All ICDP Publications with Abstracts
From parent-sysfolder "Publications" + 2 folder-levels deep
294.
The effect of hydrate content on seismic attenuation: A case study for Mallik 2L-38 well data, Mackenzie delta, Canada
Geophysical Research Letters,
31
(14)
L14609 1-4
2004
ISSN: 00948276Keywords:▾
Attenuation; Gas hydrates; Mechanical permeability; Sediments, Seismic attenuation; Squirt flow mechanism, Seismic waves, borehole geophysics; gas hydrate; permeability; seismic attenuation; seismic wave, Canada; Mackenzie Delta; North America; Northwest Territories
Abstract: ▾ Observations of velocities in sediments containing gas hydrates show that the strength of sediments increases with hydrate saturation. Hence it is expected that the attenuation of these sediments will decrease with increasing hydrate saturation. However, sonic log measurements in the Mallik 2L-38 well and cross hole tomography measurements in the Mallik field have shown that attenuation increases with hydrate saturation. We studied a range of mechanisms by which increasing hydrate saturation could cause increased attenuation. We found that a difference in permeability between the host sediment and the newly formed hydrate can produce the observed effect. We modelled attenuation in terms of Biot and squirt flow mechanisms in composite media. We have used our model to predict observed attenuations in the Mallik 2L-38 well, Mackenzie Delta, Canada. Copyright 2004 by the American Geophysical Union.
293.
Rejuvenation of the lithosphere by the Hawaiian plume
Nature,
427
(6977)
827-829
2004
ISSN: 00280836Keywords:▾
Earth atmosphere; Imaging systems; Seismology; Surface topography, Lithosphere, Oceanography, crustal thickness; lithosphere; mantle plume; seismic tomography; volcanism, article; atmosphere; heating; imaging system; island (geological); lithosphere; mechanics; plume; priority journal; sea; signal detection; temperature measurement; thickness; topography; United States; volcano, Hawaiian Islands; Pacific islands; Pacific Ocean, Hawaiia
Abstract: ▾ The volcanism responsible for creating the chain of the Hawaiian islands and seamounts is believed to mark the passage of the oceanic lithosphere over a mantle plume. In this picture hot material rises from great depth within a fixed narrow conduit to the surface, penetrating the moving lithosphere. Although a number of models describe possible plume-lithosphere interactions, seismic imaging techniques have not had sufficient resolution to distinguish between them. Here we apply the S-wave 'receiver function' technique to data of three permanent seismic broadband stations on the Hawaiian islands, to map the thickness of the underlying lithosphere. We find that under Big Island the lithosphere is 100-110 km thick, as expected for an oceanic plate 90-100 million years old that is not modified by a plume. But the lithosphere thins gradually along the island chain to about 50-60 km below Kauai. The width of the thinning is about 300 km. In this zone, well within the larger-scale topographic swell, we infer that the rejuvenation model (where the plume thins the lithosphere) is operative; however, the larger-scale topographic swell is probably supported dynamically.
292.
The nature of the alteration clays and element mobilization in Chicxulub Yaxcopoil-1 drill hole
Meteoritics and Planetary Science,
39
(SUPPL.)
A76
2004
291.
Low-velocity damaged structure of the San Andreas Fault at Parkfield from fault zone trapped waves
Geophysical Research Letters,
31
(12)
L12S06 1-5
2004
ISSN: 00948276Keywords:▾
Computer simulation; Earthquakes; Finite difference method; Structural analysis; Velocity; Waveguides; Strike-slip faults; Velocity; Water waves, Fault zones; Seismic arrays; Trapped waves; Velocity reduction, Seismology; Shear flow, low velocity zone; San Andreas Fault; seismic velocity; trapped wave, California; North America; Parkfield; United States, Damaged structures; Fault-zone trapped waves; Finite difference simulations; Large earthquakes; Linear seismic array; Low velocity zones; Micro-earthquakes; San Andreas fault
Abstract: ▾ We used dense linear seismic arrays across and along the San Andreas Fault (SAF) at Parkfield, California to record fault zone trapped waves generated by explosions and microearthquakes in 2002. Prominent trapped waves appeared at stations close to the SAF main fault trace while some energy was trapped in the north strand at the array site. Observations and 3-D finite-difference simulations of trapped waves at 2-5 Hz show evidence of a damaged core zone on the main SAF. The zone from the surface to seismogenic depths is marked by a low-velocity waveguide ∼150 m wide, in which Q is 10-50 and shear velocities are reduced by 30-40% from wall-rock velocities, with the greatest velocity reduction at shallow depth. We interpret that this distinct low-velocity zone on the main SAF is a remanent of damage due to past large earthquakes on the principal fault plane at Parkfield. A less-developed low-velocity zone may be evident on the north strand that experienced minor breaks in the 1966 M6 event. Copyright 2004 by the American Geophysical Union.
290.
ICDP-USGS workshop on deep drilling in the central Crater of the Chesapeake Bay impact structure, Virginia, USA: Proceedings volume
Document Number2004-1016
2004
289.
[English]
Stress-induced seismic velocity anisotropy and physical properties in the SAFOD pilot hole in Parkfield, CA.
Publisher
American Rock Mechanics Association (ARMA)
2004
Department of Geophysics, Stanford University, California, United States288.
Yaxcopoil-1 and the Chicxulub impact
International Journal of Earth Sciences,
93
(6)
1042-1065
2004
Abstract: ▾ CSDP core Yaxcopoil-1 was drilled to a depth of 1,511 m within the Chicxulub crater. An organic-rich marly limestone near the base of the hole (1,495 to 1,452 m) was deposited in an open marine shelf environment during the latest Cenomanian (uppermost Rotalipora cushmani zone). The overlying sequence of limestones, dolomites and anhydrites (1,495 to 894 m) indicates deposition in various carbonate platform environments (e.g., sabkhas, lagoons). A 100-m-thick suevite breccia (894-794 m) identifies the Chicxulub impact event. Above the suevite breccia is a dolomitic limestone with planktic foraminiferal assemblages indicative of Plummerita hantkeninoides zone CF1, which spans the last 300 ky of the Maastrichtian. An erosional surface 50 cm above the breccia/dolomite contact marks the K/T boundary and a hiatus. Limestones above this contact contain the first Tertiary planktic foraminifera indicative of an upper P. eugubina zone P1a(2) age. Another hiatus 7 cm upsection separates zone P1a(2) and hemipelagic limestones of planktic foraminiferal Zone P1c. Planktic foraminiferal assemblages of Zone Plc to P3b age are present from a depth of 794.04 up to 775 m. The Cretaceous carbonate sequence appears to be autochthonous, with a stratigraphic sequence comparable to late Cretaceous sediments known from outside the Chicxulub crater in northern and southern Yucatan, including the late Cenomanian organic-rich marly limestone. There is no evidence that these sediments represent crater infill due to megablocks sliding into the crater, such as major disruption of sediments, chaotic changes in lithology, overturned or deep dipping megablocks, major mechanical fragmentation, shock or thermal alteration, or ductile deformation. Breccia units that are intercalated in the carbonate platform sequence are intraformational in origin (e.g., dissolution of evaporites) and dykes are rare. Major disturbances of strata by the impact therefore appear to have been confined to within less than 60 km from the proposed impact center. Yaxcopoil-1 may be located outside the collapsed transient crater cavity, either on the upper end of an elevated and tilted horst of the terrace zone, or even outside the annular crater cavity. The Chicxulub site thus records a large impact that predates the K/T boundary impact and mass extinction. © Springer-Verlag 2004.
287.
Volatiles in glasses from the HSDP2 drill core
Geochemistry, Geophysics, Geosystems,
5
(9)
2004
ISSN: 15252027Abstract: ▾ H2O, CO2, S, Cl, and F concentrations are reported for 556 glasses from the submarine section of the 1999 phase of HSDP drilling in Hilo, Hawaii, providing a high-resolution record of magmatic volatiles over ∼200 kyr of a Hawaiian volcano's lifetime. Glasses range from undegassed to having lost significant volatiles at near-atmospheric pressure. Nearly all hyaloclastite glasses are degassed, compatible with formation from subaerial lavas that fragmented on entering the ocean and were transported by gravity flows down the volcano flank. Most pillows are undegassed, indicating submarine eruption. The shallowest pillows and most massive lavas are degassed, suggesting formation by subaerial flows that penetrated the shoreline and flowed some distance under water. Some pillow rim glasses have H2O and S contents indicating degassing but elevated CO2 contents that correlate with depth in the core; these tend to be more fractionated and could have formed by mixing of degassed, fractionated magmas with undegassed magmas during magma chamber overturn or by resorption of rising CO2-rich bubbles by degassed magmas. Intrusive glasses are undegassed and have CO2 contents similar to adjacent pillows, indicating intrusion shallow in the volcanic edifice. Cl correlates weakly with H2O and S, suggesting loss during low-pressure degassing, although most samples appear contaminated by seawater-derived components. F behaves as an involatile incompatible element. Fractionation trends were modeled using MELTS. Degassed glasses require fractionation at PH2o ≈ 5-10 bars. Undegassed low-SiO2 glasses require fractionation at PH2O ≈ 50 bars. Undegassed and partially degassed high-SiO2 glasses can be modeled by coupled crystallization and degassing. Eruption depths of undegassed pillows can be calculated from their volatile contents assuming vapor saturation. The amount of subsidence can be determined from the difference between this depth and the sample's depth in the core. Assuming subsidence at 2.5 mm/y, the amount of subsidence suggests ages of ∼500 ka for samples from the lower 750 m of the core, consistent with radiometric ages. H2O contents of undegassed low-SiO2 HSDP2 glasses are systematically higher than those of high-SiO2 glasses, and their H2O/K2O and H 2O/Ce ratios are higher than typical tholeiitic pillow rim glasses from Hawaiian volcanoes. Copyright 2004 by the American Geophysical Union.
286.
Verification of magnetostratigraphic scales of miocene core section from lake Baikal
Geologiya i Geofizika,
45
(3)
408 – 412
2004
ISSN: 00167886Abstract: ▾ The dynamics of cosmogenic 10Be contents in the BDP-96-1 (100 to 200 m) and BDP-98-2 (200 to 600 m) cores was used to test published magnetostratigraphic scales for the Miocene section and to time the core base. Only one of three alternative scales for the BDP-98 core tested against the 10Be decay of T1/2 = 1.5 myr showed a perfect consistency with the decay law. The basal age of the 600 m BDP-98 section was estimted at 8.4 myr.
285.
Tracing the protolith, UHP metamorphism, and exhumation ages of orthogneiss from the SW Sulu terrane (eastern China): SHRIMP U–Pb dating of mineral inclusion-bearing zircons
Lithos,
78
(4)
411-429
2004
ISSN: 0024-4937Keywords:▾
Orthogneiss, Mineral inclusion in zircon, UHP metamorphism, SHRIMP U–Pb dating, Chinese Continental Scientific Drilling Project
Abstract: ▾ Orthogneisses are the major country rocks hosting eclogites in the Sulu UHP terrane, eastern China. All of the analyzed orthogneiss cores from the main drilling hole of the Chinese Continental Scientific Drilling Project (CCSD-MH) have similar major and trace element compositions and a granite protolith. These rocks have relatively high LREE/HREE ratios, strong negative Eu anomalies (Eu/Eu*=0.20–0.39), and negative Ba anomalies (Ba/Ba*=0.25–0.64). Coesite and coesite-bearing UHP mineral assemblages are common inclusions in zircons separated from orthogneiss, paragneiss, amphibolite, and (retrograded) eclogite of the CCSD-MH. This suggests that the eclogite, together with its country rocks, experienced in situ ultrahigh-pressure (UHP) metamorphism. Laser Raman spectroscopy and cathodoluminescence (CL) images show that zircons from the orthogneisses are zoned and that they have distinct mineral inclusions in the different zones. Most zircons retain early magmatic cores with abundant low-pressure mineral inclusions, which are mantled with metamorphic zircon-containing inclusions of coesite and other UHP minerals. The outermost rims on these grains contain low-pressure mineral inclusions, such as quartz and albite. SHRIMP U–Pb dating of the zoned zircons gives three discrete and meaningful groups of ages: Proterozoic ages for the protolith, 227±2 Ma for the coesite-bearing mantles, and 209±3 Ma for the amphibolite facies retrograde rims. The widespread occurrence of UHP mineral inclusions in zircons from the Sulu metamorphic belt dated at about 227 Ma suggests that voluminous continental crust experienced late Triassic subduction to depths of at least 120 km and perhaps more than 200 km. Eighteen million years later, the terrane was rapidly exhumed to midcrustal levels, and the UHP rocks were overprinted by amphibolite facies metamorphism. The exhumation rate deduced from the zircon age data and previously obtained metamorphic P–T data is estimated to be 5.6–11.0 km/Ma. Such rapid exhumation of the Sulu UHP terrane may be due to the buoyancy forces produced by subduction of low-density continental material into the deep mantle.
284.
Through thick and thin
Nature,
427
(6977)
793-795
2004
ISSN: 00280836DOI:10.1038/427793a
Keywords:▾
Earth atmosphere; Sea level; Seismology, Lithosphere, Oceanography, crustal thickness; seafloor, asthenospheric upwelling; earthquake; geology; gravity; heat transfer; priority journal; short survey; United States, Hawaiian Islands; Pacific islands; Pacific Ocean, Hawaiia
Abstract: ▾ The sea floor around the Hawaiian island chain is unusually shallow. New seismic evidence suggests that this up-raised 'swell' is partly due to heating and thinning of the lithosphere beneath.
283.
The SAFOD Pilot Hole seismic array: Wave propagation effects as a function of sensor depth and source location
Geophysical Research Letters,
31
(12)
L12S07 1-5
2004
ISSN: 00948276Keywords:▾
Correlation methods; Creep; Earthquakes; Geophysics; Observatories; Site selection; Wave propagation; Creep; Shear waves; Strike-slip faults; Wave propagation, Fluid saturation; Pilot hole (PH); Seismic arrays; Seismograms, Seismology; Seismology, earthquake; San Andreas Fault; seismic wave; seismometry; wave propagation, Creep distribution; Fluid saturations; P- and S-wave velocities; Propagation effect; San Andreas fault; Source location; Surface explosions; Vertical arrays
Abstract: ▾ In July 2002 we installed a vertical array of seismometers in the San Andreas Fault Observatory at Depth (SAFOD) Pilot Hole (PH). The bottom of this 32 level, 1240 m long array of 3- components is located at a depth of ∼2100 m below ground. Surface-explosion and microearthquake seismograms recorded by the array give valuable insights into the structure of the SAFOD site. The ratios of P- and S-wave velocities.(Vp/Vs) along the array suggest the presence of two faults intersecting the PH. The Vp/Vs ratios also depend on source-location, with high values to the NW, and lower ones to the SE, correlating with high and low creep rates along the SAF, respectively. Since higher ratios can be produced by increasing fluid saturation, we suggest that this effect might account for both our observations and their correlation with the creep distribution. Copyright 2004 by the American Geophysical Union.
282.
The hazard of near-Earth asteroid impacts on earth
Earth and Planetary Science Letters,
222
(1)
1-15
2004
Abstract: ▾ Near-Earth asteroids (NEAs) have struck the Earth throughout its existence. During epochs when life was gaining a foothold ∼ 4 Ga, the impact rate was thousands of times what it is today. Even during the Phanerozoic, the numbers of NEAs guarantee that there were other impacts, possibly larger than the Chicxulub event, which was responsible for the Cretaceous-Tertiary extinctions. Astronomers have found over 2500 NEAs of all sizes, including well over half of the estimated 1100 NEAs >1 km diameter. NEAs are mostly collisional fragments from the inner half of the asteroid belt and range in composition from porous, carbonaceous- chondrite-like to metallic. Nearly one-fifth of them have satellites or are double bodies. When the international telescopic Spaceguard Survey, which has a goal of discovering 90% of NEAs >1 km diameter, is completed, perhaps as early as 2008, nearly half of the remaining impact hazard will be from land or ocean impacts by bodies 70-600 m diameter. (Comets are expected to contribute only about 1% of the total risk.) The consequences of impacts for civilization are potentially enormous, but impacts are so rare that worldwide mortality from impacts will have dropped to only about 150 per year (averaged over very long durations) after the Spaceguard goal has, presumably, ruled out near- term impacts by 90% of the most dangerous ones; that is, in the mid- range between very serious causes of death (disease, auto accidents) and minor but frightening ones (like shark attacks). Differences in perception concerning this rather newly recognized hazard dominate evaluation of its significance. The most likely type of impact events we face are hyped or misinterpreted predicted impacts or near-misses involving small NEAs. © 2004 Elsevier B.V. All rights reserved.
281.
Scientific drilling reveals geochemical heterogeneity within the Ko'olau shield, Hawai'i
Contributions to Mineralogy and Petrology,
147
(2)
162-188
2004
ISSN: 00107999Keywords:▾
geochemistry; lava; petrography; volcanic rock, Hawaii [United States]; North America; United States
Abstract: ▾ The Ko'olau Scientific Drilling Project (KSDP) was initiated to determine if the distinctive geochemistry of Ko'olau lavas is a near-surface feature. This project successfully deepened a recent, ∼351 m deep, tri-cone rotary-drilled water well by coring another ∼328 m. Three Ar-Ar plateau ages of 2.8 to 2.9 Ma from the drill core section of 103 flows confirm stratigraphic interpretations that core drilling recovered the deepest and oldest subaerially erupted lavas yet sampled from this volcano. The petrography and geochemistry of the core, and cuttings from this and another new Ko'olau water well (∼433 m deep) were determined. These analyses revealed that the geochemically distinct lavas of Ko'olau form a veneer only 175-250 in thick at the drill sites, covering flows with more typical Hawaiian tholeiite compositions. The compositional change occurred near the end of shield volcanism and is not abrupt. Thus, it is probably not related to a catastrophic event such as the collapse of the northeast flank of this volcano. The distinct geochemistry of surface Ko'olau lavas cannot be explained by melting pyroxenitic or combined pyroxenitic and peridotitic sources. Additional recycled oceanic crustal components, such as plagioclase-rich cumulates and sediment, were probably involved. As the Ko'olau volcano drifted off the Hawaiian hotspot and the overall degree of melting decreased, the proportion of melts from recycled oceanic crustal material increased relative to those from mantle peridotite. © Springer-Verlag 2004.
280.
The Corinth Rift Laboratory
Comptes Rendus - Geoscience,
336
(4-5)
235 – 241
2004
ISSN: 16310713
Publisher: Elsevier Masson SAS
Keywords:▾
Gulf of Corinth; Ionian Sea; Mediterranean Sea; rift zone; seismotectonics
279.
The Chicxulub Scientific Drilling Project (CSDP)
Meteoritics and Planetary Science,
39
(6)
787-790
2004
278.
The AIG10 drilling project (Aigion, Greece): Interpretation of the litho-log in the context of regional geology and tectonics; [Corrélation des analyses lithologiques et structurales du forage AIG10 avec l̀environnement régional tectono-géologique]
Comptes Rendus - Geoscience,
336
(4-5)
415 – 423
2004
ISSN: 16310713Keywords:▾
Gulf of Corinth; Ionian Sea; Mediterranean Sea; borehole geophysics; drilling; normal fault; rift zone; seismotectonics; stratigraphic correlation
Abstract: ▾ In the frame of the EU Project Corinth Rift Laboratory (CRL), the AIG10 borehole was successfully drilled from July until September 2002 through the Aigion normal fault in the harbour of Aigion, northern Peloponnesus, Greece. The scientific objective focuses on the investigation of fault mechanics and the relationship with fluid flow and geochemistry, fluid pressure, stress- and strain fields and earthquakes. Recognition of stratification encountered in the AIG10 borehole is based on an online analysis of well cuttings (0-708.8 m and 787.4-1001 m), core descriptions (708.8-787.4 m), monitoring of drilling parameters, as well as a preliminary geophysical well-log interpretation (0-1001 m). Geologically, the area is part of the Olonos-Pindos tectonic nappe, which is overthrusted on the Tripolitza unit during the Alpine orogeny. The litho-log of the AIG10 borehole comprises at first syn-rift deposits (graben fill). At 496 m, the Olonos-Pindos tectonic unit was encountered, however, not as expected in the platy limestones, but in the Olonos-Pindos radiolarite. The borehole has crossed at least one thrust-fault zone and a major normal fault zone at 760 m. This normal fault zone separates well-fractured platy, micritic limestone in the hangingwall from highly fractured radiolarite in the footwall, both of the Olonos-Pindos tectonic unit. The observed succession of multiple imbrication is an indicator of Alpine tectonic activity, whereas normal faulting is of the Miocene-Quaternary extension of the Gulf of Corinth, confirming our expectations gained from geologic-tectonic fieldwork. © 2004 Académie des sciences. Published by Elsevier SAS. All rights reserved.
277.
TEM study of meteorite impact glass at New Zealand Cretaceous-Tertiary sites: Evidence for multiple impacts or differentiation during global circulation?
Earth and Planetary Science Letters,
219
(3-4)
209-219
2004
Abstract: ▾ Study by transmission electron microscopy of samples from the Cretaceous-Tertiary (K-T) boundary clay at Flaxbourne River and Woodside Creek, New Zealand, has revealed the occurrence of nanometer-sized meteorite impact-derived glass. The average glass composition is exceptionally Ca-rich and is distinct from other glass found on Earth, apart from glass inferred to be of impact origin at Mexican and Haitian K-T sites. The glass shards are partially altered to montmorillonite-like smectite, with the dominant interlayer cation, Ca, reflecting the composition of the parent glass. The data imply a heterogeneous global distribution in composition of K-T boundary impact glass: Si-rich and Ca-rich in Mexico and Haiti, Si-rich in Denmark, and Ca-rich in New Zealand. This heterogeneous distribution may relate to dispersal processes similar to those used to account for the asymmetric distribution of clastic debris from the Chicxulub impact site. However, recent discovery of an impact crater of K-T boundary age in Ukraine raises the possibility of impact clusters which produce material of heterogeneous composition. © 2004 Elsevier B.V. All rights reserved.
276.
Structure and impact indicators of the Cretaceous sequence of the ICDP drill core Yaxcopoil-1, Chicxulub impact crater, Mexico
Meteoritics and Planetary Science,
39
(7)
1069-1088
2004
Abstract: ▾ As part of the ICDP Chicxulub Scientific Drilling Project, the Yaxcopoil-1 (Yax-1) bore hole was drilled 60 km south-southwest of the center of the 180 km-diameter Chicxulub impact structure down to depth of 1511 m. A sequence of 615 m of deformed Cretaceous carbonates and sulfates was recovered below a 100 m-thick unit of suevitic breccias and 795 m of post-impact Tertiary rocks. The Cretaceous rocks are investigated with respect to deformation features and shock metamorphism to better constrain the deformational overprint and the kinematics of the cratering process. The sequence displays variable degrees of impact-induced brittle damage and post-impact brittle deformation. The degree of tilting and faulting of the Cretaceous sequence was analyzed using 360°-core scans and dip-meter log data. In accordance with lithological information, these data suggest that the sedimentary sequence represents a number of structural units that are tilted and moved with respect to each other. Three main units and nine sub-units were discriminated. Brittle deformation is most intense at the top of the sequence and at 1300-1400 m. Within these zones, suevitic dikes, polymict clastic dikes, and impact melt rock dikes occur and may locally act as decoupling horizons. The degree of brittle deformation depends on lithology; massive dolomites are affected by penetrative faulting, while stratified calcarenites and bituminous limestones display localized faulting. The deformation pattern is consistent with a collapse scenario of the Chicxulub transient crater cavity. It is believed that the Cretaceous sequence was originally located outside the transient crater cavity and eventually moved downward and toward the center to its present position between the peak ring and the crater rim, thereby separating into blocks. Whether or not the stack of deformed Cretaceous blocks was already displaced during the excavation process remains an open question. The analysis of the deformation microstructure indicates that a shock metamorphic overprint is restricted to dike injections with an exception of the so called "paraconglomerate." Abundant organic matter in the Yax-1 core was present before the impact and was mobilized by impact-induced heating and suggests that >12 km3 of organic material was excavated during the cratering process. Meteoritical Society, 2004.
275.
Stress-induced seismic velocity anisotropy and physical properties in the SAFOD Pilot Hole in Parkfield, CA
Geophysical Research Letters,
31
(15)
L15S17 1-4
2004
ISSN: 00948276Keywords:▾
Drilling; Electromagnetic wave polarization; Fracture; Granite; Magnetic anisotropy; Seismic prospecting, Pilot holes; Shear velocity; Wellbores, Boreholes, borehole geophysics; in situ stress; physical property; San Andreas Fault; seismic anisotropy; seismic velocity, California; North America; Parkfield; United States; Western Hemisphere; World
Abstract: ▾ A comprehensive suite of geophysical logs was collected in the SAFOD Pilot Hole from a depth of 775 m to 2150 m in highly fractured Salinian granite. The Pilot Hole intersected numerous macroscopic fractures and faults with extremely varied orientations. Despite the highly variable orientation of the fractures and faults, the fast polarization direction of the shear waves is very consistent with the direction of maximum horizontal compression determined from wellbore breakouts and drilling induced tensile fractures. At least three major shear zones were intersected by the borehole that are characterized by anomalously low velocity and resistivity, anomalously high shear velocity anisotropy and an absence of stress-induced wellbore breakouts (which suggests anomalously low differential stress). We argue that the physical mechanism responsible for the seismic velocity anisotropy observed in the Pilot Hole is the preferential closure of fractures in response to an anisotropic stress state. Copyright 2004 by the American Geophysical Union.
274.
Stress orientations and magnitudes in the SAFOD pilot hole
Geophysical Research Letters,
31
(15)
L15S12 1-4
2004
ISSN: 00948276Keywords:▾
Compressive stress; Drilling; Fracture; Geophysical prospecting; Stress analysis, Borehole breakouts; Pilot holes; Stress fields; Stress orientations, Boreholes, borehole geophysics; in situ stress; orientation; San Andreas Fault; stress measurement, California; North America; Parkfield; United States; Western Hemisphere; World
Abstract: ▾ Borehole breakouts and drilling-induced tensile fractures in the 2.2-km-deep SAFOD pilot hole at Parkfield, CA, indicate significant local variations in the direction of the maximum horizontal compressive stress, SHmax, but show a generalized increase in the angle between SHmax and the San Andreas Fault with depth. This angle ranges from a minimum of 25 ± 10° at 1000-1150 m to a maximum of 69 ± 14° at 2050-2200 m. The simultaneous occurrence of tensile fractures and borehole breakouts indicates a transitional strike-slip to reverse faulting stress regime with high horizontal differential stress, although there is considerable uncertainty in our estimates of horizontal stress magnitudes. If stress observations near the bottom of the pilot hole are representative of stresses acting at greater depth, then they are consistent with regional stress field indicators and an anomalously weak San Andreas Fault in an otherwise strong crust. Copyright 2004 by the American Geophysical Union.
273.
Stratigraphic and sedimentological observations from seismic data across the Chicxulub impact basin
Meteoritics and Planetary Science,
39
(7)
1089-1098
2004
Abstract: ▾ Seismic data across the offshore half of the Chicxulub impact crater reveal a 145 km-diameter post-impact basin to be a thickening of Tertiary sediment, which thickens by ∼0.7 sec from the basin margin to the basin center. The basin existed long after the impact and was gradually infilled to its current flat surface. A suite of seismic horizons within the impact basin have been picked on four reflection lines across the crater. They reveal that the western and northwestern parts of the impact basin were filled first. Subsequently, there was a dramatic change in the depositional environment, indicated by an unconformable surface that can be mapped across the entire basin..A prograding shelf sequence downlaps onto this unconformity in the eastern basin. The seismic stratigraphic relationships suggest a marine regression, with sedimentation becoming gradually more passive as sediments fill the eastern part of the impact basin. The central and northeastern parts of the basin are filled last. The onshore hole Yaxcopoil-1 (Yax-1), which was drilled on the flanks of the southern basin, has been projected onto the offshore seismic data to the west of the crater center. Using dates obtained from this onshore well and regional data, approximate ages have been placed on the most significant horizons in the offshore seismic data. Our preliminary interpretation is that the western and northwestern basins were almost entirely filled by 40 Ma and that the marine regression observed in the eastern basin is early Miocene in age. Offshore seismic stratigraphic analyses and onshore data within Yax-1 suggest that the early Paleocene is highly attenuated across the impact basin. The Mesozoic section appears to be €1 km thicker offshore than onshore. We calculate that, given this offshore thickening, the volume of Mesozoic rocks that have been excavated, melted, or vaporized during impact is around 15% larger than expected from calculations that assume the offshore thickness is equal to that onshore. This has significant consequences for any environmental calculations. The current offset between the K-T boundary outside and inside the crater is ∼700 m. However, infilling of basins with sediments is usually accompanied by subsidence, and immediately following the impact, the difference would have been smaller. We calculate the original topographic offset on the K-T boundary to have been between 450 and 700 m, which is in agreement with depth-diameter scaling laws for a mixed target. © Meteoritical Society, 2004.
272.
Secondary alteration of the impactite and mineralization in the basal Tertiary sequence, Yaxcopoil-1, Chicxulub impact crater, Mexico
Meteoritics and Planetary Science,
39
(7)
1145-1167
2004
Abstract: ▾ The 65 Ma Chicxulub impact crater formed in the shallow coastal marine shelf of the Yucatán Platform in Mexico. Impacts into water-rich environments provide heat and geological structures that generate and focus sub-seafloor convective hydrothermal systems. Core from the Yaxcopoil-1 (Yax-1) hole, drilled by the Chicxulub Scientific Drilling Project (CSDP), allowed testing for the presence of an impact-induced hydrothermal system by: a) characterizing the secondary alteration of the 100 m-thick impactite sequence; and b) testing for a chemical input into the lower Tertiary sediments that would reflect aquagene hydrothermal plume deposition. Interaction of the Yax-1 impactites with seawater is evident through redeposition of the suevites (unit 1), secondary alteration mineral assemblages, and the subaqueous depositional environment for the lower Tertiary carbonates immediately overlying the impactites. The least-altered silicate melt composition intersected in Yax-1 is that of a calc-alkaline basaltic andesite with 53.4-56 wt% SiO2 (volatile-free). The primary mineralogy consists of fine microlites of diopside, plagioclase (mainly Ab 47), ternary feldspar (Ab 37 to 77), and trace apatite, titanite, and zircon. The overprinting alteration mineral assemblage is characterized by Mg-saponite, K-montmorillonite, celadonite, K-feldspar, albite, Fe-oxides, and late Ca and Mg carbonates. Mg and K metasomatism resulted from seawater interaction with the suevitic rocks producing smectite-K-feldspar assemblages in the absence of any mixed layer clay minerals, illite, or chlorite. Rare pyrite, sphalerite, galena, and chalcopyrite occur near t he base of the impactites. These secondary alteration minerals formed by low temperature (0-150 °C) oxidation and fixation of alkalis due to the interaction of glass-rich suevite with down-welling seawater in the outer annular trough intersected at Yax-1. The alteration represents a cold, Mg-K-rich seawater recharge zone, possibly recharging higher temperature hydrothermal activity proposed in the central impact basin. Hydrothermal metal input into the Tertiary ocean is shown by elevated Ni, Ag, Au, Bi, and Te concentrations in marcasite and Cd and Ga in sphalerite in the basal 25 m of the Tertiary carbonates in Yax-1. The lower Tertiary trace element signature reflects hydrothermal metal remobilization from a mafic source rock and is indicative of hydrothermal venting of evolved seawater into the Tertiary ocean from an impact-generated hydrothermal convective system. © Meteoritical Society, 2004. Printed in USA.
271.
Magnetic mineralogy of the Yaxcopoil-1 core, Chicxulub
Meteoritics and Planetary Science,
39
(6)
831-841
2004
Abstract: ▾ Core from the Yaxcopoil-1 (Yax-1) hole, drilled as a result of the Chicxulub Scientific Drilling Project (CSDP), has been analyzed to investigate the relationship between opaque mineralogy and rock magnetic properties. Twenty one samples of suevite recovered from the depth range 818-894 m are generally paramagnetic, with an average susceptibility of 2000 × 10-6 SI and have weak remanent magnetization intensities (average 0.1 A/m). The predominant magnetic phase is secondary magnetite formed as a result of low temperature (<15 °C alteration. It occurs in a variety of forms, including vesicle infillings associated with quartz and clay minerals and fine aggregates between plagioclase/diopside laths in the melt. Exceptional magnetic properties are found in a basement clast (metamorphosed quartz gabbro), which has a susceptibility of >45000 × 10-6 SI and a remanent magnetization of 77.5 A/m. Magnetic mafic basement clasts are a common component in the Yax-1 impactite sequence. The high susceptibility and remanence in the mafic basement clasts are caused by the replacement of amphiboles and pyroxenes by an assemblage with fine <1 μm magnetite, ilmenite, K-feldspar, and stilpnomelane. Replacement of the mafic minerals by the magnetic alteration assemblage occurred before impact. Similar alteration mechanisms, if operative within the melt sheet, could explain the presence of the high amplitude magnetic anomalies observed at Chicxulub. © Meteoritical Society, 2004.
270.
Heat flow in the SAFOD pilot hole and implications for the strength of the San Andreas Fault
Geophysical Research Letters,
31
(15)
L15S14 1-4
2004
ISSN: 00948276Keywords:▾
Earthquakes; Flow of fluids; Friction; Geophysical prospecting; Heating; Seismology; Thermal effects; Thermoanalysis, Heat flow; San Andreas Fault; Seismicity, Boreholes, borehole geophysics; heat flow; San Andreas Fault; strength, California; North America; Parkfield; United States; Western Hemisphere; World
Abstract: ▾ Detailed thermal measurements have been acquired in the 2.2-km-deep SAFOD pilot hole, located 1.8 km west of the SAF near Parkfield, California. Heat flow from the basement section of the borehole (770 to 2160 m) is 91 mW m-2, higher than the published 74 mW m -2 average for the Parkfield area. Within the resolution of the measurements, heat flow is constant across faults that intersect the borehole, suggesting that fluid flow does not alter the conductive thermal regime. Reanalysis of regional heat flow reveals an increase in heat flow along the SAF northwest of Parkfield. This transition corresponds to a shallowing base of seismicity and a change in fault behavior near the northern terminus of the M6 1966 Parkfield earthquake rupture. The persistence of elevated heat flow in the Coast Ranges to the west appears to rule out frictional heating on the SAF as the source of the SAFOD value.