus formosus were ubiquitous between MIS 8 and MIS 5, an interval marked by high seasonality, i.e., large differences between winter and summer insolation, and low humidity revealed by a low hydrolysis of expandable clay minerals in the watershed. Diatom concentrations peaked in the climatic optima of MIS 7e and MIS 5e and in the short periods marked by shifts to warmer conditions in the upper sections of MIS 5: MIS 5c (103-99 ky BP), MIS 5b (90-88 ky BP), and MIS 5a (84-79 ky BP) in which increased humidity resulted in enhanced hydrolysis of clay minerals. No such short similar climatic optimums were found from MIS 9a to MIS 6. Sharp climate deteriorations recorded in the diatom and clay mineral records at 107, 94, and 87 ky BP, however, occurred within 1-2 ky of cold extremes in North Atlantic sea surface temperature emphasizing the strong teleconnections between the two localities. © 2007 Elsevier B.V. All rights reserved.
Keywords: Eurasia; Lake Baikal; Russian Federation; Aulacoseira baicalensis; Aulacoseira skvortzowii; Bacillariophyta; Cyclotella; Cyclotella minuta; Stephanodiscus; climate change; diatom; interglacial; interstadial; lacustrine deposit; magnetic field; Northern Hemisphere; orbital forcing; proxy climate record; record; spectroscopy; watershed; weathering

Inorganic geochemistry of El'gygytgyn Lake sediments (northeastern Russia) as an indicator of paleoclimatic change for the last 250 kyr - Minyuk, P.S.; Brigham-Grette, J.; Melles, M.; Borkhodoev, V.Ya.; Glushkova, O.Yu. (Journal of Paleolimnology, 2007)
The inorganic geochemistry of sediments from El'gygytgyn Lake shift in phase with interpreted paleoclimatic fluctuations seen in the record over the past 250 ka. Warm periods, when the lake was seasonally ice free and fully mixed, are characterized by increased concentrations of SiO2, CaO, Na2O, K2O, and Rb, by decreased contents of TiO 2, Fe2O3, Al2O3, and MgO, and by a lower chemical index of alteration (CIA). Increased levels of SiO 2 reflect increases in limnic productivity whereas many of the other elements and the CIA likely reflect increased hydrological activity coincident with an increase in coarser sand and silt content and a decrease in clay mineral content. For cold/cooler periods when perennial lake ice cover lead to a stratifed water column and anoxic bottom waters, the opposite is generally observed suggesting a decrease in hydrological activity and an increase in post-depositional chemical alteration. Peaks in P2O3 and MnO, coincident with an increased abundance of vivianite, suggest possible linkages to the paleoproductivity of local fish fauna regardless of climate change across the region surrounding Lake El'gygytgyn. Strontium is high in concentration during warmer intervals and may also be linked to paleoproductivity. Enrichment of the post-Eemian portion of the sediment record in niobium, and yttrium appears independent of glacial-interglacial change; rather it may reflect a gradual shift in the geomorphology of the catchment, particularly the hydrology of large alluvial fans along the western side of the lake. In contrast to some lake records, changes in Zr concentration over time suggests only a weak, if any, increase in eolian sediment supply during colder periods. © 2006 Springer Science+Business Media B.V.
cited By 71
Keywords: chemical alteration; climate change; geochemistry; inorganic compound; lacustrine deposit; paleoclimate; Quaternary; sediment chemistry; Chukchi; Eurasia; Lake El' gygytgyn; Russian Federation

Geochemistry of impactites and basement lithologies from ICDP borehole LB-07A, Bosumtwi impact structure, Ghana - Coney, Louise; Reimold, Wolf Uwe; Gibson, Roger L.; Koeberl, Christian (Meteoritics and Planetary Science, 2007)
In 2004, a drilling project by the International Continental Scientific Drilling Program (ICDP) at the Bosumtwi impact crater, Ghana (1.07 Myr old and 10.5 km in diameter), obtained drill core LB-07A, which sampled impactites and underlying metasediments in the crater moat surrounding the small central uplift of the structure. The LB-07A core consists of three sequences: 82.29 m of an upper impactite sequence of alternating polymict lithic and suevitic impact breccias overlying 54.88 m of so-called lower impactite of monomict impact breccia with several suevite intercalations, and 74.53 m of meta-graywacke and altered shale of the basement, also containing a number of suevite intercalations. Major- and trace-element characteristics of all three sequences have been determined to investigate breccia formation and the role of the respective basement lithologies therein. Compositions of polymict impact breccias of the crater fill revealed by core LB-07A are compared with the compositions of the Ivory Coast tektites and the fallout suevites. The impactites of the LB-07A borehole appear well homogenized with respect to the silicate component, and little change in the ranges of many major- and trace-element differences is seen along the length of the borehole (except for Fe2O3, MgO, and CaO contents). Much scatter is observed for a number of elements, and in many cases this increases with depth. It is proposed that any variability in composition is likely the function of clast population differences (i.e., also of relatively small sample sizes). No systematic compositional difference between polymict lithic and suevitic impact breccias is evident. An indication of carbonate enrichment due to hydrothermal alteration is observed in samples from all lithologies. The impactites of the borehole generally show intermediate compositions to previously defined target rocks. The fallout suevites have comparable major element abundances, except for relatively lower MgO contents. The Ivory Coast tektites are generally similar in composition to the LB-07A suevites, but broader ranges in MgO and CaO contents are observed for the LB-07A suevites. © The Meteoritical Society, 2007. Printed in USA.

Electron microscopy of clay minerals in mudrocks from the san andreas Fault observatory at depth (SAFOD) - Schleicher, A.M.; Pluijm, B.A.; Warr, L.N.; Solum, J.G. (Scientific Drilling, 2007)
cited By 2

Estimation of velocity in the uppermost crust in a part of the western Gulf of Corinth, Greece, from the inversion of P and S arrival times using the neighbourhood algorithm - Jansk'y, Jarom'ir; Plicka, Vladim'ir; Lyon-Caen, H'elène; Novotny, Oldřich (Journal of Seismology, 2007)
We determine the velocities in an upper crustal model, composed of three homogeneous layers, for one subregion of the western part of the Gulf of Corinth, NE of the town of Aigion, Greece. We have used local events that occurred there in the year 2001 and were recorded by the Corinth Rift Laboratory Network. Weighted P and S arrival time residuals are minimized using the Neighbourhood Algorithm of Sambridge (1999), combined with the grid search for source locations. The resolution of the inversion is tested by delete-one jackknifing. The model obtained is compared with some other models derived or applied to the subregion. A fast velocity increase between depths of 5 and 7 km is confirmed as the major structural element. © Springer Science+Business Media, Inc. 2007.
Keywords: Gulf of Corinth; Ionian Sea; Mediterranean Sea; algorithm; arrival time; crustal structure; data inversion; P-wave; S-wave; seismic velocity; upper crust

Evolution of hot-spring travertine accumulation in Karlovy Vary/Carlsbad (Czech Republic) and its significance for the evolution of Tepl'a valley and Ohře/Eger rift - Vylita, Tom'aš; Ž'ak, Karel; C'ilek, V'aclav; Hercman, Helena; Mikš'ikov'a, Lucie (Zeitschrift fur Geomorphologie, 2007)
The first set of U-series ages of the hot-spring travertine from the world-famous Karlovy Vary Spa indicates repeated growth of the travertine accumulation followed by its erosion by the Tepl'a River. The obtained data are important as an estimate of minimum duration of thermal water circulation in Karlovy Vary and in understanding of the river valley evolution during the youngest geological history. The oldest 230Th/ 234U age data (230 ± 14 ka BP and 155 ± 7 ka BP) have been obtained for travertine relicts located 22 m above the present-day bottom of the valley. The extensive travertine accumulation located directly on the valley bottom was formed by gradual growth, particularly during the first half of the Weichselian period, between 100 and 50 ka BP. These data show that the valley bottom was at approximately the same elevation as it is today already during the Eemian interglacial. At the end of the Weichselian the central part of the travertine accumulation was downcut by river erosion. During Holocene the new deposition of travertine was concentrated in this wide trench, being likely repeatedly modified by river activity. The travertine accumulation started to be strongly influenced by humans since the Middle Ages. A significant part of it was quarried out for lime production. The accumulation was later covered by buildings, streets and the reinforced course of the Tepl'a River. © 2007 Gebr{uder Borntraeger.
Keywords: Central Europe; Czech Republic; Eurasia; Europe; Karlovarsky; Karlovy Vary; Tepla River; concentration (composition); dating method; Eemian; geochronology; Holocene; interglacial; landform evolution; thermal evolution; thermal spring; travertine

Exogenic Dynamics, Cratering and Surface Ages - Ivanov, B.A.; Hartmann, W.K. (Treatise on Geophysics, 2007)
To use the information about impact craters on terrestrial planets for study of planetary geology and geophysics, one should combine a wide set of processes and parameters. The chapter presents a review of impact cratering processes, estimates of average impact velocities, and impact probability for terrestrial planets. Basics of the impact crater scaling is discussed to present to date level of confidence in problems, where one needs to correlate measured size of an impact crater and mass and size of a body, created the impact structure. Scaling laws for large impact crates are discussed in comparison with results of the direct numerical modeling of impact cratering. The accumulation rate for impact craters on terrestrial planets is believed to be constant (within a factor of 2) during the last ∼3 Ga of the solar system history. Measuring the number of accumulated craters of a given size in the area of interest, one can estimate relative age of the visible surface, provided the older surfaces accumulate larger number of craters. In connection with this technique measured size-frequency distribution of impact craters is discussed, including the now widely disputable topic of secondary cratering, preventing the simple interpretation of cratering record for small craters. © 2007 Elsevier B.V. All rights reserved.
cited By 29

In situ seismic measurements in borehole LB-08A in the Bosumtwi impact structure, Ghana: Preliminary interpretation - Schmitt, Doug R.; Milkereit, Bernd; Karp, Tobias; Scholz, Christopher; Danuor, Sylvester; Meillieux, D.; Welz, M. (Meteoritics and Planetary Science, 2007)
In order to assist in the interpretation of previous seismic refraction and reflection surveys, a vertical seismic profile was acquired in the Lake Bosumtwi (Ghana) hard-rock core hole LB-08A. No seismic reflections are seen in the up-going wave field obtained, and this observation is consistent with the lack of reflectivity observed in the corresponding 2-D surface seismic profile obtained in earlier studies. Direct down-going P-waves were found both in the cased sediment column from a depth of 73 m to 239 m below the lake surface and in the open-hole hard rock section from a depth of 239 m to 451 m of LB-08A. Analysis of the observed travel times indicates a nearly constant P-wave velocity of 1520 m/s through the soft lacustrine sediments. In the hard-rock, however, the P-wave velocity rapidly increases by nearly 30% from 2600 m/s to 3340 m/s. These values are in good agreement with the gross velocity structure obtained in the earlier joint inversion of seismic reflection and refraction data. These values are low relative to those expected for the metasedimentary protoliths, an observation that has been made at other young impact structures of similar size. The low velocities, together with the fact that they increase so rapidly, is suggestive of a decreasing density of fractures and microcracks with depth. Consequently, the seismic velocity trend may provide a proxy measure of damage, and hence, the decay of the shock pressure from the impact point. Validation of this requires additional detailed studies of the porosity structure in the core. © The Meteoritical Society, 2007. Printed in USA.

Gas hydrate property measurements in porous sediments with resonant ultrasound spectroscopy - McGrail, B.P.; Ahmed, S.; Schaef, H.T.; Owen, A.T.; Martin, P.F.; Zhu, T. (Journal of Geophysical Research: Solid Earth, 2007)
Resonant ultrasound spectroscopy was used to characterize a natural geological core sample obtained from the Mallik 5L-38 gas hydrate research well at high pressure and subambient temperatures. Using deuterated methane gas to form gas hydrate in the core sample, it was discovered that resonance amplitudes are correlated with the fraction of the pore space occupied by the gas hydrate crystals. A pore water freezing model was developed that utilizes the known pore size distribution and pore water chemistry to predict gas hydrate saturation as a function of pressure and temperature. The model showed good agreement with the experimental measurements and demonstrated that pore water chemistry is the most important factor controlling equilibrium gas hydrate saturations in these sediments when gas hydrates are formed artificially in laboratory pressure vessels. With further development, the resonant ultrasound technique can provide a rapid, nondestructive, field portable means of measuring the equilibrium P-T properties and dissociation kinetics of gas hydrates in porous media, determining gas hydrate saturations, and may provide new insights into the nature of gas hydrate formation mechanisms in geologic materials. Copyright 2007 by the American Geophysical Union.
cited By 2
Keywords: core analysis; equilibrium; gas hydrate; laboratory method; measurement method; pore space; porewater; porous medium; resonance; saturation; spectroscopy

Geochemistry of impactites and basement lithologies from ICDP borehole LB-07A, Bosumtwi impact structure, Ghana - Coney, L.; Reimold, W.U.; Gibson, R.L.; Koeberl, C. (Meteoritics and Planetary Science, 2007)
In 2004, a drilling project by the International Continental Scientific Drilling Program (ICDP) at the Bosumtwi impact crater, Ghana (1.07 Myr old and 10.5 km in diameter), obtained drill core LB-07A, which sampled impactites and underlying metasediments in the crater moat surrounding the small central uplift of the structure. The LB-07A core consists of three sequences: 82.29 m of an upper impactite sequence of alternating polymict lithic and suevitic impact breccias overlying 54.88 m of so-called lower impactite of monomict impact breccia with several suevite intercalations, and 74.53 m of meta-graywacke and altered shale of the basement, also containing a number of suevite intercalations. Major- and trace-element characteristics of all three sequences have been determined to investigate breccia formation and the role of the respective basement lithologies therein. Compositions of polymict impact breccias of the crater fill revealed by core LB-07A are compared with the compositions of the Ivory Coast tektites and the fallout suevites. The impactites of the LB-07A borehole appear well homogenized with respect to the silicate component, and little change in the ranges of many major- and trace-element differences is seen along the length of the borehole (except for Fe2O3, MgO, and CaO contents). Much scatter is observed for a number of elements, and in many cases this increases with depth. It is proposed that any variability in composition is likely the function of clast population differences (i.e., also of relatively small sample sizes). No systematic compositional difference between polymict lithic and suevitic impact breccias is evident. An indication of carbonate enrichment due to hydrothermal alteration is observed in samples from all lithologies. The impactites of the borehole generally show intermediate compositions to previously defined target rocks. The fallout suevites have comparable major element abundances, except for relatively lower MgO contents. The Ivory Coast tektites are generally similar in composition to the LB-07A suevites, but broader ranges in MgO and CaO contents are observed for the LB-07A suevites. © The Meteoritical Society, 2007. Printed in USA.
cited By 17

Fluid infiltration associated with seismic faulting: Examining chemical and mineralogical compositions of fault rocks from the active Chelungpu fault - Chen, W.-m.D.; Tanaka, H.; Huang, H.-j.; Lu, C.-b.; Lee, C.-y.; Wang, C.-Y. (Tectonophysics, 2007)
In order to understand the fault zone architecture and mechanisms that caused the Chi-Chi earthquake, the Chelungpu drilling project was conducted during April 2000 through a collaborative project between Japan and Taiwan. In this study, chemical and mineralogical variations within the overall Chelungpu fault zone, including variations between less damaged host rocks, damaged zones, and fault cores caused by the Chi-Chi earthquake were examined. Slopes of TiO2 immobile isocons were consistently > 1 for analyses comparing host rocks with rocks from damaged zones or with gouges from fault cores, indicating that volume loss occurred in damaged zones and the fault cores. These results strongly imply that pervasive fluid infiltration occurred within the fault zone. Volume loss within the damaged zone and fault core is interpreted to result from a two-stage process involving: (i) coseismic mechanical wearing and/or dissolution in the fault core, and (ii) fluid infiltration within the fault zone during postseismic and interseismic periods along cracks caused by seismic failure. Semi-quantitative XRD analysis indicates that the kaolinite content consistently increases from the less damaged host rocks to the damaged zone and gouges in each fault core. Mineralogic changes indicate that pervasive acidic fluid infiltration occurred within the fault zones and reacted with the feldspars or muscovite to form kaolinite. Enrichment of kaolinite and illite found in the fault zones of southern drilling site could play some role on the slipping behavior of the southern part of the Chelungpu fault. Greater volume loss in the fault core may have resulted from moderate permeability, combined with the very fine grain nature of pulverized material in the fault core, which enhanced chemical reactions including transformation of feldspars and muscovite to clay minerals. The study results indicate that pervasive fluid infiltration occurred and changed the mineralogical and chemical architecture of fault zones caused by the cyclic earthquakes. © 2007 Elsevier B.V. All rights reserved.
cited By 21
Keywords: active fault; chemical composition; Chi-Chi earthquake 1999; core analysis; fault zone; mineralogy; Asia; Eurasia; Far East; Taiwan

Halogen systematics in the Mallik 5L-38 gas hydrate production research well, Northwest Territories, Canada: Implications for the origin of gas hydrates under terrestrial permafrost conditions - Tomaru, H.; Fehn, U.; Lu, Z.; Matsumoto, R. (Applied Geochemistry, 2007)
The authors report here halogen concentrations in pore waters and sediments collected from the Mallik 5L-38 gas hydrate production research well, a permafrost location in the Mackenzie Delta, Northwest Territories, Canada. Iodine and Br are commonly enriched in waters associated with CH4, reflecting the close association between these halogens and source organic materials. Pore waters collected from the Mallik well show I enrichment, by one order of magnitude above that of seawater, particularly in sandy layers below the gas hydrate stability zone (GHSZ). Although Cl and Br concentrations increase with depth similar to the I profile, they remain below seawater values. The increase in I concentrations observed below the GHSZ suggests that I-rich fluids responsible for the accumulation of CH4 in gas hydrates are preferentially transported through the sandy permeable layers below the GHSZ. The Br and I concentrations and I/Br ratios in Mallik are considerably lower than those in marine gas hydrate locations, demonstrating a terrestrial nature for the organic materials responsible for the CH4 at the Mallik site. Halogen systematics in Mallik suggest that they are the result of mixing between seawater, freshwater and an I-rich source fluid. The comparison between I/Br ratios in pore waters and sediments speaks against the origin of the source fluids within the host formations of gas hydrates, a finding compatible with the results from a limited set of 129I/I ratios determined in pore waters, which gives a minimum age of 29 Ma for the source material, i.e. at the lower end of the age range of the host formations. The likely scenario for the gas hydrate formation in Mallik is the derivation of CH4 together with I from the terrestrial source materials in formations other than the host layers through sandy permeable layers into the present gas hydrate zones. © 2006 Elsevier Ltd. All rights reserved.
cited By 27
Keywords: Earth atmosphere; Gas hydrates; Natural gas wells; Seawater; Sedimentology; Gas hydrate production; Gas hydrate stability zone (GHSZ); Halogen systematics; Pore waters; Halogen compounds; bromine; coastal sediment; enrichment; gas hydrate; iodine; methane; mixing; organic matter; permafrost; porewater; seawater; Canada; Mackenzie Delta; North America; Northwest Territories

Impactities as a random medium - Using variations in physical properties to assess heterogeneity within the Bosumtwi meteorite impact crater - L'Heureux, Elizabeth; Milkereit, Bernd (Meteoritics and Planetary Science, 2007)
The recent drilling of the Bosumtwi impact structure, Ghana, has provided a unique opportunity to study the petrophysical properties of a young, well-preserved impact crater. The damage induced by impact results in extensive fracturing and mixing of target materials. We discuss here a means of using sonic velocity and density logs from two boreholes through the Bosumtwi crater fill and basement to estimate the degree of heterogeneity and fracturing within the impacted target, in order to understand the discrepancy between the large impedances derived from the log data and the nonreflective zone of impactites observed in seismic sections. Based on an analysis of the stochastic fluctuations in the log data, the Bosumtwi impactites are characterized by vertical scale lengths of 2-3 m. From the resolution of the seismic data over the crater, horizontal scale lengths are estimated at <12 m. The impactites therefore fall within the quasi-homogeneous scattering regime, i.e., seismic energy will propagate through the medium with little disruption. Scale lengths as small as these are observed in the fractured basement rocks of impact structures, whereas non-impact related crystalline environments are characterized by scale lengths an order of magnitude larger. Assuming that the high-frequency fluctuations observed in the log data are more sensitive to fracture distribution than petrology, this suggests that the small scale lengths observed within impact structures are characteristic of impact-induced damage, and could be used to estimate the extent of fracturing undergone by the rocks at any depth below an impact structure. © The Meteoritical Society, 2007. Printed in USA.

Hydrothermal alteration in the Bosumtwi impact structure: Evidence from 2M1-muscovite, alteration veins, and fracture fillings - Petersen, Michael T.; Newsom, Horton E.; Nelson, Melissa J.; Moore, Duane M. (Meteoritics and Planetary Science, 2007)
Drill-core samples from the Bosumtwi impact structure (1.07 Myr old and 10.5 km in diameter) in Ghana exhibit mineralogical evidence for post-impact hydrothermal alteration. Nine samples of drill core obtained through the 2004 International Continental Scientific Drilling Project (ICDP) were studied, including an uppermost fallback layer overlying impactite breccias, and partly deformed massive meta-graywacke bedrock. The petrographic study revealed alteration veins containing secondary sericitic muscovite (comparable to 2M1-muscovite) crosscutting original bedding in meta-graywacke and forming a matrix between clasts in impactite breccias. X-ray diffraction (XRD) shows that these impactite samples are rich in 2M1-muscovite, consistent with post-impact fluid deposition and alteration. Optical analysis indicates the presence of a pre-impact stratiform chlorite in meta-graywacke samples and a secondary alteration chlorite occurring in all samples. Secondary illite was detected in upper impactites of drill core LB-08A and samples containing accretionary lapilli. The lower temperature constraint for the hydrothermal event is given by 2M1-muscovite, secondary chlorite, and illite, all of which form at temperatures greater than 280 °C. An absence of recrystallization of quartz and feldspar indicates an upper temperature constraint below 900 °C. The presence of alteration materials associated with fractures and veins in the uppermost impactites of drill cores LB-07A and LB-08A indicates that a post-impact hydrothermal system was present in and adjacent to the central uplift portion of the Bosumtwi impact structure. A sample containing accretionary lapilli obtained from drill core LB-05A exhibits limited evidence that hydrothermal processes were more widespread within the impactites on the crater floor. © The Meteoritical Society, 2007. Printed in USA.

Hydrothermal alteration in the Bosumtwi impact structure: Evidence from 2M1-muscovite, alteration veins, and fracture fillings - Petersen, M.T.; Newsom, H.E.; Nelson, M.J.; Moore, D.M. (Meteoritics and Planetary Science, 2007)
Drill-core samples from the Bosumtwi impact structure (1.07 Myr old and 10.5 km in diameter) in Ghana exhibit mineralogical evidence for post-impact hydrothermal alteration. Nine samples of drill core obtained through the 2004 International Continental Scientific Drilling Project (ICDP) were studied, including an uppermost fallback layer overlying impactite breccias, and partly deformed massive meta-graywacke bedrock. The petrographic study revealed alteration veins containing secondary sericitic muscovite (comparable to 2M1-muscovite) crosscutting original bedding in meta-graywacke and forming a matrix between clasts in impactite breccias. X-ray diffraction (XRD) shows that these impactite samples are rich in 2M1-muscovite, consistent with post-impact fluid deposition and alteration. Optical analysis indicates the presence of a pre-impact stratiform chlorite in meta-graywacke samples and a secondary alteration chlorite occurring in all samples. Secondary illite was detected in upper impactites of drill core LB-08A and samples containing accretionary lapilli. The lower temperature constraint for the hydrothermal event is given by 2M1-muscovite, secondary chlorite, and illite, all of which form at temperatures greater than 280 °C. An absence of recrystallization of quartz and feldspar indicates an upper temperature constraint below 900 °C. The presence of alteration materials associated with fractures and veins in the uppermost impactites of drill cores LB-07A and LB-08A indicates that a post-impact hydrothermal system was present in and adjacent to the central uplift portion of the Bosumtwi impact structure. A sample containing accretionary lapilli obtained from drill core LB-05A exhibits limited evidence that hydrothermal processes were more widespread within the impactites on the crater floor. © The Meteoritical Society, 2007. Printed in USA.
cited By 8

Impact cratering - Fundamental process in geoscience and planetary science - Pati, J.K.; Reimold, W.U. (Journal of Earth System Science, 2007)
Impact cratering is a geological process characterized by ultra-fast strain rates, which generates extreme shock pressure and shock temperature conditions on and just below planetary surfaces. Despite initial skepticism, this catastrophic process has now been widely accepted by geoscientists with respect to its importance in terrestrial - indeed, in planetary - evolution. About 175 impact structures have been discovered on Earth so far, and some more structures are considered to be of possible impact origin. One major extinction event, at the Cretaceous-Paleogene boundary, has been firmly linked with catastrophic impact, but whether other important extinction events in Earth history, including the so-called Mother of All Mass Extinctions at the Permian-Triassic boundary, were triggered by huge impact catastrophes is still hotly debated and a subject of ongoing research. There is a beneficial side to impact events as well, as some impact structures worldwide have been shown to contain significant (in some cases, world class) ore deposits, including the gold-uranium province of the Witwatersrand basin in South Africa, the enormous Ni and PGE deposits of the Sudbury structure in Canada, as well as important hydrocarbon resources, especially in North America. Impact cratering is not a process of the past, and it is mandatory to improve knowledge of the past-impact record on Earth to better constrain the probability of such events in the future. In addition, further improvement of our understanding of the physico-chemical and geological processes fundamental to the impact cratering process is required for reliable numerical modeling of the process, and also for the correlation of impact magnitude and environmental effects. Over the last few decades, impact cratering has steadily grown into an integrated discipline comprising most disciplines of the geosciences as well as planetary science, which has created positive spin-offs including the study of paleo-environments and paleo-climatology, or the important issue of life in extreme environments. And yet, in many parts of the world, the impact process is not yet part of the geoscience curriculum, and for this reason, it deserves to be actively promoted not only as a geoscientific discipline in its own right, but also as an important life-science discipline.
cited By 22

Impact melting in sedimentary target rocks: An assessment - Osinski, G.R.; Spray, J.G.; Grieve, R.A.F. (Special Paper of the Geological Society of America, 2007)
Despite being present in the target sequence of ∼70% of the world's known impact structures, the response of sedimentary rocks to hypervelocity impact remains poorly understood. Of particular significance is the relative importance and role of impact melting versus decomposition in carbonate and sulfate lithologies. In this work, we review experimental evidence and phase equilibria and synthesize these data with observations from studies of naturally shocked rocks from several terrestrial impact sites. Shock experiments on carbonates and sulfates currently provide contrasting and ambiguous results. Studies of naturally shocked materials indicate that impact melting is much more common in sedimentary rocks than previously thought. This is in agreement with the phase relations for calcite. A summary of the criteria for the recognition of impact melts derived from sedimentary rocks is presented, and it is hoped that this will stimulate further studies of impact structures in sedimentary target rocks. This assessment leads us to conclude that impact melting is common during hypervelocity impact into both crystalline and sedimentary rocks. However, the products are texturally and chemically distinct, which has led to much confusion in the past, particularly in terms of the recognition of impact melts derived from sedimentary rocks. © 2008 The Geological Society of America. All rights reserved.
cited By 41

Implication of seismic attenuation for gas hydrate resource characterization, Mallik, Mackenzie Delta, Canada - Bellefleur, G.; Riedel, M.; Brent, T.; Wright, F.; Dallimore, S.R. (Journal of Geophysical Research: Solid Earth, 2007)
Wave attenuation is an important physical property of hydrate-bearing sediments that is rarely taken into account in site characterization with seismic data. We present a field example showing improved images of hydrate-bearing sediments on seismic data after compensation of attenuation effects. Compressional quality factors estimated from zero-offset Vertical Seismic Profiling data acquired at Mallik, Northwest Territories, Canada, demonstrate significant wave attenuation for hydrate-bearing sediments. These results are in agreement with previous attenuation estimates obtained from sonic logs and crosshole data at different frequency intervals. The application of an inverse Q-filter to compensate attenuation effects of permafrost and hydrate-bearing sediments improved the resolution of surface 3D seismic data and its correlation with log data, particularly for the shallowest gas hydrate interval. Compensation of the attenuation effects of the permafrost likely explains most of the improvements for the shallow gas hydrate zone. Our results show that characterization of the Mallik gas hydrates with seismic data not corrected for attenuation would tend to overestimate thicknesses and lateral extent of hydrate-bearing strata and hence, the volume of hydrates in place. Copyright 2007 by the American Geophysical Union.
cited By 37
Keywords: gas hydrate; permafrost; seismic attenuation; seismic data; Canada; Mackenzie Delta; North America; Northwest Territories

Petrographic studies of fallout suevite from outside the Bosumtwi impact structure, Ghana - Boamah, Daniel; Koeberl, Christian (Meteoritics and Planetary Science, 2006)
Field studies and a shallow drilling program carried out in 1999 provided information about thickness and distribution of suevite to the north of the Bosumtwi crater rim. Suevite occurrence there is known from an ∼1.5 km2 area; its thickness is ≤15 m. The present suevite distribution is likely the result of differential erosion and does not reflect the initial areal extent of continuous Bosumtwi ejecta deposits. Here we discuss the petrographic characteristics of drill core samples of melt-rich suevite. Macroscopic constituents of the suevites are melt bodies and crystalline and metasedimentary rock (granite, graywacke, phyllite, shale, schist, and possibly slate) clasts up to about 40 cm in size. Shock metamorphic effects in the clasts include multiple sets of planar deformation features (PDFs), diaplectic quartz and feldspar glasses, lechatelierite, and ballen quartz, besides biotite with kink bands. Basement rock clasts in the suevite represent all stages of shock metamorphism, ranging from samples without shock effects to completely shock-melted material that is indicative of shock pressures up to ∼60 GPa. © The Meteoritical Society, 2006.
Keywords: Africa; Ghana; Sub-Saharan Africa; West Africa; crater; crystalline rock; ejecta; fallout; impact structure; metasedimentary rock; suevite

Petrology and thermal structure of the Hawaiian plume from Mauna Kea volcano - Herzberg, C. (Nature, 2006)
There is uncertainty about whether the abundant tholeiitic lavas on Hawaii are the product of melt from peridotite or pyroxenite/eclogite rocks. Using a parameterization of melting experiments on peridotite with glass analyses from the Hawaii Scientific Deep Project 2 on Mauna Kea volcano, I show here that a small population of the core samples had fractionated from a peridotite-source primary magma. Most lavas, however, differentiated from magmas that were too deficient in CaO and enriched in NiO (ref. 2) to have formed from a peridotite source. For these, experiments indicate that they were produced by the melting of garnet pyroxenite, a lithology that had formed in a second stage by reaction of peridotite with partial melts of subducted oceanic crust. Samples in the Hawaiian core are therefore consistent with previous suggestions that pyroxenite occurs in a host peridotite, and both contribute to melt production. Primary magma compositions vary down the drill core, and these reveal evidence for temperature variations within the underlying mantle plume. Mauna Kea magmatism is represented in other Hawaiian volcanoes, and provides a key for a general understanding of melt production in lithologically heterogeneous mantle. ©2006 Nature Publishing Group.
cited By 220
Keywords: Lithology; Melting; Rocks; Thermal plumes; Uncertain systems; Volcanoes; Hawaiian core; Mauna Kea magmatism; Pyroxenite/eclogite rocks; Tholeiitic lavas; Petrology; aluminum derivative; aluminum trioxide; calcium oxide; chromium trioxide; dipotassium oxide; glass; iron oxide; iron trioxide; magnesium oxide; manganese oxide; nitrogen oxide; oxacillin; oxide; phosphorus pentoxide; potassium derivative; silicon dioxide; titanium dioxide; core analysis; lava; magma; mantle plume; oceanic crust; peridotite; petrology; pyroxenite; thermal structure; tholeiite; article; calibration; crystallization; fractionation; garnet; melting point; peridotite; petrology; priority journal; pyroxenite; rock; volcano

Post-Chicxulub depositional and diagenetic history of the northwestern Yucatan Peninsula, Mexico - Lefticariu, M.; Perry, E.C.; Ward, W.C.; Lefticariu, L. (Sedimentary Geology, 2006)
The Chicxulub Sedimentary Basin of the northwestern Yucatan Peninsula, Mexico, which was formed because of the largest identified Phanerozoic bolide impact on Earth, became a site of deposition of dominantly marine carbonate sediments during most of the Cenozoic Era. This is a study of the filling and diagenetic history of this basin and surrounding areas. The study makes use of lithologic, biostratigraphic, petrographic, and geochemical data obtained on core samples from boreholes drilled throughout the northwestern Yucatan Peninsula. The core sample data indicate that: 1) The Chicxulub Sedimentary Basin concentrated the deposition of pelagic and outer-platform sediments during the Paleocene and Eocene, and, in places, during the Early Oligocene, as well, and filled during the Middle Miocene, 2) deeper-water limestone also is present within the Paleocene and Lower Eocene of the proposed Santa Elena Depression, which is located immediately south of the Basin, 3) shallow-water deposits are relatively more abundant outside the Basin and Depression than inside, 4) the autigenic and allogenic silicates from the Paleogene formations are the most abundant inside the Depression, 5) sediment deposition and diagenesis within the Basin also were controlled by impact crater topography, 6) the abundance of the possible features of subaerial exposure increases upward and outward from the center of the Basin, and 7) the formation of replacive low-magnesium calcite and dolomite, dedolomitization, dissolution, and precipitation of vug-filling calcite and dolomite cement have been more common outside the Basin than inside. δ18O in whole-rock (excluding vug-filling) calcite from core samples ranges from -7.14‰ to + 0.85‰ PDB. δ13C varies from -6.92‰ to +3.30‰ PDB. Both stable isotopes correlate inversely with the abundance of subaerial exposure features indicating that freshwater diagenesis has been extensive especially outside and at the edge of the Chicxulub Sedimentary Basin. δ18O and δ13C in w hole-rock (excluding vug-filling) dolomite ranges from -5.54‰ to +0.87‰ PDB and -4.63‰ to +3.38‰ PDB, respectively. Most dolomite samples have negative δ18O and positive δ13C suggesting that replacive dolomitization involved the presence of a fluid dominated by freshwater and/or an anomalously high geothermal gradient. Most dolomite XRD-determined mole percent CaCO3 varies between 51 and 56. Replacive dolomite is larger, more euhedral, and less stoichiometric inside the Chicxulub Sedimentary Basin than outside. © 2005 Elsevier B.V. All rights reserved.
cited By 25

SAFOD Penetrates the San Andreas Fault - Zoback, Mark D. (Scientific Drilling, 2006)
SAFOD, the San Andreas Fault Observatory at Depth (Fig. 1), completed an important milestone in July 2005 by drilling through the San Andreas Fault at seismogenic depth. SAFOD is one of three major components of EarthScope, a U.S. National Science Foundation (NSF) initiative being conducted in collaboration with the U.S. Geological Survey (USGS). The International Continental Scientific DrillingProgram (ICDP) provides engineering and technical support for the project as well as online access to project data and information (https://www.icdp-online.org/sites/sanandreas/news/news1.html). In 2002, the ICDP, the NSF, and the USGS provided funding for a pilot hole project at the SAFOD site. Twenty scientifi c papers summarizing the results of the pilot hole project as well as pre-SAFOD site characterization studies were published in Geophysical Research Letters (Vol.31, Nos. 12 and 15, 2004).

Rare earth element composition as evidence of the precursor material of Cretaceous-Tertiary boundary sediments at distal sections - Mart'inez-Ruiz, F.; Ortega-Huertas, M.; Rivas, P. (Chemical Geology, 2006)
The Chicxulub impact event led to a worldwide deposition of impact materials originated from target rocks and the vaporized bolide. Relative contributions of both types of material to the K/T ejecta deposits vary with distance to the crater site. At distal sites (e.g., Agost and Caravaca in the SE of Spain) a major contribution of extraterrestrial material is indicated by different impact signatures, such as Os and Cr isotope composition, abundant microkrystites, platinum group elements and other siderophile elements that are typical of extraterrestrial components. Closer settings to the Chicxulub crater, for example the Blake Nose Plateau in the North American margin, display major continental crustal rock contributions in the ejecta layer. REE compositions provide additional evidence for terrestrial vs. extraterrestrial rock contributions. Previous research has not focused specifically on REE concentrations and corresponding C1- and NASC-normalized patterns. However, normalized REE patterns are already generating supplementary insights into the nature of the original material of the K/T boundary layer. Thus, Blake Nose ejecta C1-normalized patterns indicate a derivation from continental crustal target rocks. In more distal sections REE compositions point to a probable mafic precursor and confirm that extraterrestrial materials represent a major contribution the ejecta layer. © 2006 Elsevier B.V. All rights reserved.
cited By 16

Refining the image of the San Andreas Fault near Parkfield, California using a finite difference travel time computation technique - Roecker, S.; Thurber, C.; Roberts, K.; Powell, L. (Tectonophysics, 2006)
The Parkfield Area Seismic Observatory (PASO) was a dense, telemetered seismic array that operated for nearly 2 years in a 15 km aperture centered on the San Andreas Fault Observatory at Depth (SAFOD) drill site. The main objective of this deployment was to refine the locations of earthquakes that will serve as potential targets for SAFOD drilling and in the process develop a high (for passive seismological techniques) resolution image of the fault zone structure. A challenging aspect of the analysis of this data set was the known existence of large (20-25%) contrasts in seismic wavespeed across the San Andreas Fault. The resultant distortion of raypaths could challenge the applicability of approximate ray tracing techniques. In order to test the sensitivity of our hypocenter locations and tomographic image to the particular ray tracing and inversion technique employed, we compare an initial determination of locations and structure developed using a coarse grid and an approximate ray tracer [Thurber, C., Roecker, S., Roberts, K., Gold, M., Powell, M.L., and Rittger, K., 2003. Earthquake locations and three-dimensional fault zone structure along the creeping section of the San Andreas fault near Parkfield, CA: Preparing for SAFOD, Geophys. Res. Lett., 30 3, 10.1029/2002GL016004.] with one derived from a relatively fine grid and an application of a finite difference algorithm [Hole, J.A., and Zelt, B.C., 1995. 3-D finite-difference reflection traveltimes, Geophys. J. Int., 121, 2, 427-434.]. In both cases, we inverted arrival-time data from about 686 local earthquakes and 23 shots simultaneously for earthquake locations and three-dimensional Vp and Vp/Vs structure. Included are data from an active source seismic experiment around the SAFOD site as well as from a vertical array of geophones installed in the 2-km-deep SAFOD pilot hole, drilled in summer 2002. Our results show that the main features of the original analysis are robust: hypocenters are located beneath the trace of the fault in the vicinity of the drill site and the positions of major contrasts in wavespeed are largely the same. At the same time, we determine that shear wave speeds in the upper 2 km of the fault zone are significantly lower than previously estimated, and our estimate of the depth of the main part of the seismogenic zone decreases in places by ∼ 100 m. Tests using virtual earthquakes (borehole receiver gathers of picks for surface shots) indicate that our event locations near the borehole currently are accurate to about a few tens of meters horizontally and vertically. © 2006 Elsevier B.V. All rights reserved.
cited By 65
Keywords: data inversion; fault zone; ray tracing; seismic tomography; seismic zone; travel time; California; North America; Parkfield; United States

Seismic characterization and continuity analysis of gas-hydrate horizons near Mallik research wells, Mackenzie Delta, Canada - Bellefleur, G.; Riedel, M.; Brent, T. (Leading Edge, 2006)
Gas-hydrate accumulations located onshore in Arctic permafrost regions are seen as a potential source of natural gas. Surprisingly, most of the gas hydrate found in the Mackenzie Delta and Beaufort Sea areas was indirectly discovered or inferred from conventional hydrocarbon exploration programs. One of these occurrences, the Mallik gas-hydrate field (Figure 1), has received particular attention over the last 10 years. Two internationally partnered research well programs have intersected three intervals of gas hydrates and have allowed successful extraction of subpermafrost core samples with significant gas hydrates. The gas-hydrate intervals are up to 40 m thick and have high gas-hydrate saturation, sometimes exceeding 80% of pore volume of unconsolidated clastic sediments with average porosities from 25-40%. At Mallik, the gas-hydrate intervals are located at depths of 900-1100 m and are localized on the crest of an anticline. © 2006 Society of Exploration Geophysicists.
cited By 39
Keywords: Gases; Hydration; Petroleum prospecting; Clastic sediments; Conventional hydrocarbons; Gas hydrate saturations; Hydrate accumulations; Permafrost region; Potential sources; Significant gas; Three interval; Gas hydrates; clastic sediment; gas hydrate; hydrocarbon exploration; natural gas; permafrost; porosity; supermarket; Arctic; Arctic Ocean; Beaufort Sea; Canada; Mackenzie Delta; Northwest Territories

Scanned core image database and web-sharing of Chinese continental scientific drilling project - Su, D.-C.; Wang, W.; Sun, A.-P.; Guo, L.-M.; Zhou, Y.-H. (Diqiu Kexue - Zhongguo Dizhi Daxue Xuebao/Earth Science - Journal of China University of Geosciences, 2006)
The only main hole of the Chinese Continental Scientific Drilling (CCSD) Project, the deepest scientific drilling project of all the ICDP projects, reached its depth of over 5000 m, from which more than 4400 m cores were collected. Hence the scanned core image database of CCSD project is also the largest of all the databases of ICDP projects. After various samplings, some cores have been completely damaged or disappeared. To make full use of the scanned core images, an MS ACCESS database was developed, on the basis of the original DIS system. More than 8000 images (about 100 gigabyte) have been efficiently and systematically managed and shared on the Internet with some user-friendly query program and interfaces using the ASP and VB techniques. This new information technology, proved by one year trial test, can not only store permanently and efficiently the original image data, but also help geologists to study efficiently the cores for their scientific purpose.
cited By 0
Keywords: Database systems; Image processing; Information technology; Interfaces (computer); Internet; Sampling; Scanning; Chinese Continental Scientific Drilling; Data share; Scanned core image; User friendly interface; Drilling

Seeking chemical clues to earthquakes - Wilson, E.K. (Chemical and Engineering News, 2006)
cited By 0
Keywords: Flow of fluids; Pressure effects; Real time systems; Rocks; Tectonics; Chemical clue; Frictional change; San Andreas Fault; San Andreas Fault Observatory at Depth (SAFOD); Earthquakes

Paleoclimatic variations in West Africa from a record of late Pleistocene and Holocene lake level stands of Lake Bosumtwi, Ghana - Shanahan, Timothy M.; Overpeck, Jonathan T.; Wheeler, C. Winston; Beck, J. Warren; Pigati, Jeffrey S.; Talbot, Michael R.; Scholz, Christopher A.; Peck, John; King, John W. (Palaeogeography, Palaeoclimatology, Palaeoecology, 2006)
A detailed investigation of geomorphological evidence of paleoshorelines and exposed stratigraphic sections of lake deposits, combined with a chronology based on radiocarbon dated charcoal and in-situ 14C dating of wave polished bedrock, provide important new constraints on lake level changes of Lake Bosumtwi, Ghana. Thick sequences of laminated silts, alternating with transgressive sands and deltaic gravels, attest to a long history of climatically controlled lake level variations. The post-glacial rise in lake level began sometime after 16.3 ka, reached stable levels first at 14.5 ± 0.6 ka and then rose again after ca. 14.3 ka. A significant lake level regression spanned the interval from 12.6 ± 0.3 to 11.6 ± 0.5 ka, synchronous with the Younger Dryas. Deep lake conditions were reestablished after ca. 11 ka, at which time the lake overtopped the crater. Overflow continued until 8.8 ± 0.5 ka, when another significant but short-lived regression occurred. Deep, but probably not overflowing conditions were again reestablished by > 7.2 ± 0.3 ka and continued until around 3.2 ± 0.1 ka, when lake level dropped precipitously. Multicentury late Holocene highstands occurred at 2.2 ± 0.1 and 1.7 ± 0.2 ka, although these were significantly lower than those registered in the late glacial and early Holocene. The timing of late glacial events is similar to those recorded elsewhere in Africa and the higher latitudes, and likely reflects the dominant control of high latitude northern hemisphere conditions on the African tropics during the times of large northern hemisphere ice sheets. Mid- to late-Holocene variations appear to be less coupled with changes across Africa and elsewhere, suggesting that regional forcing may be more important during warmer periods. © 2006.
Keywords: Africa; Ashanti; Ghana; Lake Bosumtwi; Sub-Saharan Africa; West Africa; climate variation; crater lake; highstand; Holocene; ice sheet; lacustrine deposit; lake level; Last Glacial; monsoon; paleoclimate; Pleistocene; radiocarbon dating; Younger Dryas

Paleoenvironmental record in Lake Baikal sediments: Environmental changes in the last 160 ky - Grygar, T.; Kadlec, J.; Pruner, P.; Swann, G.; Bezdička, P.; Hradil, D.; Lang, K.; Novotna, K.; Oberh{ansli, H. (Palaeogeography, Palaeoclimatology, Palaeoecology, 2006)
Measurement of magnetic susceptibility (MS) and diffuse reflectance spectra (DRS) were used to construct environmental proxies in a 6.5 m section of the sediment core VER98-1-13 from the Academician Ridge, Lake Baikal. The interpretation of MS and DRS was supported by X-ray diffraction and microparticle voltammetry to identify the main Fe-bearing minerals. The record of the relative paleointensity of the Earth's magnetic field was used to construct an age model showing the core interval covers the last 160 ky. The time resolution of the record was ∼1 ky. The main environmental changes were recognized by a combination of DRS and MS records and compared to marine isotopic stages in addition to the diatom record from 120 to 60 ky BP so as to demonstrate the importance of these inorganic proxies as paleoenvironmental records. A dramatic climatic deterioration between 66 and 60 ky BP was probably preceded by a dry intermediate stage between 77 and 66 ky BP. The DRS-based proxies of Fe(II) / Fe(III) ratios in the mineral assemblage, MS and/or diatom records clearly reveal 1-2 ky long environmental extremes at 110, 103, 90, 85, 77, 61, 54, 36, 27, 23, and 19 ky BP. The majority of these extremes were contemporaneous with discharges of North Atlantic and Arctic Ocean icebergs (Heinrich events) documenting a teleconnection between the North Atlantic Ocean and East Central Siberia. These sharp changes coincided with a major transition to the colder climatic stages of the last 100 ky. © 2006.
Keywords: Arctic Ocean; Atlantic Ocean; Atlantic Ocean (North); Eurasia; Lake Baikal; Russian Federation; Siberia; Bacillariophyta; climate change; environmental change; fossil record; magnetic susceptibility; paleoenvironment; reflectance; sediment analysis; X-ray diffraction

Mineral and fluid inclusions in zircon of UHP metamorphic rocks from the CCSD-main drill hole: A record of metamorphism and fluid activity - Zhang, Zeming; Shen, Kun; Xiao, Yilin; Hoefs, Jochen; Liou, J.G. (Lithos, 2006)
The Chinese Continental Scientific Drilling (CCSD) main drill hole (0-3000 m) in Donghai, southern Sulu orogen, consists of eclogite, paragneiss, orthogneiss, schist and garnet peridotite. Detailed investigations of Raman, cathodoluminescence, and microprobe analyses show that zircons from most eclogites, gneisses and schists have oscillatory zoned magmatic cores with low-pressure mineral inclusions of Qtz, Pl, Kf and Ap, and a metamorphic rim with relatively uniform luminescence and eclogite-facies mineral inclusions of Grt, Omp, Phn, Coe and Rt. The chemical compositions of the UHP metamorphic mineral inclusions in zircon are similar to those from the matrix of the host rocks. Similar UHP metamorphic P-T conditions of about 770 °C and 32 kbar were estimated from coexisting minerals in zircon and in the matrix. These observations suggest that all investigated lithologies experienced a joint in situ UHP metamorphism during continental deep subduction. In rare cases, magmatic cores of zircon contain coesite and omphacite inclusions and show patchy and irregular luminescence, implying that the cores have been largely altered possibly by fluid-mineral interaction during UHP metamorphism. Abundant H2O-CO2, H2O- or CO2-dominated fluid inclusions with low to medium salinities occur isolated or clustered in the magmatic cores of some zircons, coexisting with low-P mineral inclusions. These fluid inclusions should have been trapped during magmatic crystallization and thus as primary. Only few H2O- and/or CO2-dominated fluid inclusions were found to occur together with UHP mineral inclusions in zircons of metamorphic origin, indicating that UHP metamorphism occurred under relatively dry conditions. The diversity in fluid inclusion populations in UHP rocks from different depths suggests a closed fluid system, without large-scale fluid migration during subduction and exhumation. © 2006 Elsevier B.V. All rights reserved.
Keywords: Asia; China; Donghai; Eurasia; Far East; Jiangsu; cathodoluminescence; chemical composition; electron probe analysis; exhumation; fluid inclusion; lithology; magmatic differentiation; mineralization; P-T conditions; Raman spectroscopy; subduction; ultrahigh pressure metamorphism

Paleoclimate and human evolution workshop - Cohen, Andrew S.; Ashley, Gail M.; Potts, Richard; Behrensmeyer, Anna K.; Feibel, Craig; Quade, Jay (Eos, 2006)
Keywords: human evolution; Neogene; paleoclimate

Origin of Pleistocene-Holocene ashes of the Russian northeast based on trace and rare earth element data - Sakhno, V.G.; Bazanova, L.I.; Glushkova, O.Yu.; Melekestsev, I.V.; Ponomareva, V.V.; Surnin, A.A.; Olaf, J. (Doklady Earth Sciences, 2006)
cited By 4
Keywords: Pleistocene-Holocene boundary; rare earth element; trace element; volcanic ash; volcanic eruption

Origin and significance of clay-coated fractures in mudrock fragments of the SAFOD borehole (Parkfield, California) - Schleicher, A.M.; Solum, J.G.; Warr, L.N. (Geophysical Research Letters, 2006)
The clay mineralogy and texture of rock fragments from the SAFOD borehole at 3067 m and 3436 m measured depth (MD) was investigated by electron microscopy (SEM, TEM) and X-ray-diffraction (XRD). The washed and ultrasonically cleaned samples show slickenfiber striations and thin films of Ca-K bearing smectite that are formed on polished fault surfaces, along freshly opened fractures and within adjacent mineralized veins. The cation composition and hydration behavior of these films differ from the Namontmorillonite of the fresh bentonite drilling mud, although there is more similarity with circulated mud recovered from 3479 m MD. We propose that these thin film smectite precipitates formed by natural nucleation and crystal growth during fault creep, probably associated with the shallow circulation of low temperature aqueous fluids along this shallow portion of the San Andreas Fault. Copyright 2006 by the American Geophysical Union.
cited By 67
Keywords: Clay; Crystal growth; Rocks; Scanning electron microscopy; Textures; Thin films; Transmission electron microscopy; X ray diffraction analysis; Aqueous fluids; Drilling mud; Fault creep; Slickenfiber striations; Boreholes; bentonite; borehole; calcium; cation; clay; crystal structure; hydration; mudstone; potassium; rock property; sampling; San Andreas Fault; scanning electron microscopy; smectite; texture; transmission electron microscopy; X-ray diffraction

Orbital forcing of continental climate during the Pleistocene: a complete astronomically tuned climatic record from Lake Baikal, SE Siberia - Prokopenko, Alexander A.; Hinnov, Linda A.; Williams, Douglas F.; Kuzmin, Mikhail I. (Quaternary Science Reviews, 2006)
A new composite BDP-96 biogenic silica record over the entire Pleistocene was generated by splicing BDP-96-1 and BDP-96-2 drill cores from Lake Baikal, crosschecked against a similar record from a nearby BDP-98 drill core. A new astronomically tuned age model is proposed based on correlating peak biogenic silica responses with the timing of September perihelia. This target is derived from analysis of regional climate proxy responses during the Holocene, the last interglacial and around paleomagnetic reversals. By resolving virtually every precessional cycle during the Pleistocene, the new age model represents a major improvement compared with previously reported Lake Baikal timescales. The astronomically tuned ages of the Pleistocene paleomagnetic reversals are consistent with published dates. The minimal tuning approach we used (precession only) has also aligned high signal power in a narrow obliquity band, confirming the strong presence of orbital forcing. There are also strong ca 100-ka scale cycles, but these are not aligned with the orbital eccentricity. Despite the location of Lake Baikal in a continental interior that is highly sensitive to insolation forcing, the tuned biogenic silica record reveals a consistent phase difference of -32° (ca 4 ka) relative to insolation in the obliquity band. An inherent lag embedded in a continental proxy record, not driven by global ice volume, is an intriguing finding. Another new observation is that long-term changes in sedimentation rates in Lake Baikal appear to be related to the amplitude of orbital forcing; both amplitudes and sedimentation rates undergo significant changes during MIS 24-MIS 19 interval corresponding to the Middle Pleistocene Transition. With potential for linking continental and marine climato-stratigraphies, the new Baikal record serves a new benchmark correlation target in continental Eurasia, as an alternative to June 65°N insolation and ODP-correlated timescales. © 2006 Elsevier Ltd. All rights reserved.
Keywords: Eurasia; Lake Baikal; Russian Federation; Siberia; Astronomy; Cell culture; Correlation methods; Glaciers; Lakes; Mathematical models; Sedimentation; age determination; biogenic mineral; climate change; Holocene; insolation; magnetic reversal; orbital forcing; paleoclimate; Pleistocene; precession; proxy climate record; sedimentation rate; silica; Astronomically tuned climatic records; Biogenic silica; Pleistocene paleomagnetic reversals; Regional climate; Climate change

On Pliocaenicus costatus (Bacillariophyceae) in Lake El’gygytgyn, East Siberia - Cremer, Holger; Vijver, Bart (European Journal of Phycology, 2006)

Occurrence and palaeoenvironmental significance of aromatic hydrocarbon biomarkers in Oligocene sediments from the Mallik 5L-38 Gas Hydrate Production Research Well (Canada) - Haberer, R.M.; Mangelsdorf, K.; Wilkes, H.; Horsfield, B. (Organic Geochemistry, 2006)
The aromatic hydrocarbon biomarker distributions of thirty Oligocene sediment samples with different lithology (lignite, clay and sand) from the JAPEX/JNOC/GSC et al. Mallik 5L-38 Gas Hydrate Production Research Well, Canada, were analyzed using gas chromatography-mass spectrometry (GC-MS). The compositions vary with lithology, indicating a change in palaeoenvironmental conditions at the time of deposition. Aromatic diterpenoids of the abietane type are more abundant in the lignite samples than in the clay samples and represent a gymnosperm (e.g., conifer) dominated palaeovegetation. In contrast, in the clay samples aromatic triterpenoids are generally preserved as major constituents, indicating angiosperm dominated vegetation. The sand samples contain only minor amounts of aromatic terpenoids, but show a preference for diterpenoid gymnosperm markers. To recognise gymnosperm versus angiosperm dominated palaeoenvironments a new ratio, termed the angiosperm-gymnosperm aromatic ratio (AGAR), has been developed. Thus, the terpenoid distribution in the deltaic sediments provides information on the compositional changes in the plant community at the Mallik site (lignites) and the hinterland (clays) over time. Concomitantly, the changing dominance in the plant communities allows an insight into varying climatic conditions during the late Oligocene in the area. Additionally, the aromatic biomarker composition has been used to assess the level of thermal maturity of the organic matter in the Mallik samples and indicates a prevailing immature character. © 2006 Elsevier Ltd. All rights reserved.
cited By 61
Keywords: Angiosperm-gymnosperm aromatic ratio (AGAR); Aromatic diterpenoids; Climatic conditions; Gymnosperm markers; Aromatic hydrocarbons; Composition; Environmental impact; Gas chromatography; Hydration; Mass spectrometry; Sediments; Biomarkers; aromatic hydrocarbon; biomarker; geochemistry; Oligocene; paleoenvironment; sediment; vegetation; Canada; North America; Coniferophyta; Gymnospermae; Magnoliophyta

Numerical study of transmission signatures of gas hydrate-bearing microstructures - Kr{uger, O.S.; Saenger, E.H.; Zanoth, S.R.; Shapiro, S.A. (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)
This work is inspired by the observation, that gas hydrate bearing sediments have a high velocity in combination with high attenuation. We study numerically the influence of different gas hydrate locations within the pore space on transmitted p-waves. From the wave propagation simulations on the micro-scale it can be seen, that different positions of the gas hydrate in the pore space results in almost the same effective velocities and attenuation, as long as the gas hydrate had contact to the sediment grains. This changes in the case of a suspension, here the attenuation increases and the effective velocity decreases. The resulting p-wave versus gas hydrate saturation plot is in a qualitatively good agreement with experimental results obtained for the Mallik 2L-38 well. Copyright 2006, European Association of Geoscientists and Engineers.
cited By 0
Keywords: Computer simulation; Gas hydrates; Numerical methods; Sediments; Velocity measurement; Wave propagation; Wave transmission; Attenuation; Gas hydrate locations; Gas hydrate saturation; Sediment grains; Microstructure

Modeling the reflection coefficients and slow wave mode conversions at the top and bottom of a gas-hydrate bearing interval - Santos, J.E.; Rubino, J.G.; Ravazzoli, C.L. (SEG Technical Program Expanded Abstracts, 2006)
We analyze the reflectivity properties of the interfaces defined by a contact between a shaly sandstone and the top and bottom of a gas hydrate stability zone (GHSZ), a problem of particular interest in seismic exploration in ocean sediments and continental margins. Our com putations are based on a three phase Biot-type model predicting the existence of three compressional waves and two shear waves. We use some information from the Mallik 5L-38 Gas Hydrate Research Well. The mechanical properties of the gas hydrate bearing rocks are described with appropriate petrophysical models. We present numerical results showing the influence of the gas-hydrate saturation on the amplitude vs. angle curves and the importance of wave energy conversion from fast to slow waves. © 2005 Society of Exploration Geophysicists.
cited By 0
Keywords: Bearings (machine parts); Energy conversion; Gas hydrates; Gases; Hydration; Petroleum prospecting; Reflection; Seismic prospecting; Shear flow; Shear waves; Wave energy conversion; Compressional waves; Continental margin; Gas hydrate saturations; Gas hydrate stability zones; Numerical results; Ocean sediments; Petrophysical models; Seismic exploration; Phase interfaces

Modeling and inversion of Sonic P and S Wave velocities at the Mallik 5L-38 gas hydrate research well - Rubino, J.G.; Ravazzoli, C.L.; Santos, J.E. (Saint Petersburg 2006 International Conference and Exhibition, 2006)
Using an interval of real log data from the Mallik 5L38 Gas Hydrate Research Well, we estimate empirical relations for some mechanical properties of the gas hydrate frame. This is an important problem for geophysical exploration in gas hydrate bearing sediments, a subject which has not received as much attention as it deserves from the researchers in this field. The theoretical velocities used to model the sonic logs are computed using a Biottype formulation combined with known petrophysical models for shaley sandstones. The empirical results are also applied to obtain estimates of gas hydrate saturations at a different depth interval from sonic and density log data, showing the applicability of this approach.
cited By 0
Keywords: Exhibitions; Gases; Geophysical prospecting; Hydration; Mechanical properties; Radioactivity logging; Research; Density log; Empirical relations; Gas hydrate bearing sediments; Gas hydrate saturations; Geophysical exploration; P- and S-wave velocities; Petrophysical models; Sonic logs; Gas hydrates

Mobile element analysis by secondary ion mass spectrometry (SIMS) of impactite matrix samples from the Yaxcopoil-1 drill core in the Chicxulub impact structure - Newsom, H.E.; Nelson, M.J.; Shearer, C.K.; Dressler, B.O. (Meteoritics and Planetary Science, 2006)
The concentrations of the fluid mobile trace elements lithium, beryllium, boron, and barium were measured in samples of the altered matrix of several impactite breccias of the Yaxcopoil-1 drill core using secondary ion mass spectrometry (SIMS) to determine the extent of transport due to aqueous or hydrothermal processes. Three of the elements, Li, Be, and B, have higher concentrations in the upper suevite impact breccias than in the lower impact melt deposits by factors of 3.5, 2.2, and 1.5, respectively. Lithium and B are the most enriched elements up section, and appear to have had the greatest mobility. The similar fractionation of Li and B is consistent with fluid transport and alteration under low-temperature conditions of less than 150 °C based on published experimental studies. In contrast to Li, Be, and B, the concentration of Ba in the altered matrix materials decreases upward in the section, and the concentration of Ba in the matrix is an order of magnitude less than the bulk concentrations, likely due to the presence of barite. The origin of the elemental variations with depth may be related to different protolith compositions in the upper versus the lower impactite units. A different protolith in the altered matrix is suggested by the Mg-rich composition of the tower units versus the Al-rich composition of the upper units, which largely correlates with the mobile element variations. The possibility that vertical transport of mobile elements is due to a postimpact hydrothermal system is supported by published data showing that the sediments immediately overlying the impactites are enriched in mobile elements derived from a hydrothermal system. However, the mobile elements in the sediments do not have to originate from the underlying impactites. In conclusion, our data suggests that the impactites at this location did not experience extensive high-temperature hydrothermal processing, and that only limited transport of some elements, including Li, Be, and B, occurred. © The Meteoritical Society, 2006.
cited By 6

Mineralogical characterization of protolith and fault rocks from the SAFOD Main Hole - Solum, J.G.; Hickman, S.H.; Lockner, D.A.; Moore, D.E.; Schleicher, A.M.; Evans, J.P. (Geophysical Research Letters, 2006)
Washed cuttings provide a continuous record of the rocks encountered during drilling of the main hole of the San Andreas Fault Observatory at Depth (SAFOD). Both protolith and fault rocks exhibit a wide variety of mineral assemblages that reflect variations in some combination of lithology, P-T conditions, deformation mechanisms, and fluid composition and abundance. Regions of distinct neomineralization bounded by faults may record alteration associated with fluid reservoirs confined by faults. In addition, both smectites occurring as mixed-layer phases and serpentine minerals are found in association with active strands of the San Andreas Fault that were intersected during drilling, although their rheological influence is not yet fully known. Faults containing these mineralogical phases are prime candidates for continuous coring during Phase 3 of SAFOD drilling in the summer of 2007. Copyright 2006 by the American Geophysical Union.
cited By 97
Keywords: Deformation; Reservoirs (water); Rheology; Rock drilling; Rocks; Serpentine; Fluid reservoirs; Mineralogical phases; Smectites; Washed cuttings; Geophysics; drilling; fault; lithology; mineralogy; protolith; rock; summer; California; North America; San Andreas; United States

Seismic reflections of gas hydrate from perturbational forward modeling - Cordon, I.; Dvorkin, J.; Mavko, G. (Geophysics, 2006)
We perturb the elastic properties and attenuation in the Arctic Mallik methane-hydrate reservoir to produce a set of plausible seismic signatures away from the existing well. These perturbations are driven by the changes we impose on porosity, clay content, hydrate saturation, and geometry. The key is a data-guided, theoretical, rock-physics model that we adopt to link velocity and attenuation to porosity, mineralogy, and amount of hydrate. We find that the seismic amplitude is very sensitive to the hydrate saturation in the host sand and its porosity as well as the porosity of the overburden shale. However, changes to the amount of clay in the sand only weakly alter the amplitude. Attenuation, which may be substantial, must be taken into account during hydrate reservoir characterization because it lowers the amplitude to an extent that may affect the hydrate-volume prediction. The spatial structure of the reservoir affects the seismic reflection: A thinly-layered reservoir produces a noticeably different amplitude than a massive reservoir with the same hydrate volume. © 2006 Society of Exploration Geophysicists.
cited By 12
Keywords: Gas hydrates; Mineralogy; Organic compounds; Perturbation techniques; Petroleum reservoirs; Porosity; Reflection; Seismic amplitude; Seismic reflections; Seismology; amplitude; clay; elastic property; porosity; sand; seismic attenuation; seismic reflection; seismic velocity; seismology

Seismic evidence for asthenospheric updoming beneath the western Bohemian Massif, central Europe - Heuer, B.; Geissler, W.H.; Kind, R.; K{ampf, H. (Geophysical Research Letters, 2006)
Detailed images of the lithosphere beneath the western Bohemian Massif were obtained by analysis of more than 8500 P receiver functions. At the intersection of Regensburg-Leipzig-Rostock zone and Eger Rift, crustal thickness decreases to 26 km from approx. 31 km in the surroundings. The receiver functions display a positive phase at about 6 s delay time and a strong negative phase at 7 to 8 s, which coincides with an area of Moho updoming, CO2 mantle-derived degassing and earthquake swarm activity. These phases can be modeled by a velocity increase at 50 km and a velocity decrease at 65 km depth. The velocity decrease, observed over an area of 5300 km2, gives evidence for local asthenospheric updoming and/or a confined body of partial melt, which might be the cause for high CO2 mantle fluid flow and earthquake swarm activity in this recently nonvolcanic, intracontinental rift area. Copyright 2006 by the American Geophysical Union.
Keywords: Bohemian Massif; Central Europe; Eurasia; Europe; Carbon dioxide; Degassing; Earthquakes; Flow of fluids; Tectonics; asthenosphere; crustal thickness; earthquake swarm; fluid flow; imaging method; mantle process; Asthenospheric updoming; Carbon dioxide mantle-derived degassing; Earthquake swarm activity; Seismology

Time-lapse changes in tube and guided waves in cross-well Mallik experiment - Bakulin, A.; Korneev, V.; Watanabe, T.; Ziatdinov, S. (SEG Technical Program Expanded Abstracts, 2006)
We analyze cross-well seismic data from the Mallik experiment and demonstrate time-lapse changes in tube and guided waves. Although such changes are challenging to interpret, they are generally of a larger magnitude compared to any time-lapse signatures of the first P-wave arrivals reported elsewhere. This suggests better sensitivity of tube and guided waves to small production-related changes and their feasibility for reservoir monitoring. © 2005 Society of Exploration Geophysicists.
cited By 4
Keywords: Geophysical prospecting; Seismic waves; Seismology; Cross-well; P-wave arrival; Reservoir monitoring; Seismic datas; Guided electromagnetic wave propagation

Seismicity, deformation and seismic hazard in the western rift of Corinth: New insights from the Corinth Rift Laboratory (CRL) - Bernard, P.; Lyon-Caen, H.; Briole, P.; Deschamps, A.; Boudin, F.; Makropoulos, K.; Papadimitriou, P.; Lemeille, F.; Patau, G.; Billiris, H.; Paradissis, D.; Papazissi, K.; et al. (Tectonophysics, 2006)
This paper presents the main recent results obtained by the seismological and geophysical monitoring arrays in operation in the rift of Corinth, Greece. The Corinth Rift Laboratory (CRL) is set up near the western end of the rift, where instrumental seismicity and strain rate is highest. The seismicity is clustered between 5 and 10 km, defining an active layer, gently dipping north, on which the main normal faults, mostly dipping north, are rooting. It may be interpreted as a detachment zone, possibly related to the Phyllade thrust nappe. Young, active normal faults connecting the Aigion to the Psathopyrgos faults seem to control the spatial distribution of the microseismicity. This seismic activity is interpreted as a seismic creep from GPS measurements, which shows evidence for fast continuous slip on the deepest part on the detachment zone. Offshore, either the shallowest part of the faults is creeping, or the strain is relaxed in the shallow sediments, as inferred from the large NS strain gradient reported by GPS. The predicted subsidence of the central part of the rift is well fitted by the new continuous GPS measurements. The location of shallow earthquakes (between 5 and 3.5 km in depth) recorded on the on-shore Helike and Aigion faults are compatible with 50° and 60° mean dip angles, respectively. The offshore faults also show indirect evidence for high dip angles. This strongly differs from the low dip values reported for active faults more to the east of the rift, suggesting a significant structural or rheological change, possibly related to the hypothetical presence of the Phyllade nappe. Large seismic swarms, lasting weeks to months, seem to activate recent synrift as well as pre-rift faults. Most of the faults of the investigated area are in their latest part of cycle, so that the probability of at least one moderate to large earthquake (M = 6 to 6.7) is very high within a few decades. Furthermore, the region west to Aigion is likely to be in an accelerated state of extension, possibly 2 to 3 times its mean interseismic value. High resolution strain measurement, with a borehole dilatometer and long base hydrostatic tiltmeters, started end of 2002. A transient strain has been recorded by the dilatometer, lasting one hour, coincident with a local magnitude 3.7 earthquake. It is most probably associated with a slow slip event of magnitude around 5 ± 0.5. The pore pressure data from the 1 km deep AIG10 borehole, crossing the Aigion fault at depth, shows a 1 MPa overpressure and a large sensitivity to crustal strain changes. © 2006 Elsevier B.V. All rights reserved.
Keywords: Gulf of Corinth; Ionian Sea; Mediterranean Sea; active fault; deformation; earthquake magnitude; geophysical survey; normal fault; seismic hazard; seismicity

The Lake Pet'en Itz'a Scientific Drilling Project - Hodell, D.; Anselmetti, F.; Brenner, M.; Ariztegui, D.; Alfaro, G.; Blijdenstein, J.; Bronnimann, C.; Brady, K.; Bush, M.; Chapron, E.; Curtis, J.; Endsley, E.; et al. (Scientific Drilling, 2006)
cited By 23

Making continental crust through slab melting: Constraints from niobium-tantalum fractionation in UHP metamorphic rutile - Xiao, Yilin; Sun, Weidong; Hoefs, Jochen; Simon, Klaus; Zhang, Zeming; Li, Shuguang; Hofmann, Albrecht W. (Geochimica et Cosmochimica Acta, 2006)
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.
Keywords: continental crust; dehydration melting; eclogite; fractionation; geochemistry; metamorphism; niobium; rutile; slab; tantalum

XRF titanium, iron counting and ratio Fe/Ti for CON01-603-2 and VER98-1-14 - Demory, François; Oberh{ansli, Hedi; Nowaczyk, N; Gottschalk, Matthias; Wirth, R; Naumann, R (GFZ Data Services, 2006)

Wellbore stability study for the SAFOD borehole through the San Andreas Fault - Paul, P.; Zoback, M. (Proceedings - SPE Annual Technical Conference and Exhibition, 2006)
This paper presents a wellbore stability study of the SAFOD (San Andreas Fault Observatory at Depth) research borehole at Parkfield, central California. In the summer of 2005, the SAFOD borehole was successfully drilled through the San Andreas Fault (SAF) in an area characterized by fault creep and frequent microearthquakes. In this study we report how the analysis of wellbore failures in the upper part of the hole, geophysical logs and a model for stress gradients in the vicinity of the fault were used to estimate the mud weights required to successfully drill through the fault. Logging-while-drilling acoustic caliper data and real-time hole volume calculations both show that relatively little failure occurred while passing through the SAF. Thus, the predicted mud weight was successful in drilling a stable borehole. However, a six-arm ealiper log run after drilling was completed indicates that there was deterioration of the borehole with time. We suspeet this was caused by failures along the weak bedding planes around the borehole, exacerbated by fluid penetration and key seating. Copyright 2006, Society of Petroleum Engineers.
cited By 1
Keywords: Creep; Oil well drilling; Real time systems; Well logging; Bedding planes; Microearthquakes; San Andreas Fault (SAF); Stress gradients; Boreholes

Velocity dispersion in Vibroseis VSP data - Sun, L.F.; Milkereit, B. (SEG Technical Program Expanded Abstracts, 2006)
Seismic waves in a porous medium experience attenuation and velocity dispersion. In conventional seismic data processing, velocity dispersion is neglected partially because of insufficient or inconclusive observations. In a medium of high attenuation (Q<30), velocity dispersion is a concern. In order to detect velocity dispersion in the exploration seismic frequency band, uncorrelated Vibroseis data were utilized. We have simulated distortion of the correlation wavelet of Vibroseis data due to velocity dispersion to investigate how dispersion distorts Vibroseis data. Different methods were investigated to develop a robust method to detect and measure velocity dispersion in uncorrelated Vibroseis data. Using the CCMW (cross-correlation with a moving window) method, small velocity variations as a function of frequency were observed in the borehole Vibroseis data from the Mallik gas hydrate research wells (Mackenzie Delta, NWT, Canada). © 2005 Society of Exploration Geophysicists.
cited By 5
Keywords: Data handling; Gas hydrates; Petroleum prospecting; Porous materials; Seismic prospecting; Seismic waves; Seismology; Velocity; Cross correlations; Function of frequency; Moving window; Robust methods; Seismic data processing; Seismic frequencies; Velocity dispersion; Velocity variations; Dispersion (waves)

Uplift, thermal unrest and magma intrusion at Yellowstone caldera - Wicks, C.W.; Thatcher, W.; Dzurisin, D.; Svarc, J. (Nature, 2006)
The Yellowstone caldera, in the western United States, formed ∼640,000 years ago when an explosive eruption ejected ∼1,000 km3 of material. It is the youngest of a series of large calderas that formed during sequential cataclysmic eruptions that began ∼16 million years ago in eastern Oregon and northern Nevada. The Yellowstone caldera was largely buried by rhyolite lava flows during eruptions that occurred from ∼150,000 to ∼70,000 years ago. Since the last eruption, Yellowstone has remained restless, with high seismicity, continuing uplift/subsidence episodes with movements of ∼70 cm historically to several metres since the Pleistocene epoch, and intense hydrothermal activity. Here we present observations of a new mode of surface deformation in Yellowstone, based on radar interferometry observations from the European Space Agency ERS-2 satellite. We infer that the observed pattern of uplift and subsidence results from variations in the movement of molten basalt into and out of the Yellowstone volcanic system. © 2006 Nature Publishing Group.
cited By 128
Keywords: Earthquakes; Interferometry; Radar; Satellites; Cataclysmic eruptions; Hydrothermal activity; Volcanoes; caldera; crustal deformation; igneous intrusion; uplift; analytic method; article; interferometer; priority journal; surface property; telecommunication; thermal analysis; volcano; North America; United States; Wyoming; Yellowstone Caldera

Ultrahigh-pressure metamorphic rocks from the Chinese Continental Scientific Drilling Project: II Oxygen isotope and fluid inclusion distributions through vertical sections - Xiao, Yilin; Zhang, Zeming; Hoefs, Jochen; Kerkhof, Alfons (Contributions to Mineralogy and Petrology, 2006)
In order to reconstruct the formation and exhumation mechanisms of UHP metamorphic terrains, the Chinese Continental Scientific Drilling Program (CCSD) has been carried out in Donghai of the Dabie-Sulu ultrahigh-pressure (UHP) metamorphic belt, East China. Eclogite, gneiss, amphibolite (retrograded from eclogite), ultramafic rocks, and minor schist and quartzite have been drilled. Aiming to reveal the fluid behaviour in a vertical sequence of an UHP slab, we investigated fluid inclusion and oxygen isotope characteristics of selected drillcores from the main hole and the pilot-holes PP2 and ZK 703 of the CCSD. More than 540 laser-ablation oxygen isotope analyses on garnet, omphacite, quartz, kyanite, amphibole, phengite, rutile, epidote, amphibole, plagioclase, and biotite from various rocks in the depth range of 0-3,000 m (mainly eclogite and gneiss) show that the investigated rocks can be divided into two groups: 18O-depleted rocks (as low as δ18O = -7.4‰ for garnet) indicate interaction with cold climate meteoric waters, whereas 18O-normal rocks (with bulk δ18O > +5.6‰) have preserved the O-isotopic compositions of their protoliths. Meteoric water/rock interaction has reached depths of at least 2,700 m. Oxygen isotope equilibrium has generally been achieved. Isotopic compositions of mineral phases are homogeneous on a mm to cm scale regardless of lithology, but heterogeneous on the scale of a few metres. Oxygen isotope distributions in the vertical sections favour an in situ origin of the UHP metamorphic rocks. The very negative δ18O eclogites usually have higher hydroxyl-mineral contents than the normal δ18O rocks, indicating higher water content during UHP metamorphism. Fluid inclusion data suggest that rocks with depleted 18O compositions have had different fluid histories compared to those with normal δ18O values. Rocks with depleted 18O mainly have primary medium-to-high salinity inclusions in omphacite, kyanite and quartz, and abundant secondary low-salinity or pure water inclusions in quartz, indicating a high-salinity-brine-dominated fluid system during peak UHP metamorphism; no carbonic inclusions have been identified in these rocks. By contrast, primary very high-density CO2 inclusions are commonly found in the rocks with normal δ18O values. These observations suggest that fluid and oxygen isotope composition of minerals are related and reflect variable degrees of alterations of the Dabie-Sulu UHP metamorphic rocks. © Springer-Verlag 2006.
Keywords: Asia; China; Dabie-Sulu Belt; Donghai; Eurasia; Far East; Jiangsu; fluid inclusion; isotopic composition; metamorphic rock; mineralogy; oxygen isotope; ultrahigh pressure metamorphism

Ultrahigh pressure metamorphic rocks from the Chinese Continental Scientific Drilling Project: I. Petrology and geochemistry of the main hole (0-2,050 m) - Zhang, Zeming; Xiao, Yilin; Hoefs, Jochen; Liou, J.G.; Simon, Klaus (Contributions to Mineralogy and Petrology, 2006)
The main hole (MH) of the Chinese Continental Scientific Drilling Project (CCSD) in southern Sulu has penetrated into an ultrahigh-pressure (UHP) metamorphic rock slice which consists of orthogneiss, paragneiss, eclogite, ultramafic rock and minor schist. Recovered eclogites have a UHP metamorphic mineral assemblage of garnet + omphacite + rutile ± phengite ± kyanite ± coesite ± epidote. Ultramafic rocks contain garnet + olivine + clinopyroxene + orthopyroxene ± Ti-clinohumite ± phlogopite. Gneisses and schists contain an amphibolite-facies paragenesis, but their zircons have coesite, garnet, omphacite (or jadeite) and phengite inclusions, indicating that eclogites and gneisses have been subjected to in situ UHP metamorphism. Using available geothermobarometers, P-T estimates of 3.1-4.4 GPa and 678-816°C for eclogites were obtained. If surface outcrops and neighboring shallow drill holes are considered together, we suggest that a huge supracrustal rock slab (> 50 km long × > 100 km wide × 5 km deep) was subducted to a depth > 100 km and then exhumed to the surface. The depth interval (0-2,050 m) of the CCSD-MH can be divided into six lithological units. Unit 1 consists of alternating layers of quartz-rich and rutile-rich eclogites, with thin interlayers of gneiss and schist. Eclogites of unit 1 are characterized by Nb, Ta, Sr and Ti depletions, low Mg number and general LREE enrichment. Unit 2 comprises rutile- and ilmenite-rich eclogite and minor normal eclogite and is characterized by high TiO2, total Fe, V, Co and Sr, and very low SiO2, alkali, Zr, Ba, Nb, Ta and total REE contents, and LREE-depleted REE patterns with slightly positive Eu anomalies. Unit 3 contains ultramafic rock and minor MgO-rich eclogite. Protoliths of UHP rocks from units 1, 2 and 3 represent a layered mafic to ultramafic intrusion at crustal depth. Units 4 and 6 consist of interlayered eclogite and paragneiss; the eclogites are characterized by Th, U, Nb, Ta and Ti depletion and K enrichment and LREE-enriched REE patterns. Paragneisses show Nb, Ta, Sr and Ti depletions and LREE-enriched REE patterns occasionally with slightly negative Eu anomalies, indicating that their protoliths represent metamorphic supracrustal series. Unit 5 consists mainly of orthogneisses, showing distinct Nb, Ta, Sr and Ti depletions, and LREE-enriched REE patterns with pronounced negative Eu anomalies, suggesting granitic protoliths. In conclusion it is proposed that the southern Sulu UHP belt consists of a series of meta-supracrustal rocks, a layered mafic-ultramafic complex and granites. © Springer-Verlag 2006.
Keywords: Asia; China; Eurasia; Far East; Sulu Belt; amphibolite facies; chemical composition; geochemistry; metamorphic rock; P-T conditions; paragenesis; petrology; supracrustal rock; ultrahigh pressure metamorphism

Tube-wave monitoring at Mallik field: Comparing modeled and experimental time-lapse responses - Ziatdinov, S.; Bakulin, A.; Kashtan, B.; Korneev, V.; Sidorov, A. (SEG Technical Program Expanded Abstracts, 2006)
We apply tube-wave monitoring method to a time-lapse cross-well dataset from Mallik field. Raw waveforms are used for analysis thus avoiding any smearing of 4D response introduced by pre-processing. We perform extensive modeling that includes effects of a source borehole and confirms nature of most prominent arrivals as being tube-wave related. Modeling proves that strongest conversion of tube wave into P and S waves occurs at the sharp acoustic boundary. Data displays clear time-lapse changes in tube-wave related arrivals, while shows no change in first arrivals. Modeling suggests that to explain the data the reservoir changes have to occur at a deeper interval than previously anticipated, below the perforations. Excellent agreement between modeled and experimental data provides us with good confidence in our results. This study represents first application of tube-wave monitoring concept. © 2005 Society of Exploration Geophysicists.
cited By 0
Keywords: Geophysical prospecting; Shear waves; Tubes (components); Cross-well; Data display; First arrival; P- and S-waves; Pre-processing; Time-lapse response; Tube waves; Wave forms; Acoustics

Transmission signatures of gashydrate bearing micro-structures: A numerical study - Kr{uger, O.S.; Saenger, E.H.; Shapiro, S.A. (SEG Technical Program Expanded Abstracts, 2006)
This work is inspired by the observation, that gas hydrate bearing sediments have a high velocity in combination with high attenuation. We study numerically the influence of different gas hydrate locations within the pore space on transmitted p-waves. From the wave propagation simulations on the micro-scale it can be seen, that different positions of the gas hydrate in the pore space results in almost the same effective velocities and attenuation, as long as the gas hydrate had contact to the sediment grains. This changes in the case of a suspension, here the attenuation increases and the effective velocity decreases. The resulting p-wave versus gas hydrate saturation plot is in a qualitatively good agreement with experimental results obtained for the Mallik 2L-38 well. © 2005 Society of Exploration Geophysicists.
cited By 0
Keywords: Gases; Geophysical prospecting; Hydration; Petroleum prospecting; Seismic waves; Wave propagation; Effective velocity; Gas hydrate bearing sediments; Gas hydrate saturations; High velocity; Micro-scale; Pore space; Sediment grains; Wave propagation simulation; Gas hydrates

The Sino-Korean-Yangtze suture, the Huwan detachment, and the Paleozoic-Tertiary exhumation of (ultra)high-pressure rocks along the Tongbai-Xinxian-Dabie Mountains - Ratschbacher, Lothar; Franz, Leander; Enkelmann, Eva; Jonckheere, Raymond; P{orschke, Axel; Hacker, Bradley R.; Dong, Shuwen; Zhang, Yueqiao (Special Paper of the Geological Society of America, 2006)
There are three sutures in the Qinling-Dabie-Sulu orogen in the Tongbai-Xinxian (northern Hong'an)-northern Dabie area: the Silurian Sino-Korean craton-Erlangping intra-oceanic arc suture, the Silurian Erlangping arc-Qinling unit (microcontinent) suture, and the Early Triassic Qinling unit-Yangtze craton suture. We resolve the controversy regarding the age of the Sino-Korean craton-Yangtze craton collisionby recognizing that there was Paleozoic collision between the Qinling unit and the Sino-Korean craton and Mesozoic collision between the Qinling unit and the Yangtze craton. The Qinling unit constitutes a long and narrow microcontinent that extends through the Qinling-Dabie area and probably into the Sulu area. Its common characteristics are the Mesoproterozoic (ca. 1.0 Ga) Jinningian orogeny, ca. 0.8-0.7 Ga arc formation and rifting, and Late Silurian-Early Devonian (ca. 400 Ma) arc magmatism with concomitant regional contact metamorphism up to granulite-facies conditions (peak: 680-740 °C at 0.9-1.1 GPa). A common Proterozoic history links the Qinling microcontinent to the Yangtze craton. Its 400 Ma arc, forearc basin, and its separation from the Yangtze craton by the partly oceanic Huwan m'elange make the Qinling microcontinent distinct. The forearc basin sits on the southern part of the 400 Ma arc and underlying Proterozoic continental basement, and detrital geochronology ties it to the Qinling microcontinent basement and its arc. The Huwan m'elange is a subduction-accretion complex containing elements of the Qinling microcontinent and its arc, the Paleotethyan ocean floor, and possibly the Yangtze craton. Quartz eclogites (540-590 °C, 2.1 GPa) signify ca. 315 Ma subduction. Devonian to Permian eclogite zircon ages, 40 Ar/ 39 Ar and Rb/Sr mineral ages in the forearc and its basement, and static, Permian blueschist metamorphism in the upper-plate basement testify to subduction throughout the late Paleozoic. The ̃10-km-wide Huwan detachment bounds the high- and ultrahigh-pressure rocks of the Xinxian-Hong'an block (pressure peak at older than 240 Ma) along their northern margin. It is partly responsible for exhumation of the high- and ultrahighpressure rocks, but the entire basement core of Hong'an-Dabie orogen is also strongly deformed. The Huwan shear-zone high-strain deformation indicates passage of rocks through the lithosphere by subhorizontal N-S extension and vertical contraction, showcased by condensed Triassic isograds (420 °C and ̃0.4 GPa in the hanging wall and ̃530 °C and 2.2 GPa in the footwall). The Huwan detachment produced Triassic crustal exhumation rates of 1.9-1.4 mm/yr; synkinematic phengite grew as early as ca. 235 Ma, and the main retrograde deformation occurred at 224-195 Ma. The Tongbai-Xinxian area shows a massive 130-115 Ma cluster of cooling ages, reflecting regional cooling after granitoid injection and regional Cretaceous heating. Apatite fission-track ages cluster at 80-55 Ma and signify cooling related to transtension that coincided with rifting marked by Late Cretaceous-Eocene red bed deposition throughout eastern China. Exhumation rates of for the last 70 m.y. have been slow: ̃0.06 mm/yr. The India-Asia collision reactivated the orogen in the Eocene, particularly along the Tanlu fault zone and locally along fault zones in Tongbai-Xinxian. © 2006 Geological Society of America.
Keywords: China; Dabie Mountains; Henan; Tongbai; Xinxiang; Cooling; Deformation; Fission reactions; Geochronology; Metamorphic rocks; Phosphate minerals; Silicate minerals; Structural geology; Zircon; Holocenes; Huwan detachment; Qinling-dabie-sulu orogen; Sino-korean-yangtze suture; Ultrahigh pressure; exhumation; orogeny; Paleozoic; suture zone; Tertiary; ultrahigh pressure metamorphism; Buildings

The origin and age of biogeochemical trends in deep fracture water of the witwatersrand basin, south africa - Onstott, T.C.; Lin, L.-H.; Davidson, M.; Mislowack, B.; Borcsik, M.; Hall, J.; Slater, G.; Ward, J.; Sherwoodlollar, B.; Lippmann-Pipke, J.; Boice, E.; Pratt, L.M.; et al. (Geomicrobiology Journal, 2006)
Water residing within crustal fractures encountered during mining at depths greater than 500 meters in the Witwatersrand basin of South Africa represents a mixture of paleo-meteoric water and 2.0 –2.3 Ga hydrothermal fluid. The hydrothermal fluid is highly saline, contains abiogenic CH4 and hydrocarbon, occasionally N2, originally formed at ∼250 –300◦C and during cooling isotopically exchangedO and Hwith minerals and accrued H2,4He and other radiogenic gases. The paleo-meteoric water ranges in age from ∼10 Ka to >1.5 Ma, is of low salinity, falls along the global meteoric water line (GMWL) and is CO2 and atmospheric noble gas-rich. The hydrothermal fluid, which should be completely sterile, has probably been mixing with paleo-meteoric water for at least the past∼100 Myr, a process which inoculates previously sterile environments at depths >2.0 to 2.5 km. Free energy flux calculations suggest that sulfate reduction is the dominant electron acceptor microbial process for the high salinity fracture water and that it is 107 times that normally required for cell maintenance in lab cultures. Flux calculations also indicate that the potential bioavailable chemical energy increases with salinity, but because the fluence of bioavailable C, N and P also increase with salinity, the environment remains energy-limited. The4He concentrations and theoretical calculations indicate that the H2 that is sustaining the subsurface microbial communities (e.g. H2-utilizing SRB and methanogens) is produced by water radiolysis at a rate of ∼1nMyr−1. Microbial CH4 mixes with abiogenicCH4 to produce the observed isotopic signatures and indicates that the rate of methanogenesis diminishes with depth from∼100 at <1 kmbls, to <0.01nMyr−1 at >3 kmbls. Microbial Fe(III) reduction is limited due to the elevated pH. The δ13C of dissolved inorganic carbon is consistent with heterotrophy rather than autotrophy dominating the deeper, more saline environments. One potential source of the organic carbon may be microfilms present on the mineral surfaces. © Taylor & Francis.
cited By 33
Keywords: biogeochemistry; dissolved inorganic carbon; groundwater; methanogenesis; microbial community; sulfate-reducing bacterium; Africa; South Africa; Southern Africa; Sub-Saharan Africa; Witwatersrand; Methanobacteriales

The Cretaceous-Paleogene (K-P) boundary at Brazos, Texas: Sequence stratigraphy, depositional events and the Chicxulub impact - Schulte, P.; Speijer, R.; Mai, H.; Kontny, A. (Sedimentary Geology, 2006)
Two cores from Brazos, Texas, spanning the Cretaceous-Paleogene (K-P) boundary, are investigated by a multidisciplinary approach aiming at unraveling environmental changes and sequence stratigraphic setting. In addition, the sedimentology of the K-P event deposit and its correlation with the K-P boundary is studied. Foraminifera and nannofossil stratigraphy indicates that both cores include a latest Maastrichtian (Zone CF1-CF2) and earliest Danian (P0, Pα and P1a) shale sequence with a sandy and Chicxulub ejecta-bearing event deposit at the K-P boundary; a hiatus of unknown duration may be present by the unconformable base of the event deposit. Planktic foraminifera as well as calcareous nannofossil abundance and diversity both decline abruptly above the event deposit (K-P mass extinction), whereas benthic foraminifera show a pronounced faunal change but no mass extinction. Mineralogical and geochemical proxies suggest that-except for the sandwiched K-P event deposit-no facies change took place across the K-P boundary and no evidence for adverse an- or dysoxic sedimentary conditions following the Chicxulub impact was observed. Therefore, the interval bracketing the K-P event deposit is considered as highstand systems tract. Increased coarse detritus input and low planktic/benthic (P/B) foraminifera ratios during the earliest Paleocene (P0 and Pα) both suggest an increased coastal proximity or relative sea-level lowering, although the K-P mass extinction of planktic foraminifera might have influenced the P/B ratios as well. Consequently, the sandy shales of the early Paleocene are considered as late regressive highstand or as lowstand deposit. During P1a, shales assigned as transgressive systems tract overlie a pyrite- and glauconite-rich bioturbated transgressive surface or type-2-sequence boundary. The smectite-dominated clay assemblage, with minor illite, kaolinite and chlorite indicates semiarid-humid climates with no obvious shifts across the K-P boundary. The magnetic susceptibility signature during the Maastrichtian reveals a subtle cyclic (or rhythmic) pattern, whereas a high-amplitude cyclic pattern is present during the early Danian. The K-P event deposit shows a succession of high-energetic debris flows and turbidites derived from multiple source areas, followed by a period of decreasing current energy. Deposition was likely triggered by multiple tsunami or tempestites followed by a prolonged period of reworking and settling. The Chicxulub ejecta at the base of the K-P event deposit consists of Mg-rich smectite-as well as Fe-Mg-rich chlorite-spherules. Their mineralogical composition points to target rocks of mafic to intermediate composition, presumably situated in the northwestern sector of the Chicxulub impact structure. Besides these silicic phases, the most prominent ejecta components are limestone clasts, accretionary carbonate clasts, and microspar, suggesting that the Texas area received ejecta also from shallow, carbonate-rich lithologies at the impact site on the Yucat'an carbonate platform. The excellent correlation of Chicxulub ejecta at Brazos with ejecta found in the K-P boundary layer worldwide - along with the associated mass extinction - provides no evidence that Chicxulub predated the K-P boundary and allows for unequivocal positioning of the K-P boundary at the event deposit. © 2005 Elsevier B.V. All rights reserved.
cited By 80

Shock re-equilibration of fluid inclusions in crystalline basement rocks from the Ries crater, Germany - Elwood Madden, M.E.; Kring, D.A.; Bodnar, R.J. (Meteoritics and Planetary Science, 2006)
This study examines the effects of shock metamorphism on fluid inclusions in crystalline basement target rocks from the Ries crater, Germany. The occurrence of two-phase fluid inclusions decreases from shock stage 0 to shock stage 1, while single-phase inclusions increase, likely as a result of re-equilibration. In shock stages 2 and 3, both two-phase and single-phase inclusions decrease with increasing shock stage, indicating that fluid inclusion vesicles are destroyed due to plastic deformation and phase changes in the host minerals. However, quartz clasts entrained in shock stage 4 melts contain both single-phase and two-phase inclusions, demonstrating the rapid quenching of the melt and the heterogeneous nature of impact deformation. Inclusions in naturally shocked polycrystalline samples survive at higher shock pressures than those in single crystal shock experiments. However, fluid inclusions in both experimental and natural samples follow a similar trend in re-equilibration at low to moderate shock pressures leading to destruction of inclusion vesicles in higher shock stages. This suggests that shock processing may lead to the destruction of fluid inclusions in many planetary materials and likely contributed to shock devolatilization of early planetesimals. © The Meteoritical Society, 2006.
cited By 3

The 2005 Lake Malawi Scientific Drilling Project - Scholz, C. A.; Cohen, A. S.; Johnson, T. C.; King, J. W.; Moran, K. (Scientific Drilling, 2006)

Temperature dependence of elastic P- and S-wave velocities in porous Mt. Unzen dacite - Scheu, B.; Kern, H.; Spieler, O.; Dingwell, D.B. (Journal of Volcanology and Geothermal Research, 2006)
Laboratory measurements of elastic properties of volcanic rocks are crucial for the modelling of volcano seismic activity. Compared to the large database reported in the literature for sedimentary, igneous and metamorphic rocks, the data set for volcanic rocks is limited and mostly restricted to basalts. Data for more silica-rich rocks are sparse. In particular, velocity data for silica-rich volcanic rocks measured at elevated temperature are lacking. We measured the elastic P- and S-wave velocities and the velocity anisotropy of porous dacitic rocks from Unzen Volcano, Japan, exhibiting an open porosity of 3.3 to 24.3 vol.%. The measurements were done at temperatures of up to 600 °C and confining pressures of 100 MPa, corresponding to depths of ∼ 3000-4000 m. Samples with even higher porosities failed at the required pressures. The measurements were carried out in a cubic multi-anvil pressure apparatus, using the pulse transmission technique. In contrast to low-porosity magmatic and metamorphic rocks, the seismic velocities of the investigated volcanic rocks increased and the velocity anisotropies decreased with increasing temperature, due to further sample compaction. There is a close relationship between velocity, density and porosity. The higher the density (and the lower the porosity) the higher are the P- and S-wave velocities. These results can contribute to a better understanding of the propagation of seismic energy through the volcanic edifice. © 2005 Elsevier B.V. All rights reserved.
cited By 29
Keywords: Anisotropy; High temperature effects; Metamorphic rocks; Porosity; Seismology; Volcanic rocks; Volcanoes; Pulse transmission technique; Silica-rich rocks; Volcano seismic activity; Elastic waves; dacite; elasticity; high temperature; P-wave; S-wave; seismic anisotropy; seismology; volcanic rock; volcanology; Asia; Eurasia; Far East; Japan; Kyushu; Nagasaki [Kyushu]; Unzen Volcano

Target delamination by spallation and ejecta dragging: An example from the Ries crater's periphery - Kenkmann, T.; Ivanov, B.A. (Earth and Planetary Science Letters, 2006)
Subhorizontal shear planes (detachments) are observed in bedded limestones in the periphery of the Ries impact crater, Germany. These detachments occur at 0.8-1.8 crater radii distance from the crater center beneath deposits of the continuous ejecta blanket. Striations on detachment planes and offsets of markers indicate top-outward shearing with radial slip vectors. Detachments were found at depths between a few meters and more than 50 m beneath the target surface. The displacements along these faults range from meters to decameters and decrease with increasing depth and distance from the crater center. With increasing crater distance, detachment horizons tend to climb to shallower levels. Cross-cutting relationships to faults associated with the crater collapse indicate that detachment faulting started prior to the collapse but continued during crater modification. Numerical modeling of the cratering process shows that near-surface deformation outside the transient crater is induced by two separate mechanisms: (i) weak spallation by interference of shock and release waves near the target surface and (ii) subsequent dragging by the deposition of the ejecta curtain. Spallation causes an upward and outward directed motion of target material that increases in magnitude toward the target surface. It leads to decoupling of the uppermost target layers in the early cratering stage without totally disintegrating the rock. The subsequent arrival of the oblique impact shower of the ejecta curtain at the target surface delivers a horizontal momentum to the uppermost target area and results in a second horizontal displacement increment by dragging. With increasing depth this effect vanishes rapidly. Spallation decoupling and subsequent ejecta dragging of near-surface rocks is probably a general cratering mechanism around craters in layered targets with weak interbeds. © 2006 Elsevier B.V. All rights reserved.
cited By 30

Structure of the San Andreas fault zone at SAFOD from a seismic refraction survey - Hole, J.A.; Ryberg, T.; Fuis, G.S.; Bleibinhaus, F.; Sharma, A.K. (Geophysical Research Letters, 2006)
Refraction traveltimes from a 46-km long seismic survey across the San Andreas Fault were inverted to obtain two-dimensional velocity structure of the upper crust near the SAFOD drilling project. The model contains strong vertical and lateral velocity variations from <2 km/s to ∼6 km/s. The Salinian terrane west of the San Andreas Fault has much higher velocity than the Franciscan terrane east of the fault. Salinian basement deepens from 0.8 km subsurface at SAFOD to ∼2.5 km subsurface 20 km to the southwest. A strong reflection and subtle velocity contrast suggest a steeply dipping fault separating the Franciscan terrane from the Great Valley Sequence. A low-velocity wedge of Cenozoic sedimentary rocks lies immediately southwest of the San Andreas Fault. This body is bounded by a steep fault just northeast of SAFOD and approaches the depth of the shallowest earthquakes. Multiple active and inactive fault strands complicate structure near SAFOD. Copyright 2006 by the American Geophysical Union.
cited By 61
Keywords: Acoustic wave refraction; Earthquakes; Sedimentary rocks; Seismic prospecting; Seismic waves; Two dimensional; Velocity; San Andreas fault zone; Seismic refraction; Seismology; fault zone; San Andreas Fault; seismic refraction; seismic survey; travel time; upper crust

Strain partitioning and stress rotation at the North Anatolian fault zone from aftershock focal mechanisms of the 1999 Izmit MW = 7.4 earthquake - Bohnhoff, M.; Grosser, H.; Dresen, G. (Geophysical Journal International, 2006)
We investigate aftershock focal mechanisms of the Mw=7.4 Izmit earthquake of 1999 August 17, on the western North Anatolian fault zone (NAFZ). Spatial clustering and the orientation of 446 fault plane solutions are analysed. The Izmit mainshock occurred as a right-lateral slip on an EW-trending near-vertical fault plane. Aftershock clusters define four individual fault segments. Focal mechanisms surrounding the epicentres of the Izmit and subsequent D{uzce mainshock (Mw= 7.1, 1999 November 12) indicate predominantly strike-slip but also normal faulting. Aftershocks in the area between the Izmit and D{uzce segments are mainly related to EW-oriented normal faulting delineating a small pull-apart structure. Beneath the easternmost Sea of Marmara, alignments of aftershocks suggest branching of the NAFZ into three or more active segments that differ significantly in terms of their focal mechanism characteristics. The distribution of aftershock focal mechanisms corresponds to fault segmentation of the NAFZ in the Izmit-D{uzce region produced by coseismic slip. Areas with large amounts of coseismic slip show aftershocks that are predominantly strike-slip, but low-slip barriers show mostly normal faulting aftershocks. Stress tensor inversions of the aftershock focal mechanisms show rotations of the local stresses following the Izmit mainshock. In the Izmit-Sapanca area, the maximum horizontal compressive stress axis is horizontally rotated counter-clockwise by 8° with respect to the coseismic and long-term regional stress field. Towards the eastern end of the rupture (Karadere-D{uzce area), stresses are rotated clockwise. We conclude that the Izmit earthquake caused significant stress partitioning along the rupture. The direction of stress rotation is related to the orientation of the individual fault segments along the NAFZ. © 2006 The Authors Journal compilation © 2006 RAS.
cited By 92
Keywords: aftershock; fault zone; focal mechanism; Kocaeli earthquake 1999; strain analysis; stress analysis; Anatolia; Eurasia; Turkey

Stable isotopes in bulk carbonates and organic matter in recent sediments of Lake Qinghai and their climatic implications - Xu, Hai; Ai, Li; Tan, Liangcheng; An, ZhiSheng (Chemical Geology, 2006)
Multi-proxy indices on annual/decadal scales during the past 600 years were developed from the surface sediments of Lake Qinghai based on 210Pb and 137Cs geochronology. The δ13Ccarb, δ18Ocarb, and total carbonate content are consistent with one another in trends and their variations have been mainly ascribed to regional temperature. It is suggested that the strong evaporation contributes in modifying δ18O of lake water and δ13C of the dissolved inorganic carbon of the surface water, and appears to be responsible for the covariance between δ13Ccarb and δ18Ocarb. Photosynthesis of the aquatic plants appears to have played a significant role in determining δ15N of organic matter. The discrimination of 14N and 15N during photosynthesis may have triggered a positive linkage between temperatures and δ15Norg (after removal of the stratigraphic trend). Variations of total organic carbon (TOC), C/N ratio, and δ13Corg have been ascribed to local precipitation. TOC (after removal of the stratigraphic trend) is positively correlated with the C/N ratio (r = 0.45, α < 0.01), and negatively correlated with δ13C of organic matter (δ13Corg) (r = - 0.55, α < 0.01); while δ13Corg is negatively correlated with the C/N ratio (r = - 0.48, α < 0.01). The climatic significance of these multi-proxy indices has been verified by comparing with the meteorological records and the climates inferred from tree rings in adjacent regions. © 2006 Elsevier B.V. All rights reserved.
Keywords: Asia; China; Eurasia; Far East; Qinghai; Qinghai Lake; carbon isotope; carbonate; climate conditions; dissolved inorganic carbon; lacustrine deposit; oxygen isotope; sediment chemistry; stable isotope; total organic carbon

Source scaling relationships of microearthquakes at Parkfield, CA, determined using the SAFOD pilot hole seismic array - Imanishi, K.; Ellsworth, W.L. (Geophysical Monograph Series, 2006)
We estimate the source parameters of 34 microearthquakes at Parkfield, CA, ranging in size from M -0.2 to M 2.1, by analyzing seismograms recorded by the 32-level, 3-component seismic array installed in the SAFOD Pilot Hole. We succeeded in obtaining stable spectral ratios by stacking the ratios calculated from the moving windows taken along the record following the direct waves. These spectral ratios were modeled to determine seismic moments and corner frequencies assuming an omega-squared model. Static stress drops and apparent stresses of microearthquakes at Parkfield display moment-independent scaling in agreement with scaling laws reported for moderate and large earthquakes. It is likely that the dynamics of microearthquakes at Parkfield is macroscopically similar to that of larger tectonic earthquakes. © 2006 by the American Geophysical Union.
cited By 69
Keywords: Energy dissipation; Seismic waves; Seismology; Corner frequency; Large earthquakes; Micro-earthquakes; Moment independents; Scaling relationships; Source parameters; Spectral ratios; Tectonic earthquakes; Earthquakes

Slip zone and energetics of a large earthquake from the Taiwan Chelungpu-fault Drilling Project - Ma, K.-F.; Tanaka, H.; Song, S.-R.; Wang, C.-Y.; Hung, J.-H.; Tsai, Y.-B.; Mori, J.; Song, Y.-F.; Yeh, E.-C.; Soh, W.; Sone, H.; Kuo, L.-W.; et al. (Nature, 2006)
Determining the seismic fracture energy during an earthquake and understanding the associated creation and development of a fault zone requires a combination of both seismological and geological field data. The actual thickness of the zone that slips during the rupture of a large earthquake is not known and is a key seismological parameter in understanding energy dissipation, rupture processes and seismic efficiency. The 1999 magnitude-7.7 earthquake in Chi-Chi, Taiwan, produced large slip (8 to 10 metres) at or near the surface, which is accessible to borehole drilling and provides a rare opportunity to sample a fault that had large slip in a recent earthquake. Here we present the retrieved cores from the Taiwan Chelungpu-fault Drilling Project and identify the main slip zone associated with the Chi-Chi earthquake. The surface fracture energy estimated from grain sizes in the gouge zone of the fault sample was directly compared to the seismic fracture energy determined from near-field seismic data. From the comparison, the contribution of gouge surface energy to the earthquake breakdown work is quantified to be 6 per cent. ©2006 Nature Publishing Group.
cited By 213
Keywords: Drilling platforms; Earthquake effects; Energy dissipation; Interfacial energy; Project management; Surface phenomena; Near field; Rupture processes; Seismic fracture energy; Slip zone; Seismology; borehole; Chi-Chi earthquake 1999; drilling; fault gouge; fault slip; fault zone; grain size; seismometry; surface energy; article; earthquake; energy transfer; geology; particle size; priority journal; surface property; Taiwan; thickness; Asia; Eurasia; Far East; Taiwan

SHRIMP U-Pb zircon dating from Sulu-Dabie dolomitic marble, eastern China: Constraints on prograde, ultrahigh-pressure and retrograde metamorphic ages - Liu, Fu Lai; Gerdes, A.; Liou, J.G.; Xue, H.M.; Liang, F.H. (Journal of Metamorphic Geology, 2006)
Laser Raman spectroscopy and cathodoluminescence (CL) images show that zircon from Sulu-Dabie dolomitic marbles is characterized by distinctive domains of inherited (detrital), prograde, ultrahigh-pressure (UHP) and retrograde metamorphic growths. The inherited zircon domains are dark-luminescent in CL images and contain mineral inclusions of Qtz + Cal + Ap. The prograde metamorphic domains are white-luminescent in CL images and preserve a quartz eclogite facies assemblage of Qtz + Dol + Grt + Omp + Phe + Ap, formed at 542-693 °C and 1.8-2.1GPa. In contrast, the UHP metamorphic domains are grey-luminescent in CL images, retain the UHP assemblage of Coe + Grt + Omp + Arg + Mgs + Ap, and record UHP conditions of 739-866 °C and > 5.5 GPa. The outermost retrograde rims have dark-luminescent CL images, and contain low- P minerals such as calcite, related to the regional amphibolite facies retrogression. Laser ablation ICP-MS trace-element data show striking difference between the inherited cores of mostly magmatic origin and zircon domains grown in response to prograde, UHP and retrograde metamorphism. SHRIMP U-Pb dating on these zoned zircon identified four discrete 206Pb/238U age groups: 1823-503 Ma is recorded in the inherited (detrital) zircon derived from various Proterozoic protoliths, the prograde domains record the quartz eclogite facies metamorphism at 254-239 Ma, the UHP growth domains occurred at 238-230 Ma, and the late amphibolite facies retrogressive overprint in the outermost rims was restricted to 218-206 Ma. Thus, Proterozoic continental materials of the Yangtze craton were subducted to 55-60km depth during the Early Triassic and recrystallized at quartz eclogite facies conditions. Then these metamorphic rocks were further subducted to depths of 165-175 km in the Middle Triassic and experienced UHP metamorphism, and finally these UHP metamorphic rocks were exhumed to mid-crustal levels (about 30km) in the Late Triassic and overprinted by regional amphibolite facies metamorphism. The subduction and exhumation rates deduced from the SHRIMP data and metamorphic P-T conditions are 9-10 km Myr-1 and 6.4km Myr-1, respectively, and these rapid subduction-exhumation rates may explain the obtained P-T-t path. Such a fast exhumation suggests that Sulu-Dabie UHP rocks that returned towards crustal depths were driven by buoyant forces, caused as a consequence of slab breakoff at mantle depth. © 2006 Blackwell Publishing Ltd.
Keywords: Asia; China; Dabie-Sulu Belt; Eurasia; Far East; cathodoluminescence; dolomite; Raman spectroscopy; retrograde metamorphism; SHRIMP dating; ultrahigh pressure metamorphism; zircon

SHRIMP U-Pb zircon dating from eclogite lens in marble, Shuanghe area, Dabie UHP terrane: Restriction on the prograde, UHP and retrograde metamorphic ages - Liu, Fulai; Xue, Huaimin; Xu, Zhiqin; Liang, FengHua; Gerdes, Axel (Acta Petrologica Sinica, 2006)
Laser Raman spectroscopy and cathodoluminescence (CL) images show that zircons separated from eclogite lens in impure marble from Shuanghe area, Dabie UHP metamorphic belt, are characterized by inherited (detrital), prograde, UHP and retrograde metamorphic overgrowth domains. The inherited (detrital) zircons contain Pl + Ap and Qtz + Pl in the white-luminescent core and the dark-luminescent rim, respectively. Whereas the prograde metamorphic domains (with grey-luminescent CL images) preserve quartz eclogite-facies mineral assemblage of Qtz + Grt + Omp + Phe + Dol + Ap, formed at 588 ∼ 668°C and 1.7 ∼ 1.8CPa. In contrast, the UHP metamorphic domains (with white-luminescent CL images) retain index UHP mineral assemblage of Coe + Grt + Omp + Arg + Mgs + Ap, and record UHP condition of T = 784 ∼ 849°C and P > 5.5GPa. The outmost retrograde rims (with dark-luminescent CL images) however, contain no-UHP minerals such as quartz and calcite, related to the regional amphibolite-facies retrogression with T = 550 ∼ 720°C and P = 0.8 ∼ 1.4GPa. SHRIMP U-Pb dating on these zoned zircons identify four discrete and meaningful age groups: 2701 ∼ 1753Ma (207Pb/206Pb age) is recorded in the inherited (detrital) zircons derived from Archean-Proterozoic protolith, the prograde metamorphism of quartz eclogite-facies in the prograde domains occurred at 249 ∼ 241 Ma (206Pb/238U age), the UHP metamorphic event in the UHP mineral-bearing domains was at 239 ∼ 231Ma (206Pb/238U age), and the late amphibolite-facies retrograde overprint in the outmost rims was restricted at 219 ∼ 211 Ma (206Pb/238U age). Thus, Archean-Proterozoic continental materials of the Dabie UHP terrane were subducted to 55 ∼ 60km depth at Early Triassic and occurred quartz eclogite facies metamorphism simultaneously. Then these quartz eclogite-facies metamorphic rocks continued subduction to 165 ∼ 175km deep mande depth at Middle Triassic and experienced UHP metamorphism; The peak pressures are based on the equilibrium calculation of the reaction dolomite = magnesite + aragonite. This UHP metamorphism recorded a lower geotherm step of 4.7°C km-1 which is within what was previously considered a 'forbidden' condition within earth. Finally these UHP metamorphic rocks were exhumed to mid-crustal levels (about 30km) in the Late Triassic and overprinted regional amphibolite facies metamorphism. The subduction and exhumation rate deduced from the SHRIMP data and metamorphic P-T conditions is 11 ∼ 12km Myr-1 and 7.5 ∼ 8.1km Myr-1, respectively. Fast subduction and rapid exhumation has been considered to be one of the most important factors for the preservation of prograde, UHP and retrograde metamorphic minerals in zircons. Such a fast exhumation suggests that the Dabie UHP metamorphic rocks returned towards the crustal depths as dominant part of a buoyant sliver, caused as a consequence of slab breakoff.
Keywords: Geochronology; Lead; Levees; Luminescence; Marble; Minerals; Quartz; Shellfish; Silicate minerals; Zircon; Dabie UHP terrane; Eclogites; Laser Raman; Mineral inclusions; Shrimp u-pb dating; cathodoluminescence; eclogite; marble; P-T conditions; Raman spectroscopy; retrograde metamorphism; SHRIMP dating; ultrahigh pressure metamorphism; zircon; Metamorphic rocks

Shock-metamorphosed zircon in terrestrial impact craters - Wittmann, A.; Kenkmann, T.; Schmitt, R.T.; St{offler, D. (Meteoritics and Planetary Science, 2006)
To ascertain the progressive stages of shock metamorphism of zircon, samples from three well-studied impact craters were analyzed by optical microscopy, scanning electron microscopy (SEM), and Raman spectroscopy in thin section and grain separates. These samples are comprised of well-preserved, rapidly quenched impactites from the Ries crater, Germany, strongly annealed impactites from the Popigai crater, Siberia, and altered, variably quenched impactites from the Chicxulub crater, Mexico. The natural samples were compared with samples of experimentally shock-metamorphosed zircon. Below 20 GPa, zircon exhibits no distinct shock features. Above 20 GPa, optically resolvable planar microstructures occur together with the high-pressure polymorph reidite, which was only retained in the Ries samples. Decomposition of zircon to ZrO2 only occurs in shock stage IV melt fragments that were rapidly quenched. This is not only a result of post-shock temperatures in excess of ∼1700 °C but could also be shock pressure-induced, which is indicated by possible relics of a high-pressure polymorph of ZrO2. However, ZrO2 was found to revert to zircon with a granular texture during devitrification of impact melts. Other granular textures represent recrystallized amorphous ZrSiO4 and reidite that reverted to zircon. This requires annealing temperatures >1100 °C. A systematic study of zircons from a continuous impactite sequence of the Chicxulub impact structure yields implications for the post-shock temperature history of suevite-like rocks until cooling below ∼600 °C. © The Meteoritical Society, 2006.
cited By 131

Mapping stress and structurally controlled crustal shear velocity anisotropy in California - Boness, N.L.; Zoback, M.D. (Geology, 2006)
We present shear velocity anisotropy data from crustal earthquakes in California and demonstrate that it is often possible to discriminate structural anisotropy (polarization of the shear waves along the fabric of major active faults) from stress-induced anisotropy (polarization parallel to the maximum horizontal compressive stress). Stress directions from seismic stations located near (but not on) the San Andreas fault indicate that the maximum horizontal compressive stress is at a high angle to the strike of the fault. In contrast, seismic stations located directly on one of the major faults indicate that shear deformation has significantly altered the elastic properties of the crust, inducing shearwave polarizations parallel to the fault plane. © 2006 Geological Society of America.
cited By 144
Keywords: Crustal stress; San Andreas fault; Seismic anisotropy; Anisotropy; Compressive stress; Earthquakes; Mapping; Seismic waves; Shear stress; Tectonics; Structural geology; active fault; deformation mechanism; earthquake; elastic property; fault plane; polarization; S-wave; San Andreas Fault; seismic anisotropy; California; North America; United States

Structure of the San Andreas Fault at SAFOD from P-wave tomography and fault-guided wave mapping - Malin, P.; Shalev, E.; Balven, H.; Lewis-Kenedi, C. (Geophysical Research Letters, 2006)
Fault-guided waves reveal a low-velocity fault segment a few hundred meters southwest of the main strand of the San Andreas Fault (SAf) system. In 2004, the San Andreas Fault Observatory at Depth (SAFOD) Main Hole was drilled 2.5 km underground and 0.7 km west of the SAF surface trace. A 3-component, 4.5-Hz seismograph was installed near the bottom of this hole. This instrument recorded fault zone guided (Fg) waves originating from earthquakes along the main SAF ∼2 km north and 3 km south of the SAFOD site. This ∼5 km length corresponds to a distinctive low-velocity structure imaged in 2003 using microearthquakes recorded on the Pilot Hole array. Because this structure transmits Fg-waves from the main fault, it is probably connected to the main SAF and is most likely a major, unmapped fault. Copyright 2006 by the American Geophysical Union.
cited By 19
Keywords: Conformal mapping; Drilling; Seismic waves; Seismographs; Tomography; Velocity measurement; Fault-guided wave mapping; Low-velocity fault; Surface trace; Structural geology; earthquake; fault; fault zone; P-wave; San Andreas Fault; seismograph; tomography; California; North America; San Andreas; United States

Major and trace element compositions of melt particles and associated phases from the Yaxcopoil-1 drill core, Chicxulub impact structure, Mexico - Tuchscherer, M.G.; Reimold, W.U.; Gibson, R.L.; De Bruin, D.; Sp{ath, A. (Meteoritics and Planetary Science, 2006)
Melt particles found at various depths in impactites from the Yaxcopoil-1 borehole into the Chicxulub impact structure (Yucat'an) 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.
cited By 12

Chesapeake bay impact structure deep drilling project completes coring - Gohn, G.S.; Koeberl, C.; Miller, K.G.; Reimold, W.U.; Abramov, O.; Aleman Gonzalez, W.; Bach, N.; Blazejak, A.; Browning, J.; Bruce, T.; Budet, C.; Bybell, L.; et al. (Scientific Drilling, 2006)
cited By 13

Downcore variations of total sulphur for VER98-1-14 - Demory, François; Oberh{ansli, Hedi; Nowaczyk, N; Gottschalk, Matthias; Wirth, R; Naumann, R (GFZ Data Services, 2006)

Downcore variations of the SIRM/κLF for CON01-603-2, CON01-604-2, CON01-605-3, VER98-1-1, VER98 - Demory, F; Oberh{ansli, Hedi; Nowaczyk, N; Gottschalk, Matthias; Wirth, R; Naumann, R (GFZ Data Services, 2006)

Downcore variations of rock magnetic parameters (ARM, S-ratio and HIRM) for CON01-603-2 and VER98-1-14 - Demory, F; Oberh{ansli, Hedi; Nowaczyk, N; Gottschalk, Matthias; Wirth, R; Naumann, R (GFZ Data Services, 2006)

Down-core variations of the SIRM for CON01-603-2. - Demory, François; Nowaczyk, N; Witt, Annette; Oberh{ansli, Hedi (GFZ Data Services, 2006)

Discovery of a 25-cm asteroid clast in the giant Morokweng impact crater, South Africa - Maier, W.D.; Andreoli, M.A.G.; Mcdonald, I.; Higgins, M.D.; Boyce, A.J.; Shukolyukov, A.; Lugmair, G.W.; Ashwal, L.D.; Gr{aser, P.; Ripley, E.M.; Hart, R.J. (Nature, 2006)
Meteorites provide a sample of Solar System bodies and so constrain the types of objects that have collided with Earth over time. Meteorites analysed to date, however, are unlikely to be representative of the entire population and it is also possible that changes in their nature have occurred with time 1. Large objects are widely believed to be completely melted or vaporized during high-angle impact with the Earth2,3. Consequently, identification of large impactors relies on indirect chemical tracers, notably the platinum-group elements4. Here we report the discovery of a large (25-cm), unaltered, fossil meteorite, and several smaller fragments within the impact melt of the giant ( >70 km diameter), 145-Myr-old Morokweng crater, South Africa. The large fragment (clast) resembles an LL6 chondrite breccia, but contains anomalously iron-rich silicates, Fe-Ni sulphides, and no troilite or metal. It has chondritic chromium isotope ratios and identical platinum-group element ratios to the bulk impact melt. These features allow the unambiguous characterization of an impactor at a large crater. Furthermore, the unusual composition of the meteorite suggests that the Morokweng asteroid incorporated part of the LL chondrite parent body not represented by objects at present reaching the Earth. © 2006 Nature Publishing Group.
cited By 62

Depth versus age based on relative magnetic paleointensity correlations for all six investigated sites - Demory, François; Nowaczyk, N; Witt, Annette; Oberh{ansli, Hedi (GFZ Data Services, 2006)

Deep water scientific drilling in Lake Malawi, Africa - Moran, K.; Paulson, M.; Lengkeek, M.; Jeffery, P.; Frazer, A. (Marine Technology Society Journal, 2006)
A new deep water drilling system was developed and applied to recover deeply buried sediments for scientific analyses in one of the deep rift valley lakes of Africa - Malawi. This approach overcame the difficulty of maintaining position over a drill site in a remotely located, large, deep lake. Environmental conditions in Lake Malawi are similar to deep water marine settings and, as such, a marine approach was adopted for the Lake Malawi Drilling Project (LMDP). In February and March 2005, the modified pontoon, Viphya, successfully completed a scientific drilling expedition in Lake Malawi. This expedition recovered core at depths greater than 380 m below lake-floor in water depths as great as 600 m. The major refit of Viphya included installation of a moonpool, bridge, crew accommodations, mess, washroom, power system, dynamic positioning, and a drilling system. These major modifications required early pontoon surveys and naval architectural analyses and design work prior to their commencement. The expedition also used modified scientific coring tools with a marine geotechnical drilling rig for the first time, resulting in excellent core recovery and quality.
cited By 0
Keywords: Dynamic positioning; Environmental engineering; Lakes; Sediments; Geotechnical drilling; Pontoon surveys; Underwater drilling; Dynamic positioning; Environmental engineering; Lakes; Sediments; Underwater drilling; deep drilling; deep sea; drilling rig; environmental conditions; Africa; East African Lakes; Lake Malawi; Sub-Saharan Africa

Dating of subduction and differential exhumation of UHP rocks from the Central Dabie Complex (E-China): Constraints from microfabrics, Rb –Sr and U –Pb isotope systems - Wawrzenitz, Nicole; Romer, Rolf L.; Oberh{ansli, Roland; Dong, Shuwen (Lithos, 2006)
The correlation of deformation fabrics and metamorphic reactions with geochronologic data of UHP metamorphic rocks demonstrate that the multistage subduction and exhumation history of the Central Dabie Complex requires rapid subduction and rapid initial exhumation. Moreover, these data show that volume diffusion is not the major resetting mechanism of radiogenic isotope systems. Thus, our age data do not simply reflect a thermal/cooling history. In the investigated section, the maximum age for UHP is given by the 244±3 Ma (2σ) U –Pb age of a pre-UHP titanite phenocryst that survived UHP metamorphism and subsequent tectonometamorphic events. A minimum age for UHP is set by the 238±1 Ma (2σ) 238U –206Pb mineral isochron age of titanite and cogenetic epidote. These minerals formed from local partial melts during ascent and their age suggests fast exhumation and emplacement in the middle crust. In the period of ca. 238 –218 Ma, the UHP terrain records HT metamorphism, local partial melting, and extensive pervasive strain below the eclogite (jd+grt) stability field. Exhumation was polyphase with a first phase of fast exhumation, succeeded by episodes of HT metamorphism and concomitant deformation at deep/mid crustal level between 238 and 218 Ma. Slow exhumation related to the final emplacement of tectonic units along greenschist facies shear zones did not cease before ca. 209 –204 Ma. The resetting and homogenization of radiogenic isotope systems were aided by dissolution precipitation creep, which was the dominant deformation mechanism in quartz –feldspar rocks, in combination with fluid influx.
Keywords: UHP metamorphism; Exhumation; Deformation; U & ndash; Pb Rb & ndash; Sr Sm & ndash; Nd isotope dating; Titanite; Dabie Shan

Cratering history and Lunar Chrnology - St{offler, D.; Ryder, G.; Ivanov, B.A.; Artemieva, N.A.; Cintala, M.J.; Grieve, R.A.F. (Reviews in Mineralogy and Geochemistry, 2006)
cited By 281

Chicxulub impact event is Cretaceous/Paleogene boundary in age: New micropaleontological evidence - Arenillas, I.; Arz, J.A.; Grajales-Nishimura, J.M.; Murillo-Muñet'on, G.; Alvarez, W.; Camargo-Zanoguera, A.; Molina, E.; Rosales-Dom'inguez, C. (Earth and Planetary Science Letters, 2006)
High-resolution and quantitative planktic foraminiferal biostratigraphy from two SE Mexico stratigraphic sections (Bochil, Guayal) shows that the Chicxulub-related Complex Clastic Unit (CCU) is synchronous with the ejecta-rich airfall layer and the Cretaceous/Paleogene (K/Pg) catastrophic mass extinction horizon in the El Kef (Tunisia) and Caravaca (Spain) sections. The lowermost Danian H. holmdelensis subzone (= Biozone P0) was identified in both sections in a thin dark clay bed just above the CCU, proving that such bed is chronostratigraphically equivalent to the K/Pg boundary clay of the El Kef stratotype. These new micropaleontogical data confirm that the K/Pg impact event and the Chicxulub impact event are the same one. This contradicts the suggestion by others that the Chicxulub impact predated the K/Pg boundary by about 300 ka. © 2006 Elsevier B.V. All rights reserved.
cited By 78

Chesapeake Bay impact structure drilled - Gohn, G.S.; Koeberl, C.; Miller, K.G.; Reimold, W.U.; Cockell, C.S.; Horton Jr., J.W.; Sanford, W.E.; Voytek, M.A. (Eos, 2006)
cited By 51
Keywords: buried structure; drilling; impact structure; impactite; meteorite; Chesapeake Bay; North America; United States