All ICDP Publications with Abstracts
From parent-sysfolder "Publications" + 2 folder-levels deep
269.
Looking into a volcano: Drilling unzen
Geotimes,
49
(3)
14 – 15
2004
ISSN: 00168556268.
Composition of basaltic lavas sampled by phase-2 of the Hawaii Scientific Drilling Project: Geochemical stratigraphy and magma types
Geochemistry, Geophysics, Geosystems,
5
(3)
2004
ISSN: 15252027Abstract: ▾ This paper presents major and trace element compositions oflavas from the entire 3098 m stratigraphic section sampled by phase-2 of the Hawaii Scientific Drilling Project. The upper 245 m are lavas from Mauna Loa volcano, and the lower 2853 m are lavas and volcanoclastic rocks from Mauna Kea volcano. These intervals are inferred to represent about 100 ka and 400 ka respectively of the eruptive history of the two volcanoes. The Mauna Loa tholeiites tend to be higher in SiO2 and lower in total iron, TiO2, alkalis, and incompatible elements at a given MgO content than Mauna Kea lavas. The transition from Mauna Loa to Mauna Kea lavas is all the more pronounced because the Mauna Loa tholeiites overlie a thin sequence of postshield Mauna Kea alkalic to transitional tholeiitic lavas. The Mauna Loa tholeiites display well- developed coherent trends with MgO that are indistinguishable in most respects from modern lavas. With depth, however, there is a slight decline in incompatible element abundances, and small shifts to depleted isotopic ratios. These characteristics suggest small changes in melt production and source components over time, superimposed on shallow melt segregation. The Mauna Kea section is subdivided into a thin, upper 107 m sequence of postshield tholeiites, transitional tholeiites and alkali basalts of the Hamakua volcanics, overlying four tholeiitic magma types that are intercalated throughout the rest of the core. These four magma types are recognized on the basis of MgO-normalized SiO2 and Zr/Nb values. Type-1 lavas (high SiO 2 and Zr/Nb) are ubiquitous below the postshield lavas and are the dominant magma type on Mauna Kea. They are inter-layered with the other three lava types. Type-2 lavas (low SiO2 but high Zr/Nb) are found only in the upper core, and especially above 850 m. Type-3 lavas (low SiO2 and Zr/Nb) are very similar to tholeiites from Loihi volcano and are present only below 1974 m. There are only 3 discrete samples of type-4 lavas (high SiO2 and low Zr/Nb), which are present in the upper and lower core. The differences between these magma types are inferred to reflect changes in melt production, depth of melt segregation, and differences in plume source components over about 400 ka of Mauna Kea's eruptive history. At the start of this record, eruption rates were high, and two distinct tholeiitic magmas (type-1 and 3) were erupting concurrently. These two magmas require two distinct source components, one similar to that of modern Loihi tholeiites and the other close to that of Kilauea magmas. Subsequently, the Loihi- like source of the type-3 magmas was exhausted, and these lavas are absent from the remainder of the core. For the next 200 ka or so, the eruptive sequence consists of inter-layered type-1 and -2 lavas that are derived from a common Mauna Kea source, the major difference between the two being the depth at which the melts segregated from the source. At around 440 ka (corresponding with the transition in the core from submarine to subaerial lavas) eruption rates began to decline and low-MgO lavas are suddenly much more abundant in the record. Continuing gradual decline in melting and eruption rates was accompanied by a decline in normalized SiO2 content of the type-1 magmas, and the eventual onset of postshield magmatism. Copyright 2004 by the American Geophysical Union.
267.
Drilling through the active Aigion Fault: The AIG10 well observatory; [Forage au travers de la faille active d'Aigion : L'observatoire constitué par le puits AIG10]
Comptes Rendus - Geoscience,
336
(4-5)
395 – 406
2004
ISSN: 16310713Keywords:▾
Gulf of Corinth; Ionian Sea; Mediterranean Sea; active fault; dip; fault plane; fluid flow; hydrogeochemistry; rift zone
Abstract: ▾ The 1000 m-deep AIG10 borehole intersects the Aigion Fault within the limestone of the Pindos nappe at 760 m. It has demonstrated that the fault is dipping 60° with respect to horizontal, an angle consistent with the strength characteristics of the fault material as determined in the laboratory. It does not seem to be listric, as suggested by the location of superficial microseismic events. The fault is about 7 m thick, with a 50 cm core of clay derived from smeared radiolarite formation. The fault offsets the basement rock by 150±20 m and constitutes a hydraulic barrier that sustains a 0.5 MPa differential pressure. Below the fault, a strongly karstified limestone has been encountered down to 1000 m. It is the site of a 0.9 MPa overpressure and exhibits no temperature gradient. Water geochemistry demonstrates the continental origin of this significant flow, which obliterates the regional heat flux. The present monitoring of downhole pressure yields data on tidal variations with a resolution of 1/500 as well as pressure variations induced by teleseisms. Given preliminary results from 14C dating, the age of the fault is about 50 kyr and the mean slip rate equal about 3.5 mm yr-1. © 2004 Académie des sciences. Published by Elsevier SAS. All rights reserved.
266.
Drilling the central crater of the chesapeake bay impact structure: A first look
Eos,
85
(39)
369+377
2004
ISSN: 00963941
Publisher: American Geophysical Union
265.
Differential waveform tomography for time-lapse crosswell seismic data with application to gas hydrate production monitoring
SEG Technical Program Expanded Abstracts,
23
(1)
2323-2326
2004
ISSN: 10523812
Publisher: Society of Exploration Geophysicists
Keywords:▾
Geophysical prospecting; Petroleum prospecting; Seismic response; Seismic waves; Waveform analysis, Conventional approach; Gas production test; Gas-hydrate production; Numerical tests; Production zones; Time-lapse seismic data; Velocity changes; Waveform inversion, Gas hydrates
Abstract: ▾ To detect the change of physical properties in small areas, a series of high-resolution waveform inversions is applied to time-lapse seismic data. A procedure by use of differentiation between the time-lapse data and normalization using reference data is proposed in this study. The procedure is derived as a straight-forward extension of waveform inversion as the scatterer imaging. Through numerical tests, the proposed approach was found to be more accurate than the conventional approach in obtaining the velocity change in small areas. The method was applied to the time-lapse crosswell seismic data obtained during the Mallik 2002 gas production test. A small area showing a velocity decrease near the production zone were found using the proposed method, indicating the existence of dissociated methane gas in the sand layers. © 2004 Society of Exploration Geophysicists.
264.
Determining SAFOD area microearthquake locations solely with the Pilot Hole seismic array data
Geophysical Research Letters,
31
(12)
L12S10 1-5
2004
ISSN: 00948276Keywords:▾
Data acquisition; Earthquakes; Error analysis; Mathematical models; Observatories; Waveform analysis; Seismology; Strike-slip faults, Bootstrap method; High resolution seismic network (HRSN); Microearthquakes; Pilot hole (PH), Geophysics; Location, earthquake hypocenter; microtremor; San Andreas Fault; seismic method, Bootstrap method; High resolution seismic; Location uncertainty; San Andreas fault; Seismic arrays; Surface observation; Three component; Velocity model
Abstract: ▾ In August 2002, an array of 32 three-component geophones was installed in the San Andreas Fault Observatory at Depth (SAFOD) Pilot Hole (PH) at Parkfield, CA. As an independent test of surface-observation-based microearthquake locations, we have located such events using only data recorded on the PH array. We then compared these locations with locations from a combined set of PH and Parkfield High Resolution Seismic Network (HRSN) observations. We determined the uncertainties in the locations as they relate to errors in the travel time picks and the velocity model by the bootstrap method. Based on the PH and combined locations, we find that the "C2" cluster to the northeast of the PH has the smallest location uncertainties. Events in this cluster also have the most similar waveforms and largest magnitudes. This confirms earlier suggestions that the C2 cluster is a promising target for the SAFOD Main Hole. Copyright 2004 by the American Geophysical Union.
263.
Detailed kinematics, structure and recurrence of micro-seismicity in the SAFOD target region
Geophysical Research Letters,
31
(12)
L12S08 1-4
2004
ISSN: 00948276Keywords:▾
Creep; Geophysics; Kinematics; Seismology; Stresses; Structural analysis; Theory; Drilling platforms; Faulting; Geophysics; Kinematics; Strike-slip faults; Tectonics, Crustal volume; Micro-seismicity; Scaling properties; Stress-drop theory, Earthquakes; Earthquakes, deep drilling; earthquake recurrence; kinematics; microtremor; San Andreas Fault; seismicity, California; North America; United States, Constant stress; Fault strands; Recurrence intervals; Repeating earthquake; San Andreas fault; Scaling properties; Small earthquakes; Target regions
Abstract: ▾ Large numbers of small earthquakes recorded over 2 decades and analyzed with advanced techniques are used to characterize the detailed kinematics, structure and recurrence interval scaling properties of micro-seismicity in a 4 × 4 km lateral and 6 km deep crustal volume encompassing the region of the SAFOD deep drilling experiment. The characterization reveals that the seismically active San Andreas fault in the vicinity of SAFOD's repeating magnitude 2 target earthquakes is composed of two sub-parallel fault strands that are creeping at comparable rates and that one of the strands lies between the SAFOD drilling platform and SAFOD's target events. In the region, ∼55% of the earthquakes are members of 52 characteristically repeating earthquake sequences. The recurrence intervals of the repeating target events are consistent with the interval scaling of the other sequences. However this. scaling is contrary to that expected from standard constant stress-drop theory. Copyright 2004 by the American Geophysical Union.
262.
Crustal structure across the San Andreas Fault at the SAFOD site from potential field and geologic studies
Geophysical Research Letters,
31
(12)
L12S03 1-4
2004
ISSN: 00948276Keywords:▾
Data acquisition; Granite; Gravitational effects; Magnetic materials; Magnetic susceptibility; Mathematical models; Rock drilling; Rocks; Sandstone; Serpentine; Magnetic susceptibility; Magnetism; Rock drilling; Strike-slip faults, Crustal structure; Magnetic granitic rock; San Andreas Fault (SAF); Surface geology, Geology; Structural geology, crustal structure; geological mapping; gravity survey; magnetic survey; San Andreas Fault; serpentinite, Crustal structure; Granitic rocks; Pilot holes; Potential field; San Andreas fault; Serpentinite
Abstract: ▾ We present newly compiled magnetic, gravity, and geologic datasets from the Parkfield region around the San Andreas Fault Observatory at Depth (SAFOD) pilot hole in order to help define the structure and geophysical setting of the San Andreas Fault (SAF). A 2-D cross section of the SAF zone at SAFOD, based on new, tightly spaced magnetic and gravity observations and surface geology, shows that as drilling proceeds NE toward the SAF, it is likely that at least 2 fault bounded magnetic slivers, possibly consisting of magnetic granitic rock, serpentinite, or unusually magnetic sandstone, will be encountered. The upper 2 km of the model is constrained by an order of magnitude increase in magnetic susceptibility at 1400 m depth observed in pilot hole measurements. NE of the SAF, a flat lying, tabular body of serpentinite at 2 km depth separates two masses of Franciscan rock and truncates against the SAF.
261.
Coring the Chesapeake Bay impact crater
Geotimes,
49
(1)
22-25
2004
ISSN: 00168556Abstract: ▾ In July 1983, the shipboard scientists of Deep Sea Drilling Project Leg 95 found an unexpected bonus in a core taken 150 kilometers east of Atlantic City, N.J. At Site 612, the scientists recovered a 10-centimeter-thick layer of late Eocene debris ejected from an impact about 36 million years ago. Microfossils and argon isotope ratios from the same layer reveal that the ejecta were part of a broad North American impact debris field, previously known primarily from the Gulf of Mexico and Caribbean Sea. Since that serendipitous beginning, years of seismic reflection profiling, gravity measurements and core drilling have confirmed the source of that strewn field - the Chesapeake Bay impact crater, the largest structure of its kind in the United States, and the sixth-largest impact crater on Earth.
260.
Joint inversion of gravity and arrival time data from Parkfield: New constraints on structure and hypocenter locations near the SAFOD drill site
Geophysical Research Letters,
31
(12)
L12S041-4
2004
ISSN: 00948276Keywords:▾
Drilling; Earthquakes; Gravitational effects; Mathematical models; Perturbation techniques; Seismology; Site selection; Earthquakes; Strike-slip faults, Best-fit arrival time model; Datasets; Seismic zones, Elastic waves; Elastic waves, earthquake hypocenter; elastic wave; gravity survey; modeling; San Andreas Fault; seismic hazard, California; North America; Parkfield; United States, Arrival-time data; Earthquake location; Hypocenter location; Joint inversion; Large datasets; San Andreas fault; Shallow depths; Surface traces
Abstract: ▾ Taking advantage of large datasets of both gravity and elastic wave arrival time observations available for the Parkfield, California region, we generated an image consistent with both types of data. Among a variety of strategies, the best result was obtained from a simultaneous inversion with a stability requirement that encouraged the perturbed model to remain close to a starting model consisting of a best fit to the arrival time data. The preferred model looks essentially the same as the best-fit arrival time model in areas where ray coverage is dense, with differences being greatest at shallow depths and near the edges of the model where ray paths are few. Earthquake locations change by no more than about 100 m, the general effect being migration of the seismic zone to the northeast, closer to the surface trace of the San Andreas Fault. Copyright 2004 by the American Geophysical Union.
259.
Composition of impact melt particles and the effects of post-impact alteration in suevitic rocks at the Yaxcopoil-1 drill core, Chicxulub crater, Mexico
Meteoritics and Planetary Science,
39
(7)
1169-1186
2004
Abstract: ▾ Petrographical and chemical analysis of melt particles and alteration minerals of the about 100 m-thick suevitic sequence at the Chicxulub Yax-1 drill core was performed. The aim of this study is to determine the composition of the impact melt, the variation between different types of melt particles, and the effects of post-impact hydrothermal alteration. We demonstrate that the compositional variation between melt particles of the suevitic rocks is the result of both incomplete homogenization of the target lithologies during impact and subsequent post-impact hydrothermal alteration. Most melt particles are andesitic in composition. Clinopyroxene-rich melt particles possess lower SiO2 and higher CaO contents. These are interpreted by mixing of melts from the silicate basement with overlying carbonate rocks. Multi-stage post-impact hydrothermal alteration involved significant mass transfer of most major elements and caused further compositional heterogeneity between melt particles. Following backwash of seawater into the crater, palagonitization of glassy melt particles likely caused depletion of SiO2, A12O3, CaO, Na2O, and enrichment of K2O and FeOtot during an early alteration stage. Since glass is very susceptible to fluid-rock interaction, the state of primary crystallization of the melt particles had a significant influence on the intensity of the post-impact hydrothermal mass transfer and was more pronounced in glassy melt particles than in well-crystallized particles. In contrast to other occurrences of Chicxulub impactites, the Yax-1 suevitic rocks show strong potassium metasomatism with hydrothermal K-feldspar formation and whole rock K2O enrichment, especially in the lower unit of the suevitic sequence. A late stage of hydrothermal alteration is characterized by precipitation of silica, analcime, and Na-bearing Mg-rich smectite, among other minerals. This indicates a general evolution from a silica-undersaturated fluid at relatively high potassium activities at an early stage toward a silica-oversaturated fluid at relatively high sodium activities at later stages in the course of fluid rock interaction. © Meteoritical Society, 2004. Printed in USA.
258.
Chicxulub: Testing for post-impact hydrothermal input into the Tertiary ocean
Meteoritics and Planetary Science,
39
(7)
1223-1231
2004
Abstract: ▾ Studies of large terrestrial impact craters indicate that post-impact hydrothermal activity is a likely consequence of the crustal deformation and heating induced by such events. In the case of the Chicxulub basin, where marine conditions were re-established soon after the impact, significant fluxing of seawater-through the crust and hydrothermal venting into the water column might be anticipated. We have carried out geochemical analyses of Tertiary carbonate sediments within the Yaxcopoil-1 (Yax-1) drill hole to test for evidence of such post-impact hydrothermal circulation. Hydrothermal activity is most likely to be found close to thick layers of melt rock inside the collapsed transient cavity, and it is estimated that Yax-1 is located ∼20 km outside this cavity. Consequently, the most likely signature of hydrothermal venting into the water column would be geochemical anomalies attributable to fallout of suspended particulate matter from a submarine hydrothermal plume. Samples of Tertiary biomicrites from depths of 794.01 to 777.02 m have high concentrations of manganese, iron, phosphorous, titanium, and aluminium and low iron/manganese ratios relative to samples from higher in the stratigraphic succession. This geochemical anomaly decreases' fairly systematically between 793.13 m and 777.02 m, above which an abrupt change in geochemistry is observed. A mass balance calculation suggests that the anomaly is unlikely to be the result of a decreasing detrital input to the carbonate sediments and the nature of the element enrichments is consistent with expectations for fallout from a distal hydrothermal plume. We conclude that a post-impact hydrothermal system did develop at Chicxulub, which led to the expulsion of hydrothermal fluids into the Tertiary water column. Preliminary biostratigraphic and magnetostratigraphic dating on Yax-1 core suggest that this hydrothermal activity lasted for at least 300 ka. © Meteoritical Society, 2004. Printed in USA.
257.
Elastic velocity models for gas-hydrate-bearing sediments - A comparison
Geophysical Journal International,
159
(2)
573-590
2004
ISSN: 0956540XKeywords:▾
comparative study; hydration; marine sediment; numerical model; P-wave; S-wave; saturation; seismic reflection; seismic survey; seismic velocity
Abstract: ▾ The presence of gas hydrate in oceanic sediments is mostly identified by bottom-simulating reflectors (BSRs), reflection events with reversed polarity following the trend of the seafloor. Attempts to quantify the amount of gas hydrate present in oceanic sediments have been based mainly on the presence or absence of a BSR and its relative amplitude. Recent studies have shown that a BSR is not a necessary criterion for the presence of gas hydrates, but rather its presence depends on the type of sediments and the in situ conditions. The influence of hydrate on the physical properties of sediments overlying the BSR is determined by the elastic properties of their constituents and on sediment microstructure. In this context several approaches have been developed to predict the physical properties of sediments, and thereby quantify the amount of gas/gas hydrate present from observed deviations of these properties from those predicted for sediments without gas hydrate. We tested four models: the empirical weighted equation (WE); the three-phase effective-medium theory (TPEM); the three-phase Biot theory (TPB) and the differential effective-medium theory (DEM). We compared these models for a range of variables (porosity and clay content) using standard values for physical parameters. The comparison shows that all the models predict sediment properties comparable to field values except for the WE model at lower porosities and the TPB model at higher porosities. The models differ in the variation of velocity with porosity and clay content. The variation of velocity with hydrate saturation is also different, although the range is similar. We have used these models to predict velocities for field data sets from sediment sections with and without gas hydrates. The first is from the Mallik 2L-38 well, Mackenzie Delta, Canada, and the second is from Ocean Drilling Program (ODP) Leg 164 on Blake Ridge. Both data sets have Vp and Vs information along with the composition and porosity of the matrix. Models are considered successful if predictions from both Vp and Vs match hydrate saturations inferred from other data. Three of the models predict consistent hydrate saturations of 60-80 per cent from both Vp and Vs from log and vertical seismic profiling data for the Mallik 2L-38 well data set, but the TPEM model predicts 20 per cent higher saturations, as does the DEM model with a clay-water starting medium. For the clay-rich sediments of Blake Ridge, the DEM, TPEM and WE models predict 10-20 per cent hydrate saturation from Vp data, comparable to that inferred from resistivity data. The hydrate saturation predicted by the TPB model from Vp is higher. Using Vs data, the DEM and TPEM models predict very low or zero hydrate saturation while the TPB and WE models predict hydrate saturation very much higher than those predicted from Vp data. Low hydrate saturations are observed to have little effect on Vs. The hydrate phase appears to be connected within the sediment microstructure even at low saturations. © 2004 RAS.
256.
Chicxulub central crater structure: Initial results from physical property measurements and combined velocity and gravity modeling
Meteoritics and Planetary Science,
39
(7)
1019-1034
2004
Abstract: ▾ The Chicxulub crater in Mexico is a nearly pristine example of a large impact crater. Its slow burial has left the central impact basin intact, within which there is an apparently uneroded topographic peak ring. Its burial, however, means that we must rely on drill holes and geophysical data to interpret the crater form. Interpretations of crater structures using geophysical data are often guided by numerical modeling and observations at other large terrestrial craters. However, such endeavors are hindered by uncertainties in current numerical models and the lack of any obvious progressive change in structure with increasing crater size. For this reason, proposed structural models across Chicxulub remain divergent, particularly within the central crater region, where the deepest well is only ∼1.6 km deep. The shape and precise location of the stratigraphic uplift are disputed. The spatial extent and distribution of the allogenic impact breccias and melt rocks remain unknown, as do the lithological nature of the peak ring and the mechanism for its formation. The objective of our research is to provide a well-constrained 3D structural and lithological model across the central region of the Chicxulub crater that is consistent with combined geophysical data sets and drill core samples. With this in mind, we present initial physical property measurements made on 18 core samples from the Yaxcopoil-1 (Yax-1) drill hole between 400 and 1500 m deep and present a new density model that is in agreement with both the 3D velocity and gravity data. Future collation of petrophysical and geochemical data from Yax-1 core, as well as further seismic surveys and drilling, will allow us to calibrate our geophysical models-assigning a suite of physical properties to each lithology. An accurate 3D model of Chicxulub is critical to our understanding of large craters and to the constraining of the environmental effects of this impact. © Meteoritical Society, 2004. Printed in USA.
255.
Borehole geophysical techniques to define stratigraphy, alteration and aquifers in basalt
Journal of Applied Geophysics,
55
(1-2)
3-38
2004
ISSN: 09269851
Publisher: Elsevier
Keywords:▾
Aquifers; Basalt; Boreholes; Magnetic susceptibility; Remanence; Sedimentary rocks, Magnetic mineralogy, Stratigraphy, aquifer; basalt; borehole geophysics; groundwater exploration; mineral alteration; stratigraphy
Abstract: ▾ This paper concerns the interpretation of borehole geophysical data from basalt sequences, especially continental basalt sequences that host aquifers. Based on modifications of the rules used for interpreting borehole data from sedimentary rocks, new rules are proposed to identify the internal stratigraphy, aquifer boundaries, and alteration features in continental basalts.The value of several wireline tools is critiqued. Natural gamma logs have limited utility in basalt sequences unless anomalously high-potassium or low-potassium basalt flows and/or sedimentary interbeds exist which can act as marker beds for stratigraphic correlations. Neutron logs can usually discriminate between individual flows, flow breaks and interbeds, even in unsaturated basalts. Neutron logs and temperature logs can also be used to map aquifer thickness in basalt. Gamma-gamma density logs are usually sensitive to the density contrasts between interbeds and basalt flows, and in combination with neutron and natural gamma logs are crucial for the correct interpretation of large void spaces in basalt such as collapsed lava tubes and formerly inflated pahoehoe lobes. Basalt porosity calculated from neutron, resistivity and/or gamma-gamma density logs is commonly overestimated due to the presence of hydrous alteration minerals. Velocity and resistivity logs are best at discriminating between flows in saturated conditions. Magnetic susceptibility logs may capture magnetic mineralogy variations at a finer scale than that of flows and flow breaks and therefore should always be interpreted in combination with other logs. Non-spectral neutron-gamma logs are not useful in basalt, though spectral neutron-gamma logs have been used successfully for stratigraphic correlation and to locate pollutants. Geochemical logs or the inclination of magnetic remanence provide the best data to discriminate individual flows with a basalt sequence, and thus establish an internal stratigraphy. Other tools used alone cannot provide reliable stratigraphic information, but a combination of tools may work. We recommend the combination of natural gamma, neutron, and gamma-gamma density logs in unsaturated rocks, and these logs plus velocity and resistivity logs in saturated rocks. © 2003 Published by Elsevier B.V.
254.
Benthic foraminifera and environmental turnover across the Cretaceous/Paleogene boundary at Blake Nose (ODP Hole 1049C, Northwestern Atlantic)
Palaeogeography, Palaeoclimatology, Palaeoecology,
208
(1-2)
59-83
2004
Abstract: ▾ Sediments recovered at lower bathyal ODP Site 1049 on Blake Nose (Northwestern Atlantic) offer an opportunity to study environmental changes at the Cretaceous/Paleogene (K/P) boundary relatively close to the Chicxulub impact structure on the Yucatan peninsula, Mexico. In Hole 1049C, the boundary is located at the base of a 9-cm-thick layer with abundant spherules, considered to be impact ejecta. Uppermost Maastrichtian oozes below, and lowermost Danian pelagic oozes above the spherule-bed contain well-preserved bathyal benthic foraminifera. The spherule-bed itself, in contrast, contains a mixture of shallow (neritic) and deeper (bathyal) species, and specimens vary strongly in preservation. This assemblage was probably formed by reworking and down-slope transport triggered by the K/P impact. Across the spherule-bed (i.e., the K/P boundary) only ∼7% of benthic foraminiferal species became extinct, similar to the low extinction rates of benthic foraminifera worldwide. Quantitative analysis of benthic foraminiferal assemblages and morphogroups in the >63-μm size fraction indicates a relatively eutrophic, stable environment during the latest Maastrichtian, interrupted by a sudden decrease in the food supply to the benthos at the K/P boundary and a decrease in diversity of the faunas, followed by a stepped recovery during the earliest Danian. The recovery was probably linked to the gradual recovery of surface-dwelling primary producers. © 2004 Elsevier B.V. All rights reserved.
253.
Azimuthal anisotropy using shear dipole sonic: Insights from the AIG 10 well, Corinth Rift Laboratory; [Anisotropie azimutale par diagraphie sonique des ondes de cisaillement : Données du forage AIG 10, laboratoire du rift de Corinthe]
Comptes Rendus - Geoscience,
336
(4-5)
477 – 485
2004
ISSN: 16310713Keywords:▾
Gulf of Corinth; Ionian Sea; Mediterranean Sea; acoustic logging; borehole geophysics; fault; structural geology
Abstract: ▾ Within the Corinth Rift Laboratory, dipole sonic data have been acquired in the AIG 10 well between depths 711 and 1004 m during three passes of a sonic logging tool at three different borehole fluid pressures. Specific sections of the well reveal clear characteristics of either moderate-to-large azimuthal anisotropy (9-25%) below the Aigion Fault, e.g., 779-784 and 809-816 m, with a fast-shear azimuth directed 105°, or homogeneous isotropic medium (i.e., 735-753 m) between the casing shoe and the fault. The presence of the fault is coincident with the identification of lower velocities over an interval of approximately 12-14 m. Analysis of the data reveals that the formation is not acoustically stress-sensitive to the 3 MPa differential pressure applied. Interpretation of the sonic data with complementary image logs suggests the anisotropy is due to intrinsic fractures and bedding, and the fast-shear direction 105° is consistent with the regional maximum horizontal stress. © 2004 Académie des sciences. Published by Elsevier SAS. All rights reserved.
252.
Analysis of silicon concentration periodicity for the past 2.4 Ma in sediments from Lake Baikal site BDP 96-2
Climate Research,
26
(3)
193 – 197
2004
ISSN: 0936577X
Publisher: Inter-Research
DOI:10.3354/cr026193
Keywords:▾
Eurasia; Lake Baikal; Russian Federation; climate change; lacustrine deposit; Milankovitch cycle; orbital forcing; paleoenvironment; silicon
Abstract: ▾ The current study examines the potential cyclical nature of the elemental concentration record of silicon in sediment from Lake Baikal site BDP 96-2 in an attempt to clarify trends in periodicity observed for periods from 700 kyr BP to the present, 760 kyr BP to 1.43 Ma, and 1.47 to 2.4 Ma. The results indicate the presence of the 100 kyr cycle during the past 700 000 yr, as identified in other research. They also indicate the presence of a strong 74 kyr cycle from 760 kyr BP to 1.43 Md, with a weaker 44 kyr cycle. For the period from 1.47 to 2.4 Ma, we observed evidence of strong 465 and 155 kyr cycles, in addition to a moderate 93 kyr cycle and various lesser cycles. This new evidence represents one of the first findings of longer cycles, as predicted by classic insolation theory, and sheds some initial light on Imbrie's 400 kyr question. © Inter-Research 2004.
251.
Analysis of eleven years of deformation measured by GPS in the Corinth Rift Laboratory area; [Analyse de onze années de mesures de deformations collectées par GPS dans la zone du laboratoire du rift de corinthe]
Comptes Rendus - Geoscience,
336
(4-5)
301 – 311
2004
ISSN: 16310713Keywords:▾
crustal deformation; GPS; rift zone; seismotectonics
Abstract: ▾ GPS (Global Positioning System) data collected in the Corinth Rift during eleven campaigns between 1990 and 2001 provide velocities of 57 points with ∼1.5 mm yr-1 accuracy. Peloponnesos moves at 30 mm yr-1 to the N215° E with respect to a fixed Europe. Extension across the rift is accommodated in a narrow band offshore. Its rate increases from east to west and is 16 mm yr-1 near Aigion. Both sides of the rift behave as clockwise rotating blocks with rates of 7±0.5° M yr-1 and 2.8±0.8° M yr-1, respectively for the northern and southern blocks. After removing block rotations, the northern block shows a north-south extension rate of 120±50 nstrain yr-1, whereas the southern block indicates the internal deformation is still inside the error bar (<20 nstrain yr-1). The strain accumulation across the major faults located along the southern coast of the Corinth Gulf is less than 1 mm yr-1. This implies long recurrence periods for large earthquakes on these faults. © 2004 Académie des sciences. Published by Elsevier SAS. All rights reserved.
250.
A mechanical model of the San Andreas fault and SAFOD Pilot Hole stress measurements
Geophysical Research Letters,
31
(15)
L15S13 1-5
2004
ISSN: 00948276Keywords:▾
Friction; Geophysical prospecting; Mathematical models; Seismology; Shear stress; Stress analysis, Fault zones; Pilot holes; Stress measurements; Transpression, Boreholes, borehole geophysics; in situ stress; model; San Andreas Fault; stress measurement, California; North America; Parkfield; United States; Western Hemisphere; World
Abstract: ▾ Stress measurements made in the SAFOD pilot hole provide an opportunity to study the relation between crustal stress outside the fault zone and the stress state within it using an integrated mechanical model of a transform fault loaded in transpression. The results of this modeling indicate that only a fault model in which the effective friction is very low (<0.1) through the seismogenic thickness of the crust is capable of matching stress measurements made in both the far field and in the SAFOD pilot hole. The stress rotation measured with depth in the SAFOD pilot hole (∼28°) appears to be a typical feature of a weak fault embedded in a strong crust and a weak upper mantle with laterally variable heat flow, although our best model predicts less rotation (15°) than observed. Stress magnitudes predicted by our model within the fault zone indicate low shear stress on planes parallel to the fault but a very anomalous mean stress, approximately twice the lithostatic stress. Copyright 2004 by the American Geophysical Union.
249.
A magnetic mineral record of Late Quaternary tropical climate variability from Lake Bosumtwi, Ghana
Palaeogeography, Palaeoclimatology, Palaeoecology,
215
(1-2)
37 – 57
2004
ISSN: 00310182
Publisher: Elsevier B.V.
Keywords:▾
Africa; Ashanti; Eastern Hemisphere; Ghana; Lake Bosumtwi; Sub-Saharan Africa; West Africa; World; climate change; dust; greigite; magnetic mineral; monsoon; paleoclimate; paleolimnology; Quaternary; sedimentation; water budget
Abstract: ▾ We report magnetic hysteresis results from sediment cores obtained from Lake Bosumtwi, Ghana. As a hydrologically closed basin, the water budget of Lake Bosumtwi is extremely sensitive to changes in the precipitation/ evapotranspiration balance. Lake Bosumtwi lies in the path of the seasonal migration of the intertropical convergence zone (ITCZ); hence, the lake is ideally situated to study monsoon variability in West Africa. Five distinctive magnetic mineral zones (A-E) were identified in the 11-m-long sediment cores that span the last 26,000 calendar years. Prior to 12 calendar (cal) ka, low concentrations of multidomain, high-coercivity magnetic minerals are present. Three prominent shifts towards very high concentrations of high-coercivity iron sulfide (greigite) magnetic minerals are centered at 12,470, 17,290, and 22,600 calendar years during the last glacial period (magnetic zones D1-3). Between 12 and 3.2 cal ka, there is an abrupt shift to moderately high concentrations of mixed multidomain and single-domain, low-coercivity minerals and an organic-rich sapropel lithology. Since 3.2 cal ka, the magnetic mineral parameters reveal a shift to increased amounts of high-coercivity magnetic minerals. These magnetic mineral zones document tropical climate variability on a variety of temporal scales. Glacial age sediments have a high-coercivity magnetic mineralogy due to increased aeolian dust transport from the Sahel to Lake Bosumtwi as well as postdepositional reductive diagenesis. During the last glacial period, the increased strength of Harmattan and North African continental trade winds, the southward depression of the ITCZ, and weakened summer monsoon strength resulted in increased regional aridity and greater dust flux out of Sahel source regions. The greigite-bearing D magnetic zones correspond to brief lowstands in the level of Lake Bosumtwi and likely represent periods of intensified aridity in West Africa. The D magnetic zones closely resemble the timing and duration of Heinrich events and suggest a hemispheric-scale climatic coupling between the tropics and poles. The well-documented African humid period (AHP) is characterized by abrupt shifts in magnetic parameters between 12 and 3.2 cal ka. Dust flux to Lake Bosumtwi is inferred to be very low during this humid interval due to the strengthening of the summer monsoon. Since 3.2 cal ka, the magnetic mineral parameters suggest increased aridity as compared to the AHP. This work demonstrates that the magnetic properties of Lake Bosumtwi sediment are a sensitive recorder of abrupt climate change of global significance. © 2004 Elsevier B.V. All rights reserved.
248.
14C dating of soil samples from the Unzen volcano scientific drilling boreholes
Nuclear Instruments and Methods in Physics Research, Section B: Beam Interactions with Materials and Atoms,
223-224
(SPEC. ISS.)
560-567
2004
ISSN: 0168583XKeywords:▾
Boreholes; Carbon; Deposits; Geochronology; Leveling (machinery); Mass spectrometry; Project management; Radioactive elements; Stratigraphy; Volcanoes, Accelerator mass spectrometry (AMS); Pyroclastic eruption; Soil carbon fraction; Unzen volcano, Soils
Abstract: ▾ Soil samples were collected from the upper layers of two Unzen volcano Scientific Drilling Project boreholes and dated by radiocarbon using accelerator mass spectrometry (AMS). The humic soils underlying volcanic related lahar flow deposits at a depth of 56.42 m of the USDP-1 borehole gave a 14C age of 18.8 kaBP, suggesting that the accumulation of the lahar flow deposits was caused by the Kureishibaru pyroclastic eruption (19 kaBP) dated at the surface. The humic soils underlying a pyroclastic flow deposit at depths of 71.60-71.90 m of the USDP-2 borehole gave 14C ages of 27.5 and 29.3 kaBP at the top and bottom layers, respectively. This reveals that the initial activity of Fugendake volcano occurred around 27 kaBP and the collapse of Myokendake volcano around 29 kaBP. © 2004 Elsevier B.V. All rights reserved.
247.
Earthquake source parameters determined by the SAFOD Pilot Hole seismic array
Geophysical Research Letters,
31
(12)
L12S09 1-5
2004
ISSN: 00948276Keywords:▾
Frequencies; Function evaluation; Mathematical models; Mathematical operators; Parameter estimation; Seismology; Sensors; Tectonics; Frequency estimation; Geophysics; Seismology, Displacement amplitude spectra; Seismic arrays; Seisomograms, Earthquakes; Earthquakes, deep drilling; earthquake mechanism; San Andreas Fault; source parameters, Accurate estimation; Difference operators; Displacement amplitudes; Earthquake source parameters; Repeating earthquake; Smoothness constraints; Spectral parameters; Tectonic earthquakes
Abstract: ▾ We estimate the source parameters of #3 microearthquakes by jointly analyzing seismograms recorded by the 32-level, 3-component seismic array installed in the SAFOD Pilot Hole. We applied an inversion procedure to estimate spectral parameters for the omega-square model (spectral level and corner frequency) and Q to displacement amplitude spectra. Because we expect spectral parameters and Q to vary slowly with depth in the well, we impose a smoothness constraint on those parameters as a function of depth using a linear first-differenfee operator. This method correctly resolves corner frequency and Q, which leads to a more accurate estimation of source parameters than can be obtained from single sensors. The stress drop of one example of the SAFOD target repeating earthquake falls in the range of typical tectonic earthquakes. Copyright 2004 by the American Geophysical Union.
246.
Constraining the exhumation and burial history of the SAFOD pilot hole with apatite fission track and (U-Th)/He thermochronometry
Geophysical Research Letters,
31
(15)
L15S16 1-4
2004
ISSN: 00948276Keywords:▾
Cooling; Geophysical prospecting; Heating; Kinematics; Mathematical models; Sediments; Thermal effects; Thermoanalysis, Fission-track (FT); Pilot holes; San Andreas Fault (SAF); Thermochronometry, Boreholes, borehole geophysics; burial (geology); exhumation; fission track dating; San Andreas Fault; thermal evolution; thermochronology, California; North America; Parkfield; United States; Western Hemisphere; World
Abstract: ▾ The San Andreas Fault Observatory at Depth (SAFOD) pilot hole traverses the upper 2 km of a site 1.8 km west of the San Andreas fault (SAF) near Parkfield, California. In order to evaluate the burial and exhumation history of the site and its relationship to the kinematics and mechanics of the SAF, we use 15 apatite fission-track (FT) and 5 (U-Th)/He analyses from pilot hole samples to document their thermal history. Sample ages decrease with depth: FT and (U-Th)/He ages range from ∼60 and ∼31 Ma, respectively, in the upper 800 m of the hole to ∼3 and 1 Ma at the base of the hole (2.2 km depth, 93°C). Thermal modeling of the distribution of FT lengths indicates three events in the last 80 Ma: 1) cooling and exhumation of >60°C that culminated at ∼30 Ma; 2) reheating of ∼50°C from ∼30 to 8-4 Ma, probably as the result of basin subsidence and burial by 1-1.5 km of sediments; and 3) cooling of ∼30°C and estimated Coast Range exhumation of ∼1 km since 8-4 Ma. Copyright 2004 by the American Geophysical Union.
245.
Electrical resistivity structure at the SAFOD site from magnetotelluric exploration
Geophysical Research Letters,
31
(12)
L12S05 1-4
2004
ISSN: 00948276Keywords:▾
Algorithms; Data reduction; Geophysics; Magnetoelectric effects; Mathematical models; Seismology; Strike-slip faults, Datasets; Fault zone; Magnetotelluric exploration, Electric conductivity; Magnetotellurics, electrical resistivity; magnetotelluric method; San Andreas Fault, Constrained inversions; Electrical resistivity structures; Forward modeling; Inversion algorithm; Low resistivity; Magnetotelluric exploration; San Andreas fault; Seismic behavior
Abstract: ▾ The magnetotelluric dataset collected on the San Andreas Fault at Parkfield has been re-analyzed using superior inversion algorithms that have been developed in recent years. A combination of constrained inversion, forward modeling and synthetic inversion studies are used, and show that at the SAFOD site, the low resistivity fault zone extends to a depth of 2-3 km. An extended zone of low resistivity cast of the San Andreas Fault may be connected to the SAF at seismogenic depths. The connection increases along the SAF to the northwest and may be related to the transition from locked to creeping seismic behavior. Copyright 2004 by the American Geophysical Union.