All ICDP Publications with Abstracts
From parent-sysfolder "Publications" + 2 folder-levels deep
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.
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.