All ICDP Publications with Abstracts
From parent-sysfolder "Publications" + 2 folder-levels deep
1094.
Centimeter-scale sedimentary sequence description of Upper Cretaceous Nenjiang Formation (upper numbers 3-5): Lithostratigraphy, facies and cyclostratigraphy, based on the scientific drilling (SK1) borehole in the Songliao Basin.
Earth Science Frontiers,
18
(6)
218
2011
1093.
Centimeter-scale sedimentary sequence description of Upper Cretaceous Sifangtai Formation: Lithostratigraphy, facies and cyclostratigraphy, based on the scientific drilling (SK1) borehole in the Songliao Basin.
Earth Science Frontiers,
18
(6)
263
2011
1092.
Discussion on the geological time of Qingshankou Formation in Songliao Basin (in Chinese); 松辽盆地青山口组地质时代探讨
Petroleum Geology & Oilfield Development,
30
(4)
31-34
2011
1091.
Alaska North Slope regional gas hydrate production modeling forecasts
Marine and Petroleum Geology,
28
(2)
460-477
2011
ISSN: 02648172Keywords:▾
Alaska North Slope; Capital investment; Cash flow; Changing parameter; Commonly used; Critical value; Decision makers; Depressurizations; Development scenarios; Field development; Gas-hydrate production; Hydrate accumulations; North Slope of Alaska; Production forecasting; Production forecasts; Production rates; Production test; Production volumes; Resource development; Thermal stimulation; US Geological Survey, Forecasting; Gases; Hydration; Investments; Rating, Gas hydrates, forecasting method; gas hydrate; gas production; hydrocarbon reservoir; modeling; resource development, Alaska; North Slope; United States
Abstract: ▾ A series of gas hydrate development scenarios were created to assess the range of outcomes predicted for the possible development of the "Eileen" gas hydrate accumulation, North Slope, Alaska. Production forecasts for the "reference case" were built using the 2002 Mallik production tests, mechanistic simulation, and geologic studies conducted by the US Geological Survey. Three additional scenarios were considered: A "downside-scenario" which fails to identify viable production, an "upside-scenario" describes results that are better than expected. To capture the full range of possible outcomes and balance the downside case, an "extreme upside scenario" assumes each well is exceptionally productive.Starting with a representative type-well simulation forecasts, field development timing is applied and the sum of individual well forecasts creating the field-wide production forecast. This technique is commonly used to schedule large-scale resource plays where drilling schedules are complex and production forecasts must account for many changing parameters. The complementary forecasts of rig count, capital investment, and cash flow can be used in a pre-appraisal assessment of potential commercial viability.Since no significant gas sales are currently possible on the North Slope of Alaska, typical parameters were used to create downside, reference, and upside case forecasts that predict from 0 to 71 BM3 (2.5 tcf) of gas may be produced in 20 years and nearly 283 BM3 (10 tcf) ultimate recovery after 100 years.Outlining a range of possible outcomes enables decision makers to visualize the pace and milestones that will be required to evaluate gas hydrate resource development in the Eileen accumulation. Critical values of peak production rate, time to meaningful production volumes, and investments required to rule out a downside case are provided. Upside cases identify potential if both depressurization and thermal stimulation yield positive results. An "extreme upside" case captures the full potential of unconstrained development with widely spaced wells. The results of this study indicate that recoverable gas hydrate resources may exist in the Eileen accumulation and that it represents a good opportunity for continued research. © 2010 Elsevier Ltd.
1090.
Earthquake Rupture at Focal Depth, Part II: Mechanics of the 2004 M2.2 Earthquake Along the Pretorius Fault, TauTona Mine, South Africa
Pure and Applied Geophysics,
168
(12)
2427 – 2449
2011
1089.
Late Quaternary (24-10 ka BP) environmental history of the Neotropical lowlands inferred from ostracodes in sediments of Lago Petén Itzá, Guatemala
Journal of Paleolimnology,
46
(1)
59 – 74
2011
ISSN: 09212728Keywords:▾
Guatemala [Central America]; Lake Peten Itza; Peten; Heterocypris punctata; Ostracoda; Physocypria; abundance; aquatic ecosystem; benthos; carbon; community structure; crustacean; deglaciation; dominance; environmental change; environmental conditions; Holocene; intertidal environment; lacustrine deposit; Last Glacial Maximum; lowland environment; multivariate analysis; Neotropical Region; nitrogen; organic carbon; precipitation intensity; sediment analysis; shallow water; species richness; transfer function; water depth
Abstract: ▾ We inferred late Pleistocene and early Holocene (24-10 ka BP) environmental conditions in and around Lago Petén Itzá,Guatemala from ostracode remains in the lake sediments. Multivariate statistics were run on autecological information for 29 extant ostracode species collected in 63 aquatic ecosystems on the Yucatán Peninsula along a steep, increasing NW-S precipitation gradient and across a large altitudinal range. Conductivity and water depth are the most important factors that shape ostracode communities. Transfer functions were developed and applied to fossil ostracode assemblages in a ~76-m sediment core (PI-6, ~85 ka) taken in 71 m of water from Lago Petén Itzá, to infer past shifts in conductivity and water level. Results suggest climate was cold and wet during the Last Glacial Maximum (LGM). Alternating dry and wet conditions characterized the deglacial. Early Holocene climate was warmer and wetter. The LGM was characterized by low ostracode species richness (4 spp.) and abundance (<940 valves g-1), dominance of benthic over nektobenthic taxa, abundant Physocypria globula, conductivity as low as 190 μS cm-1, and clay-rich sediments with relatively high total organic carbon and low C/N ratios (<14), suggesting relatively deeper water at the core site associated with abundant precipitation. Greatest water depth at the core site during the LGM occurred late in the period and was ~50 m. The deglacial was characterized by drier conditions, higher ostracode species richness (6 spp.) and abundances up to 18,115 valves g-1, dominance of nektobenthic species, and presence of shallow-water and littoral-zone indicators such as Heterocypris punctata and Strandesia intrepida, conductivity up to 550 μS cm-1, C/N ratios as high as 37, and gypsum deposition. Lowest inferred lake depth at the core site during the deglacial was ~20 m. The early Holocene was characterized by high numbers of ostracode remains, up to 25,500 valves g-1, and the presence of L. opesta and P. globula. Cytheridella ilosvayi was absent from late Pleistocene sediments, suggesting it colonized northern Central America during the Holocene. © 2011 Springer Science+Business Media B.V.
1088.
International Workshop on Deep Scientific Drilling to Study Reservoir Triggered Earthquakes in Koyna, India
Journal of Geological Society of India,
77488-490
2011
1087.
Intrabasin paleoearthquake and quiescence correlation of the late Holocene Dead Sea
Journal of Geophysical Research: Solid Earth,
116
(4)
2011
ISSN: 21699313
Publisher: Blackwell Publishing Ltd
Keywords:▾
Bayesian analysis; earthquake epicenter; earthquake intensity; earthquake magnitude; fault; geochronology; Holocene; limnology; outcrop; paleoseismicity; radiocarbon dating; sedimentology; seismite, Dead Sea
Abstract: ▾ A comprehensive multisite paleoseismic archive of the late Holocene Dead Sea basin (past 2500 years) is established by constructing two age-depth chronological models of two sedimentary sections exposed at the retreating shores of the modern Dead Sea. Two new paleoseismic study sites studied are the Ein Feshkha Nature Reserve outcrop located at the northern part of the basin and close to an active underwater transverse fault and the east Ze'elim Gully outcrop at the southern part of the basin. Age-depth regression models are calculated for these sections based on atmospheric radiocarbon ages of short-lived organic debris calibrated with a Bayesian model. The uncertainties on individual model ages are smaller than 100 years. The new chronological records are compared to a laminae-counting study of the Ein Gedi core (Migowski et al., 2004) located at the central Dead Sea basin. The Ein Feshkha outcrop yielded the largest number of seismites in the studied time interval (n = 52), while lower numbers of seismites are recovered from the Ze'elim outcrop and Ein Gedi core (n = 15 and 36, respectively). The seismites show no strong dependence on the limnological-sedimentological conditions in the particular sampling sites (they coappear in both shallow and deep water environments and in different sedimentary facies). During time intervals when the chronologies are comparable it appears that the number of seismites is significantly larger in the northern part of the basin (Ein Gedi and Ein Feshkha). Seismic quiescence intervals are apparent at all three sites from 2nd-4th century A.D. and at 500-150 B.C. at Ze'elim and Ein Gedi. Several synchronous seismites appear in all sections (termed here the intrabasin seismites (IBS)). Among them: 1927, 1293, 1202/1212, 749, 551, 419, and 33 A.D. and 31 and mid-2nd century B.C. The recurrence time of the IBS from the 2nd century B.C. to the 14th century A.D. is ∼200 years, compared with ∼100 years for all earthquakes. On a diagram of epicentral distance versus magnitude, historic earthquakes that are correlated with IBS plot in a field of high local intensity. The farther and stronger IBS earthquakes require lower local intensities to be recorded. This study demonstrates that a painstaking effort is still needed for unraveling the seismic history of the Dead Sea basin. The results also indicate that such a study will likely be highly rewarding. Copyright 2011 by the American Geophysical Union.
1086.
Laboratory measurements of the seismic velocities and other petrophysical properties of the Outokumpu deep drill core samples, eastern Finland
Geophysical Journal International,
184
(1)
405 – 415
2011
ISSN: 1365246XKeywords:▾
Finland; Ita Suomen; Outokumpu; Acoustic impedance; Core drilling; Core samples; Drills; Geological surveys; Lithology; Mica; Paramagnetism; Reflection; Seismic waves; Seismology; Silicate minerals; Structural geology; Velocity; Ambient pressures; Black schist; Body waves; Crustal structure; Density data; Drill core; Eastern Finland; In-situ; Laboratory measurements; Mica schists; Microfractures; Paleoproterozoic; Petrophysical; Petrophysical properties; Poisson's ratio; Reflection coefficients; Seismic impedance; Seismic surveys; Seismic velocities; Young's Modulus; acoustic property; body wave; crustal structure; geophysics; lithology; Poisson ratio; porosity; schist; seismic survey; seismic velocity; skarn; wave propagation; Young modulus; Acoustic wave propagation
Abstract: ▾ Petrophysical, in particular seismic velocity, measurements of the Outokumpu deep drill core (depth 2.5 km) have been carried out to characterize the geophysical nature of the Paleoproterozoic crustal section of eastern Finland and to find lithological and geophysical interpretations to the distinct crustal reflectors as observed in seismic surveys. The results show that different lithological units can be identified based on the petrophysical data. The density of the samples remained nearly constant throughout the drilled section. Only diopside-tremolite skarns and black schists exhibit higher densities. The samples are dominated by the paramagnetic behaviour with occasional ferromagnetic signature caused by serpentinitic rocks. Large variations in seismic velocities, both at ambient pressure and under in situ crustal conditions are observed. The porosity of the samples, which is extremely low, is either intrinsic by nature or caused by decompaction related to fracturing during the core retrieval. It is noteworthy that these microfractures have dramatically lowered the VP and VS values. From the measured velocities and density data we have calculated the seismic impedances, Young's modulus and Poisson's ratios for the lithological units of the Outokumpu section and from these data the reflection coefficients for the major lithological boundaries, evident in the surveyed section, were determined. The data show that the strong and distinct reflections visible in wide-angle seismic surveys are caused by interfaces between diopside-tremolite skarn and either serpentinites, mica schist or black schist. © 2010 The Authors Geophysical Journal International © 2010 RAS.
1085.
Lake Malawi's response to "megadrought" terminations: Sedimentary records of flooding, weathering and erosion
Palaeogeography, Palaeoclimatology, Palaeoecology,
303
(1-4)
120-125
2011
ISSN: 00310182Keywords:▾
biogenic deposit; chemical weathering; climate change; diatom; dissolution; drought; erosion rate; flooding; lacustrine deposit; lake water; paleoclimate; phytolith; precipitation (climatology); silica; weathering, East African Lakes; Lake Malawi, Bacillariophyta
Abstract: ▾ Sediment records from the northern basin of Lake Malawi provide a means of evaluating the lake basin's response to climate change over the past 75. ky, notably to increased precipitation at the terminations of droughts. Transitions from drier to wetter conditions provide an opportunity to evaluate the system's response to climate shifts. Upon termination of drought episodes at 62 and 72. ka, enhanced precipitation and an associated increase in streampower led to enhanced physical erosion and landscapes were flooded by rising lake waters. These processes appear to have left their mark in the sedimentary record, bringing about a spike of deposition of organic matter (probably of terrestrial origin) at times of increased rainfall. This was immediately followed by a period of deposition of chemically-weathered material that had been retained on the landscape during arid times and mobilized in response to increased precipitation. After this altered material was removed (perhaps a thousand years after the transition to wetter conditions), fresher material, richer in soluble elements including nutrients, was exposed to chemical weathering, leading to substantial diatom blooms. The lag between the onset of wetter conditions and the diatom blooms is inconsistent with significant storage of bioavailable silica in soils in this system. However, biological cycling of silica, including formation and dissolution of phytoliths, may have played a role in mobilization of the silica necessary for the diatom productivity. © 2010 Elsevier B.V.
1084.
Late Pleistocene paleohydrography and diatom paleoecology of the central basin of Lake Malawi, Africa
Palaeogeography, Palaeoclimatology, Palaeoecology,
303
(1-4)
51-70
2011
ISSN: 00310182Keywords:▾
deep water; diatom; fossil assemblage; fossil record; lake level; paleoclimate; paleoecology; paleohydrology; periphyton; Pleistocene; seismic reflection, East Africa; East African Lakes; Lake Malawi, Bacillariophyta
Abstract: ▾ Analysis of sedimentary diatom assemblages (10 to 144. ka) form the basis for a detailed reconstruction of the paleohydrography and diatom paleoecology of Lake Malawi. Lake-level fluctuations on the order of hundreds of meters were inferred from dramatic changes in the fossil and sedimentary archives. Many of the fossil diatom assemblages we observed have no analog in modern Lake Malawi. Cyclotelloid diatom species are a major component of fossil assemblages prior to 35. ka, but are not found in significant abundances in the modern diatom communities in Lake Malawi. Salinity- and alkalinity-tolerant plankton has not been reported in the modern lake system, but frequently dominant fossil diatom assemblages prior to 85. ka. Large stephanodiscoid species that often dominate the plankton today are rarely present in the fossil record prior to 31. ka. Similarly, prior to 31. ka, common central-basin aulacoseiroid species are replaced by species found in the shallow, well-mixed southern basin. Surprisingly, tychoplankton and periphyton were not common throughout prolonged lowstands, but tended to increase in relative abundance during periods of inferred deeper-lake environments. A high-resolution lake level reconstruction was generated by a principle component analysis of fossil diatom and wet-sieved fossil and mineralogical residue records. Prior to 70. ka, fossil assemblages suggest that the central basin was periodically a much shallower, more saline and/or alkaline, well-mixed environment. The most significant reconstructed lowstands are ~. 600. m below the modern lake level and span thousands of years. These conditions contrast starkly with the deep, dilute, dysaerobic environments of the modern central basin. After 70. ka, our reconstruction indicates sustained deeper-water environments were common, marked by a few brief, but significant, lowstands. High amplitude lake-level fluctuations appear related to changes in insolation. Seismic reflection data and additional sediment cores recovered from the northern basin of Lake Malawi provide evidence that supports our reconstruction. © 2010 Elsevier B.V.
1083.
Late Pleistocene temperature history of Southeast Africa: A TEX86 temperature record from Lake Malawi
Palaeogeography, Palaeoclimatology, Palaeoecology,
303
(1-4)
93-102
2011
ISSN: 00310182Keywords:▾
deglaciation; Holocene; ice sheet; lacustrine deposit; Last Glacial Maximum; marine isotope stage; surface water; temperature inversion; temperature profile; tropical region; water temperature, East Africa; East African Lakes; Lake Malawi
Abstract: ▾ We present a TEX86-derived surface water temperature record for Lake Malawi that provides the first continuous continental record of temperature variability in the continental tropics spanning the past ~74kyr with millennial-scale resolution. Average temperature during Marine Isotope Stage (MIS) 5A was 26.5°C, with a range from 25.7 to 27.3°C, comparable to Holocene temperatures. MIS 4 was a relatively cold period with temperatures generally decreasing from 25.5°C at 68ka to a minimum of 20°C at ~60ka, 1.5-2°C colder than the Last Glacial Maximum (LGM). Termination of MIS 4 is characterized by a rapid increase of 3-4°C in only ~0.5kyr. Temperatures were relatively stable throughout MIS 3 at the resolution of this study, with an average of 23.8°C and a range from 25.1 to 22.9°C. The lack of millennial-scale temperature variability during MIS 3 suggests that Lake Malawi's documented response to the bipolar seesaw (Brown et al., 2007) is not reflected in its thermal history. Our temperature estimates for the LGM and Holocene are consistent with a previously published TEX86 record from Lake Malawi with a temperature of ~22.6°C for the LGM, ~25-26°C in the mid Holocene and ~25-28 for the late Holocene. In general the present extended TEX86 record indicates that temperature variability in tropical East Africa during late MIS 5 and MIS 4 was as great as that associated with the deglaciation and Holocene. A decrease in Southern Hemisphere insolation between 70 and 60ka may have played an important role in forcing temperatures during MIS 4, but after 60ka other factors, such as the extent of the polar ice sheets, or atmospheric CO2 may have forced temperature in tropical Africa to a greater extent than local summer insolation. © 2010.
1082.
Late Quaternary lake-level changes of Lake El'gygytgyn, NE Siberia
Quaternary Research,
76
(3)
441-451
2011
ISSN: 00335894Keywords:▾
Climate variability; Deep-water sediments; Holocenes; Ice cover; Impact craters; Lake levels; Lake-level changes; Late quaternary; Marine oxygen isotopes; Sediment core; SIBERIA; Siberian Arctic; Strong waves; Wave activity, Anoxic sediments; Climatology; Isotopes; Sedimentology; Water levels, Lakes, ice cover; lake level; oxygen isotope; Quaternary; sediment core; sediment transport; sedimentation; water level, Chukchi; Elgygytgyn Lake; Russian Federation; Serbia
Abstract: ▾ Lake El'gygytgyn is situated in a 3.6. Ma old impact crater in northeastern Siberia. Presented here is a reconstruction of the Quaternary lake-level history as derived from sediment cores from the southern lake shelf. There, a cliff-like bench 10. m below the modern water level has been investigated. Deep-water sediments on the shelf indicate high lake levels during a warm Mid-Pleistocene period. One period with low lake level prior to Marine Oxygen Isotope Stage (MIS) 3 has been identified, followed by a period of high lake level (10. m above present). In the course of MIS 2 the lake level dropped to -10 m At the end of MIS 2 the bench was formed and coarse beach sedimentation occurred. Subsequently, the lake level rose rapidly to the Holocene level. Changes in water level are likely linked to climate variability. During relatively temperate periods the lake becomes free of ice in summer. Strong wave actions transport sediment parallel to the coast and towards the outlet, where the material tends to accumulate, resulting in lake level rise. During cold periods the perennial lake ice cover hampers any wave activity and pebble-transport, keeping the outlet open and causing the lake level to drop. © 2011 University of Washington.
1081.
Influence of the substrate on maar-diatreme volcanoes - An example of a mixed setting from the Pali Aike volcanic field, Argentina
Journal of Volcanology and Geothermal Research,
201
(1-4)
253 – 271
2011
1080.
Late Quaternary stratigraphic analysis of the Lake Malawi Rift, East Africa: An integration of drill-core and seismic-reflection data
Palaeogeography, Palaeoclimatology, Palaeoecology,
303
(1-4)
20-37
2011
ISSN: 00310182Keywords:▾
data set; lake level; paleoclimate; sedimentary sequence; sedimentation rate; seismic reflection; seismic stratigraphy; Southern Hemisphere; surface water; total organic carbon, East Africa; East African Lakes; Lake Malawi
Abstract: ▾ Lake Malawi contains a long continuous sedimentary record of climate change in the southern hemisphere African tropics. We develop a stratigraphic framework of this basin over the last ~. 150 ka by integrating several vintages of seismic-reflection data with recently acquired drill cores. In the seismic-reflection data set, we document three lake-level cycles where progradational delta seismic facies and erosional truncation surfaces mark the basal boundary of each sequence. The clinoform packages and their down-dip, time-equivalent surfaces can be mapped throughout each basin, where each major lowstand surface was followed by a transgression and highstand. On several occasions, lake level dropped as much as 500 m below present lake level (BPLL) in the North Basin and 550 m BPLL in the Central Basin, resulting in a 97% reduction of water volume and 89% reduction of water surface area relative to modern conditions. Evidence for these lake-level fluctuations in the drill cores includes major changes in saturated bulk density, natural gamma ray values, and total organic carbon. During lowstands, density values doubled, while total organic carbon values dropped from ~. 5% to 0.2%. Coarse-grained sediment and organic matter flux into the basin were higher during transgressions, when precipitation, runoff, sediment supply, and nutrient input were high. This sedimentation pattern is also observed in seismic-reflection profiles, where coarse-grained seismic facies occur at the bases of sequences, and in the drill-core data where the highest total organic carbon values are observed immediately above lowstand surfaces. © 2009 Elsevier B.V.
1079.
Lateral transport of suevite inferred from 3D shape-fabric analysis: Evidence from the Ries impact crater, Germany
Bulletin of the Geological Society of America,
123
(11-12)
2312-2319
2011
DOI:10.1130/B30393.1
Abstract: ▾ The transport mechanism of suevite particles during impact cratering is poorly understood and was studied at the 15 Ma Ries crater in southern Germany. Two emplacement modes of suevite deposits are generally discussed: (1) fallback of plume material into the crater and its periphery upon collapse of an ejecta plume; and (2) horizontal transport of ejected material, akin to emplacement of pyroclastic deposits erupting from volcanic centers. In order to differentiate between the two emplacement modes of suevite deposition, we analyzed the shape fabrics of suevite components from two localities outside the Ries crater by fitting shape-fabric ellipsoids to measured shape-fabric ellipses and by applying high-resolution, X-ray- computed tomography to analyze the threedimensional shape and orientation of the suevite particles. We show that the preferred orientation of long axes of elongate particles is disposed either radially or concentrically with respect to the crater center. Our observations indicate that suevite material was not only derived from an ejecta plume, but was transported by lateral flow under viscous conditions upon fallback. This flow regime resembles that known from pyroclastic flows. © 2011 Geological Society of America.
1078.
Lithostratigraphy, physical properties and organic matter variability in Lake Malawi Drillcore sediments over the past 145,000years
Palaeogeography, Palaeoclimatology, Palaeoecology,
303
(1-4)
38-50
2011
ISSN: 00310182Keywords:▾
diatom; fossil record; hydrochemistry; lithostratigraphy; macrophyte; organic matter; paleoclimate; paleolimnology; Pleistocene; sediment core, East African Lakes; East African Rift; Lake Malawi, algae; Bacillariophyta
Abstract: ▾ Scientific drill cores recovered from Lake Malawi exhibit a remarkable down-core lithologic variability, and are indicative of radically changing environmental conditions forced by large-amplitude lake-level shifts over the past 150kyr. Here we present detailed lithologic and sedimentary fabric descriptions of the key sedimentary units, along with down-core physical properties data, down-core organic matter geochemistry (TOC, C/N, and δ13C data sets), and images and descriptions from core sections and from sediment smear slide microscopy. These data reveal a fundamental change in Lake Malawi's limnology and regional climate at ca. 60-70ka. Prior to this time the lake was characterized by large-amplitude variations in lake level and water chemistry, but after 60ka the lake remained comparatively high, and the central basin drill site accumulated mainly organic-rich, laminated sediments. Organic matter sources changed dramatically during the different lake stages. During major lake high stands, a mixed assemblage of algal (diatom-dominated), woodland and aquatic macrophyte (C4-pathway), and grassland (C3-pathway) organic matter was deposited, whereas during extreme low lake stages (water depths <200m), when saline, alkaline lakes persisted in the basin, sediments with minimal amounts of algal-dominated organic matter accumulated and were preserved. © 2010 Elsevier B.V.
1077.
Low strength of deep San Andreas fault gouge from SAFOD core
Nature,
472
(7341)
82-86
2011
ISSN: 00280836Keywords:▾
magnesium; mineral; silicon dioxide, borehole; creep; deformation mechanism; fault gouge; historical record; San Andreas Fault; saponite; serpentine; vertical distribution, article; geography; laboratory; priority journal; strength; United States, California; Parkfield; United States
Abstract: ▾ The San Andreas fault accommodates 28-"34-‰mm-‰yr ĝ̂'1 of right lateral motion of the Pacific crustal plate northwestward past the North American plate. In California, the fault is composed of two distinct locked segments that have produced great earthquakes in historical times, separated by a 150-km-long creeping zone. The San Andreas Fault Observatory at Depth (SAFOD) is a scientific borehole located northwest of Parkfield, California, near the southern end of the creeping zone. Core was recovered from across the actively deforming San Andreas fault at a vertical depth of 2.7-‰km (ref. 1). Here we report laboratory strength measurements of these fault core materials at in situ conditions, demonstrating that at this locality and this depth the San Andreas fault is profoundly weak (coefficient of friction, 0.15) owing to the presence of the smectite clay mineral saponite, which is one of the weakest phyllosilicates known. This Mg-rich clay is the low-temperature product of metasomatic reactions between the quartzofeldspathic wall rocks and serpentinite blocks in the fault. These findings provide strong evidence that deformation of the mechanically unusual creeping portions of the San Andreas fault system is controlled by the presence of weak minerals rather than by high fluid pressure or other proposed mechanisms. The combination of these measurements of fault core strength with borehole observations yields a self-consistent picture of the stress state of the San Andreas fault at the SAFOD site, in which the fault is intrinsically weak in an otherwise strong crust. © 2011 Macmillan Publishers Limited. All rights reserved.
1076.
Low-temperature deformation in calcite veins of SAFOD core samples (San Andreas Fault) - Microstructural analysis and implications for fault rheology
Tectonophysics,
509
(1-2)
107-119
2011
ISSN: 00401951Keywords:▾
Amorphous phasis; Calcite grains; Calcite veins; Complex compositions; Crystal plasticity; Deformation process; Differential stress; Dislocation densities; Equivalent stress; Fault core; Fault gouge; Friction coefficients; Frictional sliding; Intermediate principal stress; Lattice strain; Laue diffraction; Line density; Low friction; Low temperature deformations; Lower bounds; Lower stress; Maximum stress; Mean values; Microstructural analysis; Overburden stress; Piezometer; Pilot holes; Residual strains; SAFOD; San Andreas Fault; Scale stress; Siltstones; Upper Bound, Amorphous materials; Calcite; Deformation; Dislocations (crystals); Dissolution; Elasticity; Estimation; Friction; Light transmission; Microstructure; Quartz; Strain; Structural geology; Transmission electron microscopy; Twinning, Carbonate minerals, borehole breakout; calcite; deformation mechanism; dissolution; fault gouge; low temperature; microstructure; precipitation (chemistry); pressure effect; San Andreas Fault; sandstone; siltstone; transmission electron microscopy; twinning, California; San Andreas; United States
Abstract: ▾ The microstructures of four core samples from the San Andreas Fault Observatory at Depth (SAFOD) were investigated with optical and transmission electron microscopy. These samples, consisting of sandstone, siltstone, and fault gouge from phase III of the drilling campaign (3141-3307m MD), show a complex composition of quartz, feldspar, clays, and amorphous material. Microstructures indicate intense shearing and dissolution-precipitation as main deformation processes. The samples also contain abundant veins filled with calcite. Within the inspected veins the calcite grains exhibit different degrees of deformation with evidence for twinning and crystal plasticity. Dislocation densities (ranging from≈3·1012m-2 to ≈3·1013m-2) and twin line densities (≈22mm-1-165mm-1) are used as paleo-piezometers. The corresponding estimates of differential stresses vary between 33 and 132MPa, deduced from dislocation density and 92-251MPa obtained from twin density, possibly reflecting chronologically different maximum stress states and/or grain scale stress perturbations. Mean values of stress estimates are 68±46MPa and 168±60MPa, respectively, where estimates from dislocation density may represent a lower bound and those from twin density an upper bound. The stress estimates are also compatible with residual lattice strains determined with microfocus Laue diffraction yielding equivalent stresses of 50-300MPa in twinned calcite. The lower stress bound agrees with stress estimates from borehole breakout measurements performed in the pilot hole. From these data and assuming hydrostatic pore pressure and a low intermediate principal stress close to the overburden stress, frictional sliding of the San Andreas Fault at the SAFOD site is constrained to friction coefficients between 0.24 and 0.31. These low friction values may be related to the presence of clays, talc, and amorphous phases found in the fault cores and support the hypothesis of a weak San Andreas Fault. © 2011 Elsevier B.V.
1075.
Magnetic anomalies and rock magnetism of basalts from Reykjanes (SW-Iceland)
Studia Geophysica et Geodaetica,
55
(1)
109-130
2011
ISSN: 00393169
Publisher: Kluwer Academic Publishers
Keywords:▾
active fault; basalt; fissure; geomagnetic field; geomagnetism; lava; lithology; magnetic anomaly; magnetic mineral; magnetic property; magnetic susceptibility; oxidation; remanent magnetization; rift zone; stratigraphy; tholeiite; titanomagnetite, Atlantic Ocean; Iceland; Reykjanes Peninsula; Reykjanes Ridge
Abstract: ▾ This study presents rock magnetic properties along with magnetic field measurements of different stratigraphic and lithologic basalt units from Reykjanes, the southwestern promontory of the Reykjanes peninsula, where the submarine Reykjanes Ridge passes over into the rift zone of southwestern Iceland. The basaltic fissure eruptions and shield lava of tholeiitic composition (less than 11500 a old) show a high natural remanent magnetization (NRM, Jr) up to 33.6 A/m and high Koenigsberger ratio (Q) up to 52.2 indicating a clear dominance of the NRM compared to the induced part of the magnetization. Pillow basalts and picritic shield lava show distinctly lower Jr values below 10 A/m. Magnetic susceptibility (κ) ranges for all lithologies from 2.5 to 26 × 10-3 SI. Heterogeneously distributed titanomagnetite with small grain sizes is the main carrier of magnetization. Magnetic susceptibility vs. temperature (κ-T) curves reveal multiple Curie temperatures from 35 to 570 °C suggesting different Ti-concentrations in titanomagnetite. A minor oxidation to titanomaghemite is indicated by the irreversibility of some of the κ-T curves. Intra flow variation of the magnetic minerals is relatively high depending on crystallization history and resulting primary composition and amount of titanomagnetite as well as high-temperature oxidation. The total geomagnetic field was measured for regional field variations along three profiles normal to the spreading zone at Reykjanes. These measurements along with the rock magnetic data and field observations were used for modeling the geological subsurface. The models are in agreement with a feeder dyke system related to the youngest Stampahraun 4 fissure eruption in the western part and a hydrothermally active fault system in the eastern part of Reykjanes. Furthermore, topographic features like fault scarps, pillow basalt - hyaloclastite ridges and shield lava are considered. © 2011 Institute of Geophysics of the ASCR, v.v.i.
1074.
Erratum for "Trace element diffusion and viscous flow in potassium-rich trachytic and phonolitic melts", authors Harald Behrens and Matthias Hahn, published in Chemical Geology 259 (2009) 63-77
Chemical Geology,
288
(1-2)
86
2011
ISSN: 000925411073.
Inter-laboratory comparison of oxygen isotope compositions from biogenic silica
Geochimica et Cosmochimica Acta,
75
(22)
7242-7256
2011
ISSN: 00167037Keywords:▾
biogenic mineral; calibration; comparative study; isotopic analysis; isotopic composition; laboratory method; opal; oxygen isotope; scanning electron microscopy, Bacillariophyta
Abstract: ▾ Several techniques have been introduced in the last decades for the dehydration and release of O2 from biogenic silica (opal-A) for oxygen-isotope analysis. However, only one silica standard is universally available: a quartz standard (NBS28) distributed by the IAEA, Vienna. Hence, there is a need for biogenic silica working standards. This paper compares the existing methods of oxygen-isotope analyses of opal-A and aims to characterize additional possible working standards to calibrate the δ18O values of biogenic silica. For this purpose, an inter-laboratory comparison was organized. Six potential working standard materials were analysed repeatedly against NBS28 by eight participating laboratories using their specific analytical methods. The materials cover a wide range of δ18O values (+23 to +43‰) and include diatoms (marine, lacustrine), phytoliths and synthetically-produced hydrous silica. To characterize the proposed standards, chemical analyses and imaging by scanning electron microscopy (SEM) were also performed. Despite procedural differences at each laboratory, all methods are in reasonable agreement with a standard deviation (SD) for δ18O values between 0.3‰ and 0.9‰ (1σ). Based on the results, we propose four additional biogenic silica working standards (PS1772-8: 42.8‰; BFC: 29.0‰; MSG60: 37.0‰; G95-25-CL leaves: 36.6‰) for δ18O analyses, available on request through the relevant laboratories. © 2011 Elsevier Ltd.
1072.
Lithology and internal structure of the San Andreas fault at depth based on characterization of Phase 3 whole-rock core in the San Andreas Fault Observatory at Depth (SAFOD) borehole
Earth and Planetary Science Letters,
310
(1-2)
131-144
2011
ISSN: 0012821XKeywords:▾
Casing deformation; Cataclasite; Clay gouges; Deformation process; During phase; Fault gouge; Fault zone; Fault-related rocks; Fluid-rock interaction; Host rocks; Internal structure; Lizardite; matrix; Microseismicity; North American; Pacific plates; Protoliths; SAFOD; San Andreas Fault; Serpentinite; Siltstones; Ultrafine; Whole-rock geochemistry, Buildings; Coatings; Creep; Iron ores; Kaolinite; Lithology; Minerals; Observatories; Plates (structural components); Rock drilling; Rocks; Sandstone; Sedimentology; Serpentine, Structural geology, borehole; creep; deformation; drilling; fault gouge; fault zone; host rock; lithology; observatory; Pacific plate; San Andreas Fault; sandstone; seismicity; shale
Abstract: ▾ We characterize the lithology and structure of the spot core obtained in 2007 during Phase 3 drilling of the San Andreas Fault Observatory at Depth (SAFOD) in order to determine the composition, structure, and deformation processes of the fault zone at 3. km depth where creep and microseismicity occur. A total of approximately 41. m of spot core was taken from three separate sections of the borehole; the core samples consist of fractured arkosic sandstones and shale west of the SAF zone (Pacific Plate) and sheared fine-grained sedimentary rocks, ultrafine black fault-related rocks, and phyllosilicate-rich fault gouge within the fault zone (North American Plate). The fault zone at SAFOD consists of a broad zone of variably damaged rock containing localized zones of highly concentrated shear that often juxtapose distinct protoliths. Two zones of serpentinite-bearing clay gouge, each meters-thick, occur at the two locations of aseismic creep identified in the borehole on the basis of casing deformation. The gouge primarily is comprised of Mg-rich clays, serpentinite (lizardite ± chrysotile) with notable increases in magnetite, and Ni-Cr-oxides/hydroxides relative to the surrounding host rock. The rocks surrounding the two creeping gouge zones display a range of deformation including fractured protolith, block-in-matrix, and foliated cataclasite structure. The blocks and clasts predominately consist of sandstone and siltstone embedded in a clay-rich matrix that displays a penetrative scaly fabric. Mineral alteration, veins and fracture-surface coatings are present throughout the core, and reflect a long history of syn-deformation, fluid-rock reaction that contributes to the low-strength and creep in the meters-thick gouge zones. © 2011 Elsevier B.V.
1071.
Imaging permafrost velocity structure using high resolution 3D seismic tomography
Geophysics,
76
(5)
B187-B198
2011
ISSN: 00168033
Publisher: Society of Exploration Geophysicists
Keywords:▾
Lakes; Permafrost; Seismic prospecting; Seismic waves; Seismology; Surface structure; Tomography, Case history; Complex near surfaces; Frequency attenuation; Low and high frequencies; Systematic correlation; Tomographic inversion; Travel time tomography; Velocity analysis, Velocity, arrival time; imaging method; P-wave; permafrost; seismic data; seismic survey; seismic tomography; three-dimensional modeling; travel time; velocity structure, Canada; Mackenzie Delta; Northwest Territories
Abstract: ▾ A 3D seismic survey (Mallik 3D), covering 126 km2 in the Mackenzie Delta area of Canada's north, was conducted by industry in 2002. Numerous lakes and marine inundation create a complex near-surface structure in the permafrost terrain. Much of the near subsurface remains frozen but significant melt zones exist particularly from perennially unfrozen water bodies. This results in an irregular distribution of permafrost ice creating a complex pattern of low and high frequency near-surface velocity variations which induce significant traveltime distortions in surface seismic data. A high resolution 3D traveltime tomography study was employed to map the permafrost velocity structure utilizing first-arrival traveltimes picked from 3D seismic shot records. Approximately 900,000 traveltime picks from 3167 shots were used in the inversion. Tomographic inversion of the first-arrival traveltimes resulted in a smooth velocity model for the upper 200 m of the subsurface. Ray coverage in the model is excellent down to 200 m providing effective control for estimating velocities through tomographic inversion. Resolution tests conducted through horizontal and vertical checkerboard tests confirm the robustness of the velocity model in detailing small scale velocity variations. Well velocities were used to validate tomographic velocities. The tomographic velocities do not show systematic correlation with well velocities. The velocity model clearly images the permafrost velocity structure in lateral and vertical directions. It is inferred from the velocity model that the permafrost structure in the near subsurface is discontinuous. Extensions of surface water bodies in depth, characterized by low P-wave velocities, are well imaged by the velocity model. Deep lakes with unfrozen water, inferred from the tomographic velocity model, correlate with areas of strong amplitude blanking and frequency attenuation observed in processed reflection seismic stack sections. © 2011 Society of Exploration Geophysicists.
1070.
Geomagnetic field intensity and inclination records from Hawaii and the Réunion Island: Geomagnetic implications
Physics of the Earth and Planetary Interiors,
187
(3-4)
170 – 187
2011
ISSN: 00319201Keywords:▾
Hawaii [United States]; United States; Calluna vulgaris; Electric generators; Seismology; Best fit; Core-mantle boundary; Expected values; Geomagnetic fields; Geomagnetic modeling; Hawaii; Heat-flow; Lateral variations; Model results; Paleointensity; Rayleigh number; Seismic tomography; Subaerial flows; Thermal boundary; Time-periods; amplitude; core-mantle boundary; geomagnetic field; heat flow; numerical model; Rayleigh number; seismic tomography; Geomagnetism
Abstract: ▾ We report on new paleointensity and inclination records obtained in Hawaii from 386 samples drilled in 137 subaerial flows of the HSDP2 long basaltic core that we have combined in a composite record with the other Hawaiian results to produce a unique and accurate lava record of absolute geomagnetic field intensity and inclination at Hawaii for the last 405kyr. These data are considered together with published results from La Réunion Island, for about the same time period in order to compare them to model results. In Hawaii, when recognized excursional periods are omitted, the average VADM (VDM) value is about 8.2×(8.4)×1022Am2 and the inclination is on the average 29.8°, i.e. about 6° shallower than the expected value at this location (GAD value). In the Réunion Island, we selected published results in which both inclination and intensities values are obtained using modern methods and strict criteria. The average VADM (VDM) is 9.4±2.3 (8.7±2.3)1022Am2 and the average inclination value (when transitional data are ignored) is -44.4±2.4°, about 7° steeper than the GAD value. These results, compared with model intensities predicted by dynamo solutions that incorporate lateral variations in core-mantle boundary heat-flow derived from seismic tomography, are too high for both localities, but they decrease as the amplitude of thermal boundary anomalies increases. With strong boundary heat-flow, the model inclination anomaly at Hawaii fits with the data but at La Réunion, model inclination anomalies have the wrong sign. This comparison indicates that strong boundary heat-flow anomalies give the best fit to the data and that future improvements will be obtained by proportionally increasing both the Rayleigh number and the amplitude of heat-flow variations. © 2011 Elsevier B.V.