All ICDP Publications with Abstracts
From parent-sysfolder "Publications" + 2 folder-levels deep
244.
High resolution temperature measurements in the borehole Yaxcopoil-1, Mexico
Meteoritics and Planetary Science,
39
(6)
813-819
2004
Abstract: ▾ Within the frame of the International Continental Deep Drilling Program (ICDP) and as a part of the Chicxulub Scientific Drilling Project (CSDP), high resolution temperature measurements were performed in the borehole Yaxcopoil-1 (Yax-1). The temperature was logged to the depth of 858 m seven times between March 6-19, 2002, starting 10 days after the hole was shut in and mud circulation ceased. Successive logs revealed only small temperature variations in time and space, indicating a fast temperature recovery to almost undisturbed conditions prior to the first log. From these logs, a mean temperature gradient of ∼37 mK/m was determined below the uppermost 250 m. Another temperature log was recorded on May 24, 2003 (15 months after the shut in) to a depth of 895 m. The obtained temperature profile is very similar to the 2002 profile, with an insignificantly higher mean gradient below 250 m that may indicate a long-term return to the pre-drilling temperature. The temperature in the uppermost part of the hole bears signs of considerable influence of a convective contribution to the vertical thermal heat transfer. The depth extent of the convection seems to have deepened from 150 m in March 2002 to 230 m in May 2003. Based on the observed temperature gradient and the rock types encountered in the borehole above 670 m, the conducted heat flow is expected to be in the range 65-80 mW/m2. © Meteoritical Society, 2004.
243.
Is the transition impact to post-impact rock complete? Some remarks based on XRF scanning, electron microprobe, and thin section analyses of the Yaxcopoil-1 core in the Chicxulub crater
Meteoritics and Planetary Science,
39
(7)
1113-1126
2004
Abstract: ▾ The transition from impact to post-impact rocks in the Yaxcopoil-1 (Yax-1) core is marked by a 2 cm-thick clay layer characterized by dissolution features. The clay overlies a 9 cm-thick hardground, overlying a 66 cm-thick crossbedded unit, consisting of dolomite sandstone alternating with thin micro-conglomerates layers with litho- and bioclasts and the altered remains of impact glass, now smectite. The micro-conglomerates mark erosion surfaces. Microprobe and backscatter SEM analysis of the dolomite rhombs show an early diagenetic, complex-zoned, idiomorphic overgrowth, with Mn-rich zones, possibly formed by hot fluids related to cooling melt sheet in the crater. The pore spaces are filled with several generations of coelestite, barite, K-feldpar, and sparry calcite. XRF core scanning analysis detected high Mn values in the crossbedded sediments but no anomalous enrichment of the siderophile elements Cr, Co, Fe, and Ni in the clay layer. Shocked quartz occurs in the crossbedded unit but is absent in the clay layer. The basal Paleocene marls are strongly dissolved and do not contain a basal Paleocene fauna. The presence of a hardground, the lack of siderophile elements, shocked quartz, or Ni-rich spinels in the clay layer, and the absence of basal Paleocene biozones PO and Pa all suggest that the top of the ejecta sequence and a significant part of the lower Paleocene is missing. Due to the high energy sedimentation infill, a hiatus at the top of the impactite is not unexpected, but there is nothing in the biostratigraphy, geochemistry, and petrology of the Yax-1 core that can be used to argue against the synchroneity of the end-Cretaceous mass-extinctions and the Chicxulub crater. © Meteoritical Society, 2004.
242.
Evidence for ocean water invasion into the Chicxulub crater at the Cretaceous/Tertiary boundary
Meteoritics and Planetary Science,
39
(7)
1233-1247
2004
Abstract: ▾ The possibility of ocean water invasion into the Chicxulub crater following the impact at the Cretaceous/Tertiary boundary was investigated based on examination of an impactite between approximately 794.63 and 894.94 m in the Yaxcopoil-1 (Yax-1) core. The presence of cross lamination in the uppermost part of the impactite suggests the influence òf an oçean current at least during the sedimentation of this interval. Abundant occurrence of nannofossils of late Campanian to early Maastrichtian age in the matrices of samples from the upper part of the impactite suggests that the carbonate sediments deposited on the inner rim margin and outside the crater were eroded and transported into the crater most likely by ocean water that invaded the crater after its formation. The maximum grain size of limestone lithics and vesicular melt fragments, and grain and bulk chemical compositions show a cyclic variation in the upper part of the impactite. The upward fining grain size and the absence of erosional contact at the base of each cycle suggest that the sediments were derived from resuspension of units elsewhere in the crater, most likely by high energy currents association with ocean water invasion. © Meteoritical Society, 2004. Printed in USA.
241.
Introduction to special section: Preparing for the San Andreas Fault Observatory at Depth
Geophysical Research Letters,
31
(12)
L12S01 1-4
2004
ISSN: 00948276Keywords:▾
earthquake; San Andreas Fault; seismic hazard, California; North America; United States
240.
Integrated deep drilling, coring, downhole logging, and data management in the Chicxulub Scientific Drilling Project (CSDP), Mexico
Meteoritics and Planetary Science,
39
(6)
791-797
2004
Abstract: ▾ Impact structures in the solar system are mainly recognized and explored through remote sensing and, on Earth, through geophysical deep sounding. To date, a continuous scientific sampling of large impact craters from cover rocks to target material has only seldom been performed. The first project to deep-drill and core into one of the largest and well-preserved terrestrial impact structures was executed in the winter of 2001/2002 in the 65 Myr-old Chicxulub crater in Mexico using integrated coring sampling and in situ measurements. The combined use of different techniques allows a three-dimensional insight and a better understanding of impact processes. Here, we report the integration of conventional rotary drilling techniques with wireline mining coring technology that was applied to drill the 1510 m-deep Yaxcopoil-1 (Yax-1) well about 40 km southwest of Mérida, Yucatán, Mexico. During the course of the project, we recovered approximately 900 m of intact core samples including the transitions of reworked ejecta to post-impact sediments, and that one from large blocks of tilted target material to impact-generated rocks, i.e., impact melt breccias and suevites. Coring was complemented by wireline geophysical measurements to obtain a continuous set of in situ petrophysical data of the borehole walls. The data acquired is comprised of contents of a natural radioactive element, velocities of compressional sonic waves, and electrical resistivity values. All the digital data sets, including technical drilling parameters, initial scientific sample descriptions, and 360° core pictures, were distributed during the course of the operations via Internet and were stored in the ICDP Drilling Information System (http://www.icdp-online.org), serving the global community of cooperating scientists as a basic information service. © Meteoritical Society, 2004.
239.
Impactites of the Yaxcopoil-1 drilling site, Chicxulub impact structure: Petrography, geochemistry, and depositional environment
Meteoritics and Planetary Science,
39
(6)
857-878
2004
Abstract: ▾ The impact breccias encountered in drill hole Yaxcopoil-1 (Yax-1) in the Chicxulub impact structure have been subdivided into six units. The two uppermost units are redeposited suevite and suevite, and together are only 28 m thick. The two units below are interpreted as a ground surge deposit similar to a pyroclastic flow in a volcanic regime with a fine-grained top (unit 3; 23 m thick; nuée ardente) and a coarse breccia (unit 4;∼15 m thick) below. As such, they consist of a mélange of clastic matrix breccia and melt breccia. The pyroclastic ground surge deposit and the two units 5 and 6 below are related to the ejecta curtain. Unit 5 (∼24 m thick) is a silicate impact melt breccia, whereas unit 6 (10 m thick) is largely a carbonate melt breccia with some clastic-matrix components. Unit 5 and 6 reflect an overturning of the target stratigraphy. The suevites of units 1 and 2 were deposited after emplacement of the ejecta curtain debris. Reaction of the super-heated breccias with seawater led to explosive activity similar to phreomagmatic steam explosion in volcanic regimes. This activity caused further brecciation of melt and melt fragments. The fallback suevite deposit of units 1 and 2 is much thinner than suevite deposits at larger distances from the center of the impact structure than the 60 km of the Yax-1 drill site. This is evidence that the fallback suevite deposit (units 1 and 2) originally was much thicker. Unit 1 exhibits sedimentological features suggestive of suevite redeposition. Erosion possibly has occurred right after the K/T impact due to seawater backsurge, but erosion processes spanning thousands of years may also have been active. Therefore, the top of the 100 m thick impactite sequence at Yaxcopoil, in our opinion, is not the K/T boundary. © Meteoritical Society, 2004.
238.
Impact-related dike breccia lithologies in the ICDP drill core Yaxcopoil-1, Chicxulub impact structure, Mexico
Meteoritics and Planetary Science,
39
(6)
931-954
2004
Abstract: ▾ Petrographic descriptions of three dike breccia lithologies from drill core Yaxcopoil-1 (Yax-1) are presented. They occur within allochthonous units of displaced sedimentary megablocks of the Chicxulub impact structure. The suevitic dike breccias are the uppermost dike lithology. They contain melt rock particles and melt injections into the dike groundmass. Shock features occur ubiquitously and indicate a strong thermal annealing. Flow textures suggest a highly energetic emplacement process, possibly during the excavation stage as a ground-surge related deposit. The impact melt rock dikes are present in a strongly brecciated megablock interval as flow textured, anastomozing veinlets of impact melt rock that were altered to clay minerals. The melt impregnated a dolomitic host rock, indicating a low viscosity and, thus, high initial temperatures. Brecciation of the impact melt rock dikes occurred while they were still below the glass transition temperature, suggesting that dynamic conditions prevailed shortly after the emplacement process. Major element data indicates that the impact melt rock dikes differ in composition from the homogenized impact melt rock of Chicxulub. This could point to an emplacement during the late compression or early excavation stages of cratering. The clastic polymict dike breccias are coeval with pervasive brittle fracturing of the host rocks. They bear clasts including some crystalline basement and possible melt rock particles in a fine-grained dolomite matrix with turbulent flow textures. Fabric and texture indicate a granular flow at ambient pressures. Such conditions could be envisaged for the excavation phase while the transient cavity grew and fractures opened. © Meteoritical Society, 2004.
237.
Impact lithologies and their emplacement in the Chicxulub impact crater: Initial results from the Chicxulub Scientific Drilling Project, Yaxcopoil, Mexico
Meteoritics and Planetary Science,
39
(6)
879-897
2004
Abstract: ▾ The Chicxulub Scientific Drilling Project (CSDP), Mexico, produced a continuous core of material from depths of 404 to 1511 m in the Yaxcopoil-1 (Yax-1) borehole, revealing (top to bottom) Tertiary marine sediments, polymict breccias, an impact melt unit, and one or more blocks of Cretaceous target sediments that are crosscut with impact-generated dikes, in a region that lies betweeon the peak ring and final crater rim. The impact melt and breccias in the Yax-1 borehole are 100 m thick, which is approximately 1/5 the thickness of breccias and melts exposed in the Yucatán-6 exploration hole, which is also thought to be located between the peak ring and final rim of the Chicxulub crater. The sequence and composition of impact melts and breccias are grossly similar to those in the Yucatán-6 hole. Compared to breccias in other impact craters, the Chicxulub breccias are incredibly rich in silicate melt fragments (up to 84% versus 30 to 50%, for example, in the Rieś). The melt in the Yax-1 hole was produced largely from the silicate basement lithologies that lie beneath a 3 km- thick carbonate platform in the target area. Small amounts of immiscible molten carbonate were ejected with the silicate melt, and clastic carbonate often forms the matrix of the polymict breccias. The melt unit appears to have been deposited while molten but brecciated after solidification. The melt fragments in the polymict breccias appear to have solidified in flight, before deposition, and fractured during transport and deposition. © Meteoritical Society, 2004.
236.
Hydrologic measurements in wells in the Aigion area (Corinth Gulf, Greece): Preliminary results Greece): Preliminary results; [Mesures hydrologiques dans les puits de la région d'Aigion (golfe de Corinthe, Grèce) : Résultats préliminaires]
Comptes Rendus - Geoscience,
336
(4-5)
385 – 393
2004
ISSN: 16310713Keywords:▾
Gulf of Corinth; Ionian Sea; Mediterranean Sea; aquifer characterization; flow measurement; hydrological regime; rift zone; seismotectonics; tide
Abstract: ▾ Two wells have been equipped in 2002, in order to provide complementary hydrological information within the framework of the European project devoted to the Corinth Rift Laboratory. In this distensive tectonic domain, temporal series of flow (Neratzes well) and of piezometric fluid level (Trapeza well) have been recorded in 2002. We present here a first analysis of the data. As far as Trapeza is concerned, we present the first results of the calibration of the well, using the tidal and barometric analyses. The strain sensitivity of the well, as well as the aquifer hydraulic conductivity, is derived from tides, whereas the specific storativity of the aquifer and its confinement degree are estimated from the barometric response. These preliminary results show that the aquifer has got properties that make possible the observation of coupled tectonic/hydrological processes. In addition, the time series let appear some fast and permanent variations of level. We analyse these phenomena and detail the various assumptions, which could explain these processes. As far as Neratzes is concerned, we present the first data measured from the flowmeter. Contrary to the piezometric data, the flow measurements are not conventional. At the end of this first year of measurement, we prove that we are able to record the tides effect using the flowmeter. A prospective analysis of all these results will be drawn. © 2004 Académie des sciences. Published by Elsevier SAS. All rights reserved.
235.
High-rate real-time GPS network at Parkfield: Utility for detecting fault slip and seismic displacements
Geophysical Research Letters,
31
(15)
L15S20 1-4
2004
ISSN: 00948276Keywords:▾
Acoustic noise; Atmospheric spectra; Global positioning system; Natural frequencies; Neural networks, Master station; Real time relative; San andreas fault, Real time systems, borehole geophysics; fault slip; GPS; monitoring system; San Andreas Fault; seismicity, California; North America; Parkfield; United States; Western Hemisphere; World
Abstract: ▾ A network of 13 continuous GPS stations near Parkfield, California has been converted from 30 second to 1 second sampling with positions of the stations estimated in real-time relative to a master station. Most stations are near the trace of the San Andreas fault, which exhibits creep. The noise spectra of the instantaneous 1 Hz positions show flicker noise at high frequencies and change to frequency independence at low frequencies; the change in character occurs between 6 to 8 hours. Our analysis indicates that 1-second sampled GPS can estimate horizontal displacements of order 6 mm at the 99% confidence level from a few seconds to a few hours. High frequency GPS can augment existing measurements in capturing large creep events and postseismic slip that would exceed the range of existing creepmeters, and can detect large seismic displacements. Copyright 2004 by the American Geophysical Union.
234.
Hydrothermal alteration in the core of the Yaxcopoil-1 borchole, Chicxulub impact structure, Mexico
Meteoritics and Planetary Science,
39
(7)
1199-1221
2004
Abstract: ▾ Petrographic, electron microprobe, and Raman spectrometric analyses of Yaxcopoil-1 core samples from the Chicxulub crater indicate that the impact generated a hydrothermal system. Relative textural and vein crosscutting relations and systematic distribution of alteration products reveal a progression of the hydrothermal event in space and time and provide constraints on the nature of the fluids. The earliest calcite, halite, and gaylussite suggest that the impactite sequence was initially permeated by a low temperature saline brine. Subsequent development of a higher temperature hydrothermal regime is indicated by thermal metamorphic diopside-hedenbergite (Aeg3Fs18-33En32-11Wo47-53) after primary augite and widespread Na-K for Ca metasomatic alkali exchange in plagioclase. Hydrothermal sphene, apatite, magnetite ± (bornite), as well as early calcite (combined 3 to 8 vol%) were introduced with metasomatic feldspar. A lower temperature regime characterized by smectite after probable primary glass, secondary chlorite, and other pre-existing mafic minerals, as well as very abundant calcite veins and open-space fillings, extensively overprinted the early hydrothermal stage. The composition of early and late hydrothermal minerals show that the solution was chlorine-rich (Cl/F>10) and that its Fe/Mg ratio and oxidation state increased substantially (4 to 5 logfO2 units) as temperature decreased through time. The most altered zone in the impactite sequence occurs 30 m above the impact melt. The lack of mineralogical zoning about the impact melt and convective modeling constraints suggest that this unit was too thin at Yaxcopoil-1 to provide the necessary heat to drive fluids and implies that the hydrothermal system resulted from the combined effects of a pre-existing saline brine and heat that traveled to the Yaxcopoil-1 site from adjacent areas where the melt sheet was thicker. Limonite after iron oxides is more common toward the top of the sequence and suggests that the impactite section was subjected to weathering before deposition of the Tertiarý marine cover. In addition, scarce latest anatase stringers, chalcopyrite, and barite in vugs, francolite after apatite, and recrystallized halite are the likely products of limited post-hydrothermal ambient-temperature diagenesis, or ocean and/or meteoric water circulation. © Meteoritical Society, 2004. Printed in USA.
233.
First petrographic results on impactites from the Yaxcopoil-1 borehole, Chicxulub structure, Mexico
Meteoritics and Planetary Science,
39
(6)
899-930
2004
Abstract: ▾ The ICDP Yaxcopoil-1 (Yax-1) borehole located 60 km south-southwest of the center of the Chicxulub impact structure intercepted an interval of allogenic impactites (depth of 795-895 m). Petrographic analysis of these impactites allows them to be differentiated into five units based on their textural and modal variations. Unit 1 (795-922 m) comprises an apparently reworked, poorly sorted and graded, fine-grained, clast-supported, melt fragment-bearing suevitic breccia. The interstitial material, similar to units 2 and 3, is permeated by numerous carbonate veinlets. Units 2 (823-846 m) and 3 (846-861 m) are groundmass-supported breccias that comprise green to variegated angular and fluidal melt particles. The groundmass of units 2 and 3 comprises predominantly fine-grained calcite, altered alkali element-, Ca-, and Si-rich cement, as well as occasional lithic fragments. Unit 4 (861-885 m) represents a massive, variably devitrified, and brecciated impact melt rock. The lowermost unit, unit 5 (885-895 m), comprises highly variable proportions of melt rock particles (MRP) and lithic fragments in a fine-grained, carbonate-dominated groundmass. This groundmass could represent either a secondary hydrothermal phase or a carbonate melt phase, or both. Units 1 and 5 contain well-preserved foraminifera fossils and a significantly higher proportion of carbonate clasts than the other units. All units show diagnostic shock deformation features in quartz and feldspar clasts. Our observations reveal that most felsic and all mafic MRP are altered. They register extensive K-metasomatism. In terms of emplacement, we suggest that units 1 to 3 represent fallout suevite from a collapsing impact plume, whereby unit 1 was subsequently reworked by resurging water. Unit 4 represents a coherent impact melt body, the formation of which involved a significant proportion of crystalline basement. Unit 5 is believed to represent an initial ejecta/ground-surge deposit. © Meteoritical Society, 2004.
232.
Geochemistry of drill core samples from Yaxcopoil-1, Chicxulub impact crater, Mexico
Meteoritics and Planetary Science,
39
(6)
979-1001
2004
Abstract: ▾ The chemical composition of suevites, displaced Cretaceous target rocks, and impact-generated dikes within these rocks from the Yaxcopoil-1 (Yax-1) drill core, Chicxulub impact crater, Mexico, is reported and compared with the data from the Yucatán 6 (Y6) samples. Within the six suevite subunits of Yax-1, four units with different chemical compositions can be distinguished: a) upper/lower sorted and upper suevite (depth of 795-846 m); b) middle suevite (depth of 846-861 m); c) brecciated impact melt rock (depth of 861-88-5 m); and d) lower suevite (depth of 885-895 m). The suevite sequence (a), (b), and (d) display an increase of the CaO content and a decrease of the silicate basement component from top to bottom. In contrast, the suevite of Y6 shows an inverse trend. The different distances of the Yax-1 and Y6 drilling sites from the crater center (∼60, and ∼47 km, respectively) lead to different suevite sequences. Within the Cretaceous rocks of Yax-1, a suevitic dike (depth of ∼916 m) does not display chemical differences when compared with the suevite, while an impact melt rock dike (depth of ∼1348 m) is significantly enriched in immobile elements. A elastic breccia dike (depth of ∼1316 m) is dominated by material derived locally from the host rock, while the silicate-rich component is similar to that found in the suevite. Significant enrichments of the K2O content were observed in the Yax- 1 suevite and the impact-generated dikes. All impactites of Yax-1 and Y6 are mixtures of a crystalline basement and a carbonate component from the sedimentary cover. An anhydrite component in the impactites is missing (Yax-1) or negligible (Y6).
231.
Geochemistry of Cenozoic microtektites and clinopyroxene-bearing spherules
Geochimica et Cosmochimica Acta,
68
(19)
3971 – 4006
2004
ISSN: 00167037Keywords:▾
Cenozoic; clinopyroxene; geochemistry; spherule; tektite
Abstract: ▾ We have determined the major and trace element compositions of 176 individual microtektites/spherules from the Australasian, Ivory Coast, and North American microtektite and clinopyroxene-bearing (cpx) spherule layers. Trace element contents for up to 30 trace elements were determined by instrumental neutron activation analysis (INAA), and major element compositions were determined using energy dispersive X-ray (EDX) analysis in combination with a scanning electron microscope (SEM). In addition, petrographic data were obtained for the cpx spherules using the SEM and EDX. This is the first trace element study of individual Australasian microtektites, and the data revealed the presence of a previously unrecognized group of Australasian microtektites with high contents of Ni (up to 471 ppm). In previous studies the high-Mg (HMg) Australasian microtektites were thought to be related to the HMg Australasian tektites, but our trace element data suggest that the high-Ni (HNi) Australasian microtektites, rather than the high-Mg microtektites, are related to the high-Mg Australasian tektites. We find that Cenozoic microtektites/spherules from a given layer can be distinguished from microtektites/spherules from other layers as a group, but it is not always possible to determine which layer an individual microtektite/spherule came from based only on trace element compositions. The cpx spherules and most of the microtektites have Cr, Co, and Ni contents that are higher than the average contents of these elements in the upper continental crust, suggesting the presence of a meteoritic component. The highest Cr, Co, and Ni contents are found in the cpx spherules (and low-Si cpx-related microtektites). Unetched to slightly etched cpx spherules have Ni/Cr and Ni/Co ratios that generally lie along mixing curves between the average upper continental crust and chondrites. The best fit appears to be with an LL chondrite. The moderately to heavily etched cpx spherules have values that lie off the mixing curves in a direction that suggests Ni loss, probably as a result of solution of a Ni-rich phase (olivine?). The Ni-rich Australasian microtektites also have Ni values that lie close to mixing curves between the average upper continental crust and chondrites. However, both the cpx spherules and HNi Australasian microtektites appear to have Ir (and to a lesser extent Au) contents that are much too low to have Ni/Ir ratios similar to chondritic values. We have no explanation for the low-Ir and -Au contents except to speculate that they may be the result of a complex fractionation process. The Ivory Coast and North American microtektites do not have high enough siderophile element contents to reach any firm conclusions regarding the presence of, or nature of, a meteoritic component in them. Trace element compositions are consistent with derivation of the Cenozoic microtektite/spherule layers from upper continental crust. The normal Australasian microtektites appear to have been derived from a graywacke or lithic arenite with a range in clay and quartz content. The source rock for the high-Mg Australasian microtektites is not known, but the HMg microtektites do not appear to be normal Australasian microtektites that were simply contaminated by meteorites or ultramafic rocks. The average Ivory Coast microtektite composition can be matched with a mixture of target rocks at the Bosumtwi crater. The average composition of the North American microtektites suggests an arkosic source rock, but with graywacke and quartz-rich end members. However, we could not match the composition of the North American microtektites with lithologies in impact breccias recovered from the Chesapeake Bay impact structure that is believed to be the source crater. Likewise, we could not match the composition of the cpx spherules with mixtures of basement rocks and overlying sedimentary deposits (for which compositional data are available) at the Popigai impact crater that may be the source crater for the cpx spherules. This may be because the cpx spherules were derived, in large part, from clastic surface rocks (sandstones and shales) for which no compositional data are available. Copyright © 2004 Elsevier Ltd.
230.
Gas-hydrate concentration estimated from P- and S-wave velocities at the Mallik 2L-38 research well, Mackenzie Delta, Canada
Journal of Applied Geophysics,
56
(1)
73-78
2004
ISSN: 09269851Keywords:▾
Computer simulation; Phosphorus; Rocks; Seismology; Sulfur; Well logging, Dry-rocks; Wave velocities, Gas hydrates, gas hydrate; seismic velocity; seismic wave; vertical seismic profile; well logging, Canada; Mackenzie Delta; North America; Northwest Territories
Abstract: ▾ We estimate the concentration of gas hydrate at the Mallik 2L-38 research site using P- and S-wave velocities obtained from well logging and vertical seismic profiles (VSP). The theoretical velocities are obtained from a generalization of Gassmann's modulus to three phases (rock frame, gas hydrate and fluid). The dry-rock moduli are estimated from the log profiles, in sections where the rock is assumed to be fully saturated with water. We obtain hydrate concentrations up to 75%, average values of 37% and 21% from the VSP P- and S-wave velocities, respectively, and 60% and 57% from the sonic-log P- and S-wave velocities, respectively. The above averages are similar to estimations obtained from hydrate dissociation modeling and Archie methods. The estimations based on the P-wave velocities are more reliable than those based on the S-wave velocities. © 2004 Elsevier B.V. All rights reserved.
229.
Foraminiferal biostratigraphy and paleoenvironmental reconstruction at the Yaxcopoil-1 drill hole, Chicxulub crater, Yucatán Peninsula
Meteoritics & Planetary Science,
39
(7)
1099-1111
2004
Abstract: ▾ Abstract The Yaxcopoil-1 (Yax-1) drill hole comprises Cretaceous limestones and calcarenites, the K/P boundary cocktail unit (including impact breccia), and a Danian marly clay layer overlain by calcareous marls. The biostratigraphy, paleobathymetry, and environmental turnover across the K/P interval were inferred after analyzing the planktic and benthic foraminiferal assemblages. The Cretaceous samples only contain a few poorly preserved planktic foraminifera of a middle Campanian to Maastrichtian age, while low-diversity benthic foraminiferal assemblages suggest a sufficient nutrient supply to the sea floor and a shallow neritic, occasionally stressed environment. The impact breccia and the redeposited suevite are overlain by a 46 cm-thick dolomitic calcareous sandstone unit that contains scarce, reworked planktic foraminiferal specimens. This unit probably represents the uppermost part of the initial infill of the crater. The uppermost centimeters of this unit are bioturbated, and its top represents a hiatus that spans at least the G. cretacea, Pv. eugubina, and part of the P. pseudobulloides biozones. This unit is overlain by a 3–4 cm-thick marly clay layer that represents a condensed layer. Benthic foraminiferal assemblages suggest a low food supply to the sea floor and environmental instability during the deposition of the marly clay layer. The increase in diversity of the assemblages indicates that the environmental conditions improved and stabilized from the G. compressa biozone toward the A. uncinata (P2) biozone. The Danian planktic and benthic foraminiferal assemblages indicate a deeper, probably bathyal environment.
228.
Fluid inclusion evidence for impact-related hydrothermal fluid and hydrocarbon migration in Creataceous sediments of the ICDP-Chicxulub drill core Yax-1
Meteoritics and Planetary Science,
39
(7)
1187-1197
2004
Abstract: ▾ Fluid inclusions studies in quartz and calcite in samples from the ICDP-Chicxulub drill core Yaxcopoil-1 (Yax-1) have revealed compelling evidence for impact-induced hydrothermal alteration. Fluid circulation through the melt breccia and the underlying sedimentary rocks was not homogeneous in time and space. The formation of euhedral quartz crystals in vugs hosted by Cretaceous limestones is related to the migration of hot (>200 °C), highly saline, metal-rich, hydrocarbon-bearing brines. Hydrocarbons present in some inclusions in quàrtz are assumed to derive from cracking of pre-impact organic matter. The center of the crater is assumed to be the source of the hot quartz-forming brines. Fluid inclusions in abundant newly-formed calcite indicate lower cyrstallization temperatures (75-100 °C). Calcite crystallization is likely related to a later stage of hydrothermal alteration. Calcite precipitated from saline fluids, most probably from formation water. Carbon and oxygen isotope compositions and REE distributions in calcites and carbonate host rocks suggest that the calcite-forming fluids have achieved close equilibrium conditions with the Cretaceous limestones. The precipitation of calcite may be related to the convection of local pore fluids, possibly triggered by impact-induced conductive heating of the sediments. © Meteoritical Society, 2004. Printed in USA.
227.
First results of the CRLN seismic network in the western Corinth Rift: Evidence for old-fault reactivation; [Premiers résultats du réseau sismique (CRLN) de la partie ouest du rift de Corinthe : Évidence de la réactivation d'une ancienne faille]
Comptes Rendus - Geoscience,
336
(4-5)
343 – 351
2004
ISSN: 16310713
Publisher: Elsevier Masson SAS
Keywords:▾
Gulf of Corinth; Ionian Sea; Mediterranean Sea; earthquake; fault slip; reactivation; rift zone; seismotectonics
Abstract: ▾ The 12 stations Corinth Rift Laboratory Seismological Network (CRLNET) aims at monitoring the seismicity (Ml>1) in the CRL area and at constraining the geometry of active structures at depth. Two years of microseismicity (2000-2001) recorded by the CRLNET in the Aigion area shows: (1) background seismicity inside the Corinth rift at depth of 4.5-11 km, deepening towards the north and no activity in the upper 4 km of the crust - this seismicity is not clearly related to major faults observed at the surface -; (2) a swarm, 6 km south of the city of Aigion, associated with the Mw=4.2, 8 April 2001 earthquake. This earthquake occurred at 6 km depth, on a SW-NE oriented fault dipping 40° to the northwest and corresponds to normal faulting with a right lateral component of slip. It likely occurred on an old structure reactivated in the present stress field. © 2003 Académie des sciences. Published by Elsevier SAS. All rights reserved.
226.
Glass in the submarine section of the HSDP2 drill core, Hilo, Hawaii
Geochemistry, Geophysics, Geosystems,
5
(7)
2004
ISSN: 15252027Abstract: ▾ The Hawaii Scientific Drilling Project recovered-3 km of basalt by coring into the flank of Mauna Kea volcano at Hilo, Hawaii. Rocks recovered from deeper than-1 km were deposited below sea level and contain considerable fresh glass. We report electron microprobe analyses of 531 glasses from the submarine section of the core, providing a high-resolution record of petrogenesis over ca. 200 Kyr of shield building of a Hawaiian volcano. Nearly all the submarine glasses are tholeiitic. SiO2 contents span a significant range but are bimodally distributed, leading to the identification of low-SiO2 and high-SiO2 magma series that encompass most samples. The two groups are also generally distinguishable using other major and minor elements and certain isotopic and incompatible trace element ratios. On the basis of distributions of high-and low-SiO2 glasses, the submarine section of the core is divided into four zones. In zone 1 (1079-1950 mbsl), most samples are degassed high-SiO2 hyaloclastites and massive lavas, but there are narrow intervals of low-SiO2 hyaloclastites. Zone 2 (-1950-2233 mbsl), a zone of degassed pillows and hyaloclastites, displays a continuous decrease in silica content from bottom to top. In zone 3 (2233-2481 mbsl), nearly all samples are undegassed low-SiO2 pillows. In zone 4 (2481-3098 mbsl), samples are mostly high-SiO2 undegassed pillows and degassed hyaloclastites. This zone also contains most of the intrusive units in the core, all of which are undegassed and most of which are low-SiO 2. Phase equilibrium data suggest that parental magmas of the low-SiO2 suite could be produced by partial melting of fertile peridotite at 30-40 kbar. Although the high-SiO2 parents could have equilibrated with harzburgite at 15-20 kbar, they could have been produced neither simply by higher degrees of melting of the sources of the low-SiO 2 parents nor by mixing of known dacitic melts of pyroxenite/eclogite with the low-SiO2 parents. Our hypothesis for the relationship between these magma types is that as the low-SiO2 magmas ascended from their sources, they interacted chemically and thermally with overlying peridotites, resulting in dissolution of orthopyroxene and clinopyroxene and precipitation of olivine, thereby generating high-SiO2 magmas. There are glasses with CaO, Al2O3, and SiO2 contents slightly elevated relative to most low-SiO2 samples; we suggest that these differences reflect involvement of pyroxene-rich lithologies in the petrogenesis of the CaO-Al2O3-enriched glasses. There is also a small group of low-SiO2 glasses distinguished by elevated K2O and CaO contents; the sources of these samples may have been enriched in slab-derived fluid/melts. Low-SiO2 glasses from the top of zone 3 (2233-2280 mbsl) are more alkaline, more fractionated, and incompatible-element-enriched relative to other glasses from zone 3. This excursion at the top of zone 3, which is abruptly overlain by more silica-rich tholeiitic magmas, is reminiscent of the end of Mauna Kea shield building higher in the core. Copyright 2004 by the American Geophysical Union.
225.
Fine-scale structure of the San Andreas fault zone and location of the SAFOD target earthquakes
Geophysical Research Letters,
31
(12)
L12S02 1-4
2004
ISSN: 00948276Keywords:▾
Data acquisition; Error analysis; Geophysics; Mathematical models; Seismology; Tomography; Velocity measurement; Earthquakes; Faulting; Geophysics; Seismology; Strike-slip faults, Epicenter; Inversion codes; Tomographic analysis, Earthquakes; Location, earthquake; microstructure; San Andreas Fault; seismic hazard; seismic tomography; velocity structure, California; North America; United States, Absolute location errors; Fine-scale structures; Inversion methods; Inversion results; Potential targets; San Andreas fault; Velocity contrasts; Velocity structure
Abstract: ▾ We present results from the tomographic analysis of seismic data from the Parkfield area using three different inversion codes. The models provide a consistent view of the complex velocity structure in the vicinity of the San Andreas, including a sharp velocity contrast across the fault. We use the inversion results to assess our confidence in the absolute location accuracy of a potential target earthquake. We derive two types of accuracy estimates, one based on a consideration of the location differences from the three inversion methods, and the other based on the absolute location accuracy of "virtual earthquakes." Location differences are on the order of 100-200 m horizontally and up to 500 m vertically. Bounds on the absolute location errors based on the "virtual earthquake" relocations are ∼ 50 m horizontally and vertically. The average of our locations places the target event epicenter within about 100 m of the SAF surface trace. Copyright 2004 by the American Geophysical Union.
224.
Seismic tomography of the lithosphere with body waves
Pure and Applied Geophysics,
160
(3-4)
717-737
2003
ISSN: 00334553
Publisher: Birkhauser Verlag AG
Keywords:▾
body wave; earthquake; lithospheric structure; seismic tomography; seismic velocity; teleseismic wave
Abstract: ▾ A pair of papers in 1976 lead-authored by Kei Aki heralded the beginning of the field of seismic tomography of the lithosphere. The 1976 paper by Aki, Christofferson, and Husebye introduced a simple and approximate yet elegant technique for using body-wave arrival times from teleseismic earthquakes to infer the three-dimensional (3-D) seismic velocity heterogeneities beneath a seismic array or network (teleseismic tomography). Similarly, a 1976 paper by Aki and Lee presented a method for inferring 3-D structure beneath a seismic network using body-wave arrival times from local earthquakes (local earthquake tomography). Following these landmark papers, many dozens of papers and numerous books have been published presenting exciting applications of and/or innovative improvements to the methods of teleseismic and local earthquake tomography, many by Aki's students. This paper presents a brief review of these two types of tomography methods, discussing some of the underlying assumptions and limitations. Thereafter some of the significant methodological developments are traced over the past two and a half decades, and some of the applications of tomography that have reaped the benefits of these developments are highlighted. One focus is on the steady improvement in structural resolution and inference power brought about by the increased number and quality of seismic stations, and in particular the value of utilizing shear waves. The paper concludes by discussing exciting new scientific projects in which seismic tomography will play a major role - the San Andreas Fault Observatory at Depth (SAFOD) and USArray, the initial components of Earthscope.
223.
Summary of recent research in Long Valley Caldera, California
Journal of Volcanology and Geothermal Research,
127
(3-4)
165-173
2003
ISSN: 03770273
Publisher: Elsevier
Keywords:▾
Earthquakes; Hydrology; Pressure effects; Seismology; Thermal effects, Hydrothermal fluids, Volcanoes, borehole; caldera; geodesy; hydrothermal system; research; volcanic earthquake; volcano, California; Long Valley Caldera; United States
Abstract: ▾ Since 1978, volcanic unrest in the form of earthquakes and ground deformation has persisted in the Long Valley caldera and adjacent parts of the Sierra Nevada. The papers in this special volume focus on periods of accelerated seismicity and deformation in 1980, 1983, 1989-1990, and 1997-1998 to delineate relations between geologic, tectonic, and hydrologic processes. The results distinguish between earthquake sequences that result from relaxation of existing stress accumulation through brittle failure and those in which brittle failure is driven by active intrusion. They also indicate that in addition to a relatively shallow (7-10-km) source beneath the resurgent dome, there exists a deeper (∼15-km) source beneath the south moat. Analysis of microgravimety and deformation data indicates that the composition of the shallower source may involve a combination of silicic magma and hydrothermal fluid. Pressure and temperature fluctuations in wells have accompanied periods of crustal unrest, and additional pressure and temperature changes accompanying ongoing geothermal power production have resulted in land subsidence. The completion in 1998 of a 3000-m-deep drill hole on the resurgent dome has provided useful information on present and past periods of circulation of water at temperatures of 100-200°C within the crystalline basement rocks that underlie the post-caldera volcanics. The well is now being converted to a permanent geophysical monitoring station. © 2003 Elsevier B.V. All rights reserved.
222.
Structural properties of rift-related normal faults: The case study of the Gulf of Corinth, Greece
Journal of Geodynamics,
36
(1-2)
275 – 303
2003
ISSN: 02643707
Publisher: Elsevier Ltd
Keywords:▾
Greece; Gulf of Corinth; deformation; faulting; neotectonics; rift zone; seismicity; tectonics
Abstract: ▾ In the frame of the Corinth Rift Laboratory project, which aims to understand the relation between stress, strain and fluid flow in an extensional context, three normal faults have been studied and sampled in the south-western sector of the Gulf of Corinth. Two of these faults (the Pirgaki and Helike faults) juxtapose carbonate pre-rift sediments and syn-rift conglomerates at outcrop, and the third (the Aigion fault) has been drilled and cored at 760 m depth. The distance between the faults is around 4 km, and the offsets are approximately 1000, 700 and 170 m, respectively. The fault damage zones and cores of the analysed faults exhibit different geometry, size and textural characteristics. In general, it has been assessed that fault cores behave as near-impermeable barriers to transverse fluid flow, whereas highly fractured damage zones act mostly as conduits for nearly along-strike flow. Different types of fault rocks are exposed along the main faults; these include cataclasite, ultra-cataclasite, fault breccia and gouge, with random or foliated fabrics. Along the Pirgaki fault zone only, fault rock micro-fabrics have been related to multiple tectonic episodes. The results of our work also emphasise that the current conditions of deformation, in this sector of the Gulf of Corinth, are responsible for the development of a system of extension fractures that is only partially sealed. These fractures mainly control the permeability structure of the analysed fault zones and furnish information on the stress field acting in the area. Furthermore, their orientation and distribution indicate that they are consistent with a deformation pattern resulting from active extension. © 2003 Elsevier Ltd. All rights reserved.
221.
Paleointensity in Hawaiian scientific drilling project hole (HSDP2): Results from submarine basaltic glass
Geochemistry, Geophysics, Geosystems,
4
(2)
2003
ISSN: 15252027
Publisher: Blackwell Publishing Ltd
Abstract: ▾ Paleointensity estimates based on the high quality Thellier-Thellier data from the early Brunhes (420-780 ka) are rare (only 30 in the published literature). The Second Hawaiian Scientific Drilling Project (HSDP2) drill hole recovered submarine volcanics spanning the approximate time period of 420-550 ka. These are of particular interest for absolute paleointensity studies owing to the abundance of fresh submarine basaltic glass, which can preserve an excellent record of ancient geomagnetic field intensity. We present here new results of Thellier-Thellier paleointensity experiments that nearly double the number of reliable paleointensity data available for the early Brunhes. We also show that the magnetizations of the associated submarine basalts are dominated by viscous magnetizations and therefore do not reflect the true ancient geomagnetic field intensity at the time of extrusion. The viscous contamination is particularly severe because of a combination of low blocking temperatures in the basalts and relatively high temperatures in the deeper parts of the drill core. Our new data, when placed on the approximate timescale available for HSDP and HSDP2, are at odds with other contemporaneous paleointensity data. The discrepancy can be reconciled by adjusting the HSDP timescales to be younger by about 35 kyr. © 2003 by the American Geophysical Union.
220.
Review of ground temperatures in the Mallik field area: A constraint to the methane hydrate stability
Bulletin of the Geological Survey of Canada
(544)
45-56
2003
ISSN: 00687626Abstract: ▾ Analysis of data from 32 industrial exploration wells in the Mallik field and surrounding area in the Mackenzie Delta-Beaufort Sea region allowed construction of temperature-depth profiles using regionalheat-flowvalues, temperature at the base ofice-bearing permafrost, and model so thermal conductivity with depth. An analysis of the stability conditions for methane hydrate showed that it is stable in the Mallik field area and that the depth to the base of the methane hydrate stability zone can be as deep as 1500±100minareas of thick permafrost.The depth to the base of the methane hydrate stabilityzone, calculated in this study using reconstructed temperature-depth profiles, was found in a majority of the wells to be 50-150 m deeper than that previously determined using linear temperature profiles and a constant thermal conductivity with depth.