Earthquake source parameters determined by the SAFOD Pilot Hole seismic array - Imanishi, K.; Ellsworth, W.L.; Prejean, S.G. (Geophysical Research Letters, 2004)
We estimate the source parameters of #3 microearthquakes by jointly analyzing seismograms recorded by the 32-level, 3-component seismic array installed in the SAFOD Pilot Hole. We applied an inversion procedure to estimate spectral parameters for the omega-square model (spectral level and corner frequency) and Q to displacement amplitude spectra. Because we expect spectral parameters and Q to vary slowly with depth in the well, we impose a smoothness constraint on those parameters as a function of depth using a linear first-differenfee operator. This method correctly resolves corner frequency and Q, which leads to a more accurate estimation of source parameters than can be obtained from single sensors. The stress drop of one example of the SAFOD target repeating earthquake falls in the range of typical tectonic earthquakes. Copyright 2004 by the American Geophysical Union.
cited By 53
Keywords: Frequencies; Function evaluation; Mathematical models; Mathematical operators; Parameter estimation; Seismology; Sensors; Tectonics; Frequency estimation; Geophysics; Seismology; Displacement amplitude spectra; Seismic arrays; Seisomograms; Earthquakes; Earthquakes; deep drilling; earthquake mechanism; San Andreas Fault; source parameters; Accurate estimation; Difference operators; Displacement amplitudes; Earthquake source parameters; Repeating earthquake; Smoothness constraints; Spectral parameters; Tectonic earthquakes

Constraining the exhumation and burial history of the SAFOD pilot hole with apatite fission track and (U-Th)/He thermochronometry - Blythe, A.E.; d'Alessio, M.A.; B{urgmann, R. (Geophysical Research Letters, 2004)
The San Andreas Fault Observatory at Depth (SAFOD) pilot hole traverses the upper 2 km of a site 1.8 km west of the San Andreas fault (SAF) near Parkfield, California. In order to evaluate the burial and exhumation history of the site and its relationship to the kinematics and mechanics of the SAF, we use 15 apatite fission-track (FT) and 5 (U-Th)/He analyses from pilot hole samples to document their thermal history. Sample ages decrease with depth: FT and (U-Th)/He ages range from ∼60 and ∼31 Ma, respectively, in the upper 800 m of the hole to ∼3 and 1 Ma at the base of the hole (2.2 km depth, 93°C). Thermal modeling of the distribution of FT lengths indicates three events in the last 80 Ma: 1) cooling and exhumation of >60°C that culminated at ∼30 Ma; 2) reheating of ∼50°C from ∼30 to 8-4 Ma, probably as the result of basin subsidence and burial by 1-1.5 km of sediments; and 3) cooling of ∼30°C and estimated Coast Range exhumation of ∼1 km since 8-4 Ma. Copyright 2004 by the American Geophysical Union.
cited By 15
Keywords: Cooling; Geophysical prospecting; Heating; Kinematics; Mathematical models; Sediments; Thermal effects; Thermoanalysis; Fission-track (FT); Pilot holes; San Andreas Fault (SAF); Thermochronometry; Boreholes; borehole geophysics; burial (geology); exhumation; fission track dating; San Andreas Fault; thermal evolution; thermochronology; California; North America; Parkfield; United States; Western Hemisphere; World

Electrical resistivity structure at the SAFOD site from magnetotelluric exploration - Unsworth, M.; Bedrosian, P.A. (Geophysical Research Letters, 2004)
The magnetotelluric dataset collected on the San Andreas Fault at Parkfield has been re-analyzed using superior inversion algorithms that have been developed in recent years. A combination of constrained inversion, forward modeling and synthetic inversion studies are used, and show that at the SAFOD site, the low resistivity fault zone extends to a depth of 2-3 km. An extended zone of low resistivity cast of the San Andreas Fault may be connected to the SAF at seismogenic depths. The connection increases along the SAF to the northwest and may be related to the transition from locked to creeping seismic behavior. Copyright 2004 by the American Geophysical Union.
cited By 98
Keywords: Algorithms; Data reduction; Geophysics; Magnetoelectric effects; Mathematical models; Seismology; Strike-slip faults; Datasets; Fault zone; Magnetotelluric exploration; Electric conductivity; Magnetotellurics; electrical resistivity; magnetotelluric method; San Andreas Fault; Constrained inversions; Electrical resistivity structures; Forward modeling; Inversion algorithm; Low resistivity; Magnetotelluric exploration; San Andreas fault; Seismic behavior

High resolution temperature measurements in the borehole Yaxcopoil-1, Mexico - Wilhelm, H.; Heidinger, P.; Šafanda, J.; Čerm'ak, V.; Burkhardt, H.; Popov, Yu. (Meteoritics and Planetary Science, 2004)
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.
cited By 20

Introduction to special section: Preparing for the San Andreas Fault Observatory at Depth - Hickman, S.; Zoback, M.; Ellsworth, W. (Geophysical Research Letters, 2004)
cited By 110
Keywords: earthquake; San Andreas Fault; seismic hazard; California; North America; United States

Evidence for ocean water invasion into the Chicxulub crater at the Cretaceous/Tertiary boundary - Goto, K.; Tada, R.; Tajika, E.; Bralower, T.J.; Hasegawa, T.; Matsui, T. (Meteoritics and Planetary Science, 2004)
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.
cited By 43

Impactites of the Yaxcopoil-1 drilling site, Chicxulub impact structure: Petrography, geochemistry, and depositional environment - Dressler, B.O.; Sharpton, V.L.; Schwandt, C.S.; Ames, D. (Meteoritics and Planetary Science, 2004)
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; nu'ee ardente) and a coarse breccia (unit 4;∼15 m thick) below. As such, they consist of a m'elange 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.
cited By 51

Impact-related dike breccia lithologies in the ICDP drill core Yaxcopoil-1, Chicxulub impact structure, Mexico - Wittman, A.; Kenkmann, T.; Schmitt, R.T.; Hecht, L.; St{offler, D. (Meteoritics and Planetary Science, 2004)
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.
cited By 35

Impact lithologies and their emplacement in the Chicxulub impact crater: Initial results from the Chicxulub Scientific Drilling Project, Yaxcopoil, Mexico - Kring, D.A.; Horz, F.; Zurcher, L.; Urrutia Fucugauchi, J. (Meteoritics and Planetary Science, 2004)
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 Yucat'an-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 Yucat'an-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 Rie's). 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.
cited By 62

ICDP-USGS workshop on deep drilling in the central Crater of the Chesapeake Bay impact structure, Virginia, USA: Proceedings volume - Edwards, Lucy E.; Wright Horton Jr., W.J.J.; Gohn, Gregory S. (2004)

Hydrothermal alteration in the core of the Yaxcopoil-1 borchole, Chicxulub impact structure, Mexico - Zurcher, L.; Kring, D.A. (Meteoritics and Planetary Science, 2004)
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 Tertiar'y 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.
cited By 72

Hydrologic measurements in wells in the Aigion area (Corinth Gulf, Greece): Preliminary results Greece): Preliminary results; [Mesures hydrologiques dans les puits de la r'egion d'Aigion (golfe de Corinthe, Grèce) : R'esultats pr'eliminaires] - L'eonardi, V'eronique; Gavrilenko, Pierre (Comptes Rendus - Geoscience, 2004)
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 Acad'emie des sciences. Published by Elsevier SAS. All rights reserved.
Keywords: Gulf of Corinth; Ionian Sea; Mediterranean Sea; aquifer characterization; flow measurement; hydrological regime; rift zone; seismotectonics; tide

High-rate real-time GPS network at Parkfield: Utility for detecting fault slip and seismic displacements - Langbein, J.; Bock, Y. (Geophysical Research Letters, 2004)
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.
cited By 105
Keywords: 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

Integrated deep drilling, coring, downhole logging, and data management in the Chicxulub Scientific Drilling Project (CSDP), Mexico - Wohlgemuth, L.; Bintakies, E.; Conze, R.; Harms, U. (Meteoritics and Planetary Science, 2004)
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 M'erida, Yucat'an, 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.
cited By 14

Heat flow in the SAFOD pilot hole and implications for the strength of the San Andreas Fault - Williams, C.F.; Grubb, F.V.; Galanis Jr., S.P. (Geophysical Research Letters, 2004)
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.
cited By 67
Keywords: 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

Fluid inclusion evidence for impact-related hydrothermal fluid and hydrocarbon migration in Creataceous sediments of the ICDP-Chicxulub drill core Yax-1 - L{uders, V.; Rickers, K. (Meteoritics and Planetary Science, 2004)
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.
cited By 33

Glass in the submarine section of the HSDP2 drill core, Hilo, Hawaii - Stolper, E.; Sherman, S.; Garcia, M.; Baker, M.; Seaman, C. (Geochemistry, Geophysics, Geosystems, 2004)
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.
cited By 94

Fine-scale structure of the San Andreas fault zone and location of the SAFOD target earthquakes - Thurber, C.; Roecker, S.; Zhang, H.; Baher, S.; Ellsworth, W. (Geophysical Research Letters, 2004)
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.
cited By 136
Keywords: 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

First results of the CRLN seismic network in the western Corinth Rift: Evidence for old-fault reactivation; [Premiers r'esultats du r'eseau sismique (CRLN) de la partie ouest du rift de Corinthe : 'Evidence de la r'eactivation d'une ancienne faille] - Lyon-Caen, H'elène; Papadimitriou, Panayotis; Deschamps, Anne; Bernard, Pascal; Makropoulos, Kostas; Pacchiani, Francesco; Patau, Geneviève (Comptes Rendus - Geoscience, 2004)
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 Acad'emie des sciences. Published by Elsevier SAS. All rights reserved.
Keywords: Gulf of Corinth; Ionian Sea; Mediterranean Sea; earthquake; fault slip; reactivation; rift zone; seismotectonics

First petrographic results on impactites from the Yaxcopoil-1 borehole, Chicxulub structure, Mexico - Tuchscherer, M.G.; Reimold, W.U.; Koeberl, C.; Gibson, R.L.; Bruin, D. (Meteoritics and Planetary Science, 2004)
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.
cited By 33

Foraminiferal biostratigraphy and paleoenvironmental reconstruction at the Yaxcopoil-1 drill hole, Chicxulub crater, Yucat'an Peninsula - ARZ, Jos'e A.; ALEGRET, Laia; ARENILLAS, Ignacio (Meteoritics & Planetary Science, 2004)
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.

Gas-hydrate concentration estimated from P- and S-wave velocities at the Mallik 2L-38 research well, Mackenzie Delta, Canada - Carcione, J.M.; Gei, D. (Journal of Applied Geophysics, 2004)
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.
cited By 58
Keywords: 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

Geochemistry of Cenozoic microtektites and clinopyroxene-bearing spherules - Glass, Billy P.; Huber, Heinz; Koeberl, Christian (Geochimica et Cosmochimica Acta, 2004)
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.
Keywords: Cenozoic; clinopyroxene; geochemistry; spherule; tektite

Geochemistry of drill core samples from Yaxcopoil-1, Chicxulub impact crater, Mexico - Schmitt, R.T.; Wittmann, A.; St{offler, D. (Meteoritics and Planetary Science, 2004)
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 Yucat'an 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).
cited By 37

Seismic tomography of the lithosphere with body waves (Pure and Applied Geophysics, 2003)
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.
cited By 15
Keywords: body wave; earthquake; lithospheric structure; seismic tomography; seismic velocity; teleseismic wave

Noble gases in olivine phenocrysts from drill core samples of the Hawaii Scientific Drilling Project (HSDP) pilot and main holes (Mauna Loa and Mauna Kea, Hawaii) - Althaus, Tilmann; Niedermann, Samuel (Geochemistry, Geophysics, Geosystems, 2003)
[1] We have determined concentrations and isotopic compositions of all noble gases in olivine phenocrysts from the Hawaii Scientific Drilling Project (HSDP) drill core, comprising Mauna Loa lavas in the top 247 m and Mauna Kea lavas down to the preliminary depth of 3109 m. Our aim was to describe the long-term isotopic evolution of noble gases over a significant time fraction of the active life of a major Hawaiian volcano. The He isotopic signature displays a clear temporal trend: 3He/4He ratios increase from MORBlike 9 RA in the youngest lavas to 15 RA in the Mauna Loa section and from ̃7 RA to ̃12 RA in the subaerial Mauna Kea deposits. They remain close to 12 RA in most of the submarine Mauna Kea samples, except for a few excursions with 3He/4He ratios of up to 21 RA in borehole depths between 2000 and 2600 m. The average 3He/4He ratio of 12 RA is lower than that observed in recent eruptions of Kilauea and Loihi seamount and supports the idea of a concentrically zoned Hawaiian plume [Kurz et al., 1996]. The Ne isotopic signature does not show a temporal evolution. It remains plume-like (plotting close to the Loihi-Kilauea correlation line in a Ne three-isotope diagram) over the whole Mauna Kea section in those samples which are not dominated by air-like Ne. Maximum 20Ne/22Ne and 21Ne/22Ne ratios reach 12.10 ± 0.36 and 0.0360 ±0.0042, respectively. 40Ar/36Ar ratios vary widely between 360 and ̃3300 in the ≥1000°C release steps due to variable atmospheric contributions. In at least one sample, a 40Ar/ 36Ar ratio of 14,300 ±910 demonstrates the presence of a MORB-like Ar component. Kr and Xe isotopic compositions are atmospheric throughout. We discuss several possibilities on how to explain the isotopic trends of the noble gases and their correlation to other geochemical parameters. Simple admixture of MORB-like noble gases to the plume component cannot account for the observations. We favor a model involving early melt extraction from the outer plume sections, followed by radiogenic ingrowth and, possibly, some interaction with ambient mantle material. © 2003 by the American Geophysical Union.

Open fissure mineralization at 2600 m depth in Long Valley Exploratory Well (California) – insight into the history of the hydrothermal system - Fischer, M.; R{oller, K.; K{uster, M.; St{ockhert, B.; McConnell, V.S. (Journal of Volcanology and Geothermal Research, 2003)
Long Valley Exploratory Well, drilled into the Resurgent Dome at Long Valley Caldera (California) to explore the potential of geothermal power in an active magmatic system, achieved temperatures of only ca. 100°C at 2500 –3000 m depth, well below the range expected atop an active magma chamber. Open fissures encountered at 2600 m depth are coated by mm-sized idiomorphic quartz crystals with first- and second-order growth discontinuities. Specific growth defects indicating rapid crystallization reflect sudden changes in SiO2 supersaturation. Fluid inclusions contain low salinity (0 –5 wt% NaCl) and low CO2 (<3 mole%) aqueous fluids, with V –L homogenization temperatures of 300 –350°C, indicating trapping at more than 200°C above the ambient temperatures measured within the borehole today. Fluid composition and inclusion density varies between and within the growth zones, reflecting progressive changes in the hydrothermal system during crystallization. Episodic crystallization from supersaturated fluids is interpreted to reflect sudden changes in the convection pattern, presumably induced by seismic activity, with a more recent and dramatic reorganization resulting in convective cooling. The quartz crystals are sensitive recorders of the earlier higher temperature history, unaffected by the present-day situation.
Crustal Unrest in Long Valley Caldera, California: New interpretations from geophysical and hydrologic monitoring and deep drilling
Keywords: Long Valley Caldera; hydrothermal system; thermal history; fluid inclusions; quartz crystallization; open fissures

Operation overview of the 2002 mallik gas hydrate production research well program at the Mackenzie delta in the Canadian arctic - Takahashi, H.; Yonezawa, T.; Fercho, E. (Proceedings of the Annual Offshore Technology Conference, 2003)
The first production test of methane hydrate layers was carried out at the Mackenzie Delta in the Canadian Arctic. Three wells, named JAPEX/JNOC/GSC et al. Mallik 3L-38, 4L-38 and 5L-38, were drilled through hydrate layers at depths of approximately 900-1100m beneath 640m of permafrost on a line at 40m spacing. Coring, logging, various scientific experiments and production testing were performed over a 79-day period from December 2001 to March 2002. This research project was funded by participants from Japan, Canada, US, Germany and India. The Geological Survey of Canada (GSC) was the coordinator of the science program and Japan National Oil Corporation (JNOC) along with Japan Petroleum Exploration Co. (JAPEX) were the operators. JAPEX Canada Ltd., a subsidiary of JAPEX, acted as the legal operator in Canada and Canadian Petroleum Engineering Inc. (CPE) was retained to provide JAPEX with operational management for the remote arctic environment. The production testing was achieved using two testing methods. Six short duration depressurization tests were conducted using Schlumberger's Modular Dynamic Formation Tester (MDT). A longer duration, thermal stimulation test was carried out by circulating hot fluid past the hydrate layers. An extensive research program was conducted to monitor the formation response to the testing. Cross well tomography surveys were conducted from the observation wells before and during the testing. The post-test wireline log response was compared with pre-test log. The wellbore temperatures were monitored on all wells using fiber-optic cables. These data are valuable for improving the reservoir simulation models to assess gas hydrate production. A practical model is essential for a country such as Japan that has been seeking to economically develop deep water marine gas hydrates off its coast. This paper describes the field operation, explaining how all the programs were accomplished successfully in the limited operational period. The full research gas hydrate research program being conducted at the Mallik site is ongoing with a scheduled public release of the results expected on or before August 2004. © 2003, Offshore Technology Conference
cited By 16
Keywords: Gas hydrates; Gas industry; Gasoline; Hydration; Offshore oil well production; Offshore oil wells; Offshore technology; Oil well logging; Well testing; Gas-hydrate production; Geological Survey of Canada; Japan national oil corporations; Operational management; Petroleum exploration; Reservoir simulation model; Scientific experiments; Thermal stimulation tests; Software testing

Oxygen isotope geochemistry of the second HSDP core - Wang, Z.; Kitchen, N.E.; Eiler, J.M. (Geochemistry, Geophysics, Geosystems, 2003)
Oxygen isotope ratios were measured in olivine phenocrysts (̃1 mm diameter), olivine microphenocrysts (generally ̃100-200 mm diameter), glass, and/or matrix from 89 samples collected from depths down to 3079.7 m in the second, and main, HSDP core (HSDP-2). Olivine phenocrysts from 11 samples from Mauna Loa and 34 samples from the submarine section of Mauna Kea volcano have δ18O values that are similar to one another (5.11 ± 0.10%, 1s, for Mauna Loa; 5.01 ± 0.07%, for submarine Mauna Kea) and within the range of values typical of olivines from oceanic basalts (δ18O of ̃5.0 to 5.2%). In contrast, δ18O values of olivine phenocrysts from 20 samples taken from the subaerial section of Mauna Kea volcano (278 to 1037 mbsl) average 4.79 ± 0.13%. Microphenocrysts in both the subaerial (n = 2) and submarine (n = 24) sections of Mauna Kea are on averagẽ0.2% lower in δ18O than phenocrysts within the same stratigraphic interval; those in submarine Mauna Kea lavas have an average δ18O of 4.83 ± 0.11%. Microphenocrysts in submarine Mauna Kea lavas and phencrysts in Mauna Loa lavas are the only population of olivines considered in this study that are typically in oxygen isotope exchange equilibrium with coexisting glass or groundmass. These data confirm the previous observation that the stratigraphic boundary between Mauna Loa and Mauna Kea lavas defines a shift from normal to unusually low δ18O values. Significantly, they also document that the distinctive 18O-depleted character of subaerial Mauna Kea lavas is absent in phenocrysts of submarine Mauna Kea lavas. Several lines of evidence suggest that little if any of the observed variations in δ18O can be attributed to subsolidus alteration or equilibrium fractionations accompanying partial melting or crystallization. Instead, they reflect variable proportions of an 18O-depleted source component or contaminant from the lithosphere and/or volcanic edifice that is absent in or only a trace constituent of subaerial Mauna Loa lavas, a minor component of submarine Mauna Kea lavas, and a major component of subaerial Mauna Kea lavas. Relationships between the δ18O of phenocrysts, microphenocrysts, and glass or groundmass indicate that this component (when present) was added over the course of crystallization-differentiation. This process must have taken place in the lithosphere and most likely at depths of between ̃5 and 15 km. We conclude that the low-δ18O component is either a contaminant from the volcanic edifice that was sampled in increasingly greater proportions as the volcano drifted off the center of the Hawaiian plume or a partial melt of low-δ18O, hydrothermally altered perdotites in the shallow Pacific lithosphere that increasingly contributed to Mauna Kea lavas near end of the volcano's shield building stage. The first of these alternatives is favored by the difference in δ18O between subaerial and submarine Mauna Kea lavas, whereas the second is favored by systematic differences in radiogenic and trace element composition between higher and lower δ18O lavas. Copyright 2003 by the American Geophysical Union.
cited By 31

Paleointensity in Hawaiian scientific drilling project hole (HSDP2): Results from submarine basaltic glass - Tauxe, L.; Love, J.J. (Geochemistry, Geophysics, Geosystems, 2003)
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.
cited By 29

Quasi-continuous depth profiles of rock magnetization from magnetic logs in the HSDP-2 borehole, Island of Hawaii - Steveling, E.; Stoll, J.B.; Leven, M. (Geochemistry, Geophysics, Geosystems, 2003)
A quasi-continuous magnetic log has been obtained in the Hawaii Scientific Drilling Project 2 (HSDP-2) between 600 m and 1800 m, which corresponds to a time interval of approximately 350 ka to 480 ka. A tri-axial borehole magnetometer was employed to measure the horizontal and vertical magnetic fields. Measurements were taken in downhole and uphole runs, with a good correlation between the two. In a first step the logs were corrected for the transfer function of the employed low-pass filter and then for the logging depths. To calculate rock magnetizations from magnetic components, we used a multidisk cylindrical model for the penetrated rocks. The disk thickness corresponds with 0.1 m to the logging sampling rate. Magnetic borehole logging in the HSDP-2 hole has established the following: Massive lava flows can be distinguished from those with prevailing hyaloclastites and enables us to supplement the lithology, especially in depth intervals with poor core recovery. The inclinations of rock magnetization derived from the magnetic log agree well with those measured in core samples from HSDP-2 hole. The same applies to magnitudes of magnetizations from logging, as the sum of induced and remanent magnetizations, with laboratory determinations of the remanent magnetizations of core samples. We observe a distinct discrepancy between the local present-day geocentric axial dipole (GAD) inclination of 35.6° and the mean logging inclination of 22.7°. Furthermore, a systematic inclination decrease with depth is observed. Logging and core inclinations in HSDP-2 can be brought into agreement with core inclinations in the HSDP-1 pilot hole by shifting depths of HSDP-1 100 m downward. The correlation of inclination data between the two boreholes and the known age-depth relation of HSDP-1 is used to reexamine the age versus depth model curve for Mauna Kea of Sharp et al. (manuscript in preparation, 2003). We identify tentatively two logged inclination minima with excursion of the geomagnetic field in Brunhes, namely, Levantine at 360-370 ka and Unknown at 400-420 ka. © 2003 by the American Geophysical Union.
cited By 10

REE inverse modeling of HSDP2 basalts: Evidence for multiple sources in the Hawaiian plume - Feigenson, M.D.; Bolge, L.L.; Carr, M.J.; Herzberg, C.T. (Geochemistry, Geophysics, Geosystems, 2003)
The rare earth element (REE) concentrations of lavas from the Hawaiian Scientific Drilling Project (HSDP2) can be used to provide additional constraints on phase equilibria and the nature of the Hawaiian source. Major element analyses separate Mauna Kea lavas into two distinct populations, a high-silica and a low-silica suite. The low-silica samples can be separated stratigraphically into an upper low-silica alkalic series and a low-silica tholeiitic group that occurs deeper in the section. These contrasting groups could result from different extents of source partial fusion, or lithologically distinct source regions, or some combination of both factors. Petrologic modeling is performed to calculate that primary magma compositions contain about 20% MgO, and can be formed by 8-15% melting of a depleted mantle source for low-silica alkalic and high-silica lavas, respectively. The low-silica tholeiites could be generated by higher degrees of melting of a more fertile source. REE ratios and various isotopic systems reinforce the division of the low-SiO2 samples into the upper alkalic series, characterized by high Gd/Yb, and the deeper low-silica tholeiitic group, with low Gd/Yb. REE inverse modeling of fractionation-corrected basalts is consistent with lower degrees of melting to generate the late-stage alkalic lavas, with garnet present as a residual phase. The relatively constant Gd/Yb for low-silica tholeiites suggests that garnet is not an important residual phase during partial melting, implying higher extents of melting. The low-silica tholeiites are characterized by relatively enriched isotopic signatures that are consistent with contributions from a primitive source or from recycled subduction components. High 3He/4He associated with the lowsilica lavas could derive from primitive mantle, mass transfer from the core, or from a refractory lithospheric contribution to a recycled subduction package. However, the combination of major element, REE and isotopic data suggests that the deeper low-silica suite is sampling the relatively fertile, interior part of the Hawaiian plume, whereas the high-silica lavas are extracted from the more depleted periphery; later alkalic lavas are generated from a depleted source as the volcano moves off the hot spot. © 2003 by the American Geophysical Union.
cited By 43

Review of ground temperatures in the Mallik field area: A constraint to the methane hydrate stability - Majorowicz, J.A.; Smith, S.L. (Bulletin of the Geological Survey of Canada, 2003)
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.
cited By 1

The hydromechanical behaviour of a fracture: An in situ experimental case study - Cornet, F.H.; Li, L.; Hulin, J.-P.; Ippolito, I.; Kurowski, P. (International Journal of Rock Mechanics and Mining Sciences, 2003)
Relationships between flow distributions and mechanical opening in a single natural fracture are investigated in situ through field experiments, at a scale of about 1 m, in a granitic quarry. Experiments have been conducted at various injection flow-rates while the normal stress applied to the fracture was controlled by hydraulic flat jacks. Variations of the collected flow-rate (monitored through contiguous flow collectors distributed along the fracture periphery) with the injection pressure are fully reproducible. They show that the fracture opens only above a threshold pressure which increases with the externally applied stress. This threshold is non-zero with no applied flat jacks pressure which raises questions on the reliability of hydraulic jacking techniques for the measurement of the normal stress on preexisting fractures. It is shown that equivalent hydraulic aperture and mechanical opening are comparable only above a critical mean fracture opening estimated to be around 15-20 μm for the tested granite. For mean fracture openings smaller than this value, the standard time scale used for stress measurements distorts the results. It is also shown that channeling effects may control flow away from the injection hole so that the hydraulic jacking stress measurement technique may overestimate the mean normal stress acting on the fracture plane by as much as 4 MPa. It is concluded that hydraulic testing techniques for normal stress measurements should not include results from the fracture opening phase. Moreover, criteria should be established for validating results from the closing phase in order to demonstrate the absence of channelling effects. Finally, it is shown that, because of the elastic response of the rock, water injection in a fracture system decreases the interstitial pressure ahead of the increasing pressure front associated with the water flow. © 2003 Elsevier Ltd. All rights reserved.
Keywords: Elasticity; Fracture; Granite; Quarrying; Water; Hydromechanical behavior; fracture flow; hydromechanics; in situ stress; rock mechanics; stress measurement; Rock mechanics

Summary of recent research in Long Valley Caldera, California - Sorey, M.L.; McConnell, V.S.; Roeloffs, E. (Journal of Volcanology and Geothermal Research, 2003)
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.
cited By 21
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

Temperatures at the base of the Laurentide Ice Sheet inferred from borehole temperature data - Rolandone, F.; Mareschal, J.-C.; Jaupart, C. (Geophysical Research Letters, 2003)
We use temperature profiles from 4 deep (>1600 m) boreholes across Canada to determine ground surface temperature histories (GSTH's) through and after the Last Glacial Maximum (LGM). Inversion yields the temperature history at the base of the glacier and the surface temperature evolution after the glacial retreat. The results indicate geographic differences in basal temperature history across the Ice Sheet. During the Last Glacial Maximum, temperatures at the base of the Ice Sheet were lower in eastern Canada, at the southeastern edge of the glacier, than in central Canada, southwest of the glacier center. At all sites, basal temperatures were above the melting point of ice during and after the LGM, which may explain the highly unstable character of the Ice Sheet. The GSTH's are consistent with information on the history of the Laurentide ice sheet and provide quantitative constraints on glacier flow dynamics.
cited By 27
Keywords: Boreholes; Glaciers; Ice; Melting; Atmospheric temperature; Glacial geology; Surface properties; Melting points; Borehole temperature; Geographic difference; Ground surface temperature; Last Glacial Maximum; Laurentide ice sheets; Surface temperatures; Temperature history; Temperature profiles; Geophysics; Ice; basal temperature; Last Glacial Maximum; Laurentide Ice Sheet; paleoclimate; North America

The 320 kyr Pb isotope evolution of Mauna Kea lavas recorded in the HSDP-2 drill core - Eisele, J{urgen; Abouchami, Wafa; Galer, Stephen J.G.; Hofmann, Albrecht W. (Geochemistry, Geophysics, Geosystems, 2003)
[1] We analyzed Pb isotopic compositions of 50 samples from the HSDP-2 drill hole, covering the time interval 180 to 550 kyr B.P. in the stratigraphic record of Mauna Kea. All analyses were corrected for instrumental bias using a triple-spike technique. The aims of this study are to document temporal changes in sources contributing to Mauna Kea and to investigate how these may relate to the chemical structure of the Hawaiian plume. Lead isotopic compositions of the lavas have 206Pb/204Pb ratios ranging from 18.41 to 18.63, 207Pb/204Pb from 15.47 to 15.49, and 208Pb/ 204Pb from 37.97 to 38.22. In 207Pb/204Pb- 206Pb/204Pb space, the samples display a broad linear array, while three distinct arrays are found in 208Pb/ 204Pb-206Pb/204Pb space. These arrays can clearly be distinguished by their 208Pb/204Pb ratios and are referred to as Kea-lo8, Kea-mid8, and Kea-hi8. The 206Pb/204Pb isotope ratios exhibit rapid shifts by ̃0.2 over 100 m depth intervals, and jumps from one Pb isotope array to another and back in less than ̃100 m depth. Despite these rapid Pb isotope fluctuations, a particular Pb isotope array dominates over periods of several tens to hundreds of kiloyears. We interpret the Pb isotope arrays found in HSDP-2 in terms of mixing of end-members lying along the radiogenic and unradiogenic extensions of the arrays. At the radiogenic extension the three HSDP-2 arrays converge to a common end-member. The lower extensions of the arrays diverge in three directions, each with different 208Pb/ 204Pb ratios. This topology suggests that the HSDP-2 arrays were produced by mixing of at least four end-members. The origin of these end-members was investigated using Monte Carlo simulations of a Pb isotope evolution model. The simulations suggest that the common radiogenic end-member of the three Pb isotope arrays contains material with elevated m values and has a relatively young age (<1.5 Ga). Such a signature can be plausibly interpreted in terms of the presence of recycled oceanic crust in the source. The HSDP-2 Kea-lo8, Keamid8, and Kea-hi8 Pb isotope arrays dominate over different time periods and can be related to the displacement of Mauna Kea relative to the plume center over time. The Kea-lo8 array is present between ̃180 and 370 ka and samples more peripheral parts of the plume, while the Kea-mid8 and Kea-hi8 arrays occur in the deeper parts of the core (̃370 to 550 kyr ago), when Mauna Kea was closer to the plume center. Over the time intervals when each array dominates, we derive corresponding lengths of materials in the source by integrating the estimated upwelling velocity across the plume. These calculations suggest Pb isotope heterogeneities of at least several tens of kilometers in vertical length within the Hawaiian plume. The Pb isotope arrays may correspond to relatively small-scale heterogeneities derived from the D″ layer in the lower mantle. © 2003 by the American Geophysical Union.

The Gulf of Corinth: An active half graben? - Moretti, Isabelle; Sakellariou, D.; Lykousis, V.; Micarelli, L. (Journal of Geodynamics, 2003)
The Gulf of Corinth is often considered as a typical example of a more or less simple half graben with major border faults to the south and a flexure of the northern shore. This paper reviews new data, especially subsurface data, from both onshore and offshore, compiled or acquired through the Corinth Rift Laboratory EEC project. This data indicate that (1) the Gulf of Corinth is bordered both north and south by active faults; (2) there is a lot of them, and not only the one bordering the Peloponnese coastline are still active; and (3) distinct opening phases may be recognized on the area. During the first phase, the depocenter was located near the northern shore, whereas during the most recent phase, and only in the centre and the eastern sector, the depocenter moved towards the southern shore. Furthermore, active tectonic, in this western sector, is characterised by a general uplift of the Peloponnese that leads to the formation of new faults, i.e. the Doumena, Aigion and Helike faults and to selective reactivation of older ones such as the Pirgaki fault. We suggest that this current tectonic phase started about 150-120 000 years ago in the Aigion area and about 350 000 years ago eastward (Corinth-city) and that it represents a third opening phase during the growth of the Gulf of Corinth. © 2003 Elsevier Ltd. All rights reserved.
Keywords: Greece; Gulf of Corinth; active fault; deformation; faulting; graben; neotectonics; rift zone; tectonics

The heat transfer in the region of the Mauna Kea (Hawaii) - constraints from borehole temperature measurements and coupled thermo-hydraulic modeling - B{uttner, Grit; Huenges, Ernst (Tectonophysics, 2003)
The objectives of this paper are an understanding of the thermal and hydraulic field because of a negative temperature gradient and cold temperatures in the 1-km-deep borehole of the Hawaiian Scientific Drilling Project (HSDP), located near the coast line. The temperature pattern is attributed to a superposition of thermal and hydraulic processes. In the deeper borehole (HSDP-2, depth 3.1 km) detailed temperature monitoring was performed. Temperature measurements reveal two different thermal regimes. The upper part is characterised by cold temperatures and a negative temperature gradient similar to those observed in the shallow pilot borehole. Below 1100 m, increasing temperatures are observed. Different processes, such as topographically driven groundwater flow, ingress of salt water and conductive heat flow are investigated by numerical modeling. A pure conductive scenario fails to match the temperature measurements, implying that both borehole sections are overprinted by advective conditions. Coupled fluid and heat flow modeling with solute transport yield results that agree with observed temperatures. The results of these simulations suggest that meteoric water flow from the mountain range controls the thermal conditions in the upper part of the borehole. Below this level, the thermal regime is additionally affected by circulation of salt water from the nearby ocean. Each of these flow systems has been observed at other locations: topographically driven fresh water at locations with pronounced topography and ingress of salt water is typical for islands or coastal areas. At Hawaii, they coincide and influence each other, resulting in a salt water interface occurring at greater depth than expected. © 2003 Elsevier B.V. All rights reserved.
Keywords: Hawaii; Mauna Kea; United States; borehole; heat flow; heat transfer; hydraulic property; temperature gradient; thermal structure

The mechanics of unrest at Long Valley caldera, California. 2. Constraining the nature of the source using geodetic and micro-gravity data - Battaglia, M.; Segall, P.; Roberts, C. (Journal of Volcanology and Geothermal Research, 2003)
We model the source of inflation of Long Valley caldera by combining geodetic and micro-gravity data. Uplift from GPS and leveling, two-color EDM measurements, and residual gravity change determinations are used to estimate the intrusion geometry, assuming a vertical prolate ellipsoidal source. The U.S. Geological Survey occupied the Long Valley gravity network six times from 1980 to 1985. We reoccupied this network twice, in the summer of 1998 (33 stations), and the summer of 1999 (37 stations). Before gravity data can be used to estimate the density of the intrusion, they must be corrected for the effect of vertical deformation (the free-air effect) and changes in the water table. We use geostatistical techniques to interpolate uplift and water table changes at the gravity stations. The inflation source (a vertical prolate ellipsoid) is located 5.9 km beneath the resurgent dome with an aspect ratio equal to 0.475, a volume change from 1982 to 1999 of 0.136 km3 and a density of around 1700 kg/m3. A bootstrap method was employed to estimate 95% confidence bounds for the parameters of the inflation model. We obtained a range of 0.105-0.187 km3 for the volume change, and 1180-2330 kg/m3 for the density. Our results do not support hydrothermal fluid intrusion as the primary cause of unrest, and confirm the intrusion of silicic magma beneath Long Valley caldera. Failure to account for the ellipsoidal nature of the source biases the estimated source depth by 2.9 km (a 33% increase), the volume change by 0.019 km3 (a 14% increase) and the density by about 1200 kg/m3 (a 40% increase). © 2003 Elsevier B.V. All rights reserved.
cited By 113
Keywords: Deformation; Gravitational effects; Microgravity processing; Ellipsoidal sources; Geology; caldera; geodetic datum; geometry; geostatistics; GPS; gravity; modeling; uplift; California; Long Valley Caldera; United States

The suevite of drill hole Yucatàn 6 in the Chicxulub impact crater - Claeys, P.; Heuschkel, S.; Lounejeva-Baturina, E.; Sanchez-Rubio, G.; St{offler, D. (Meteoritics and Planetary Science, 2003)
The suevite breccia of the Chicxulub impact crater, Yucatàn, Mexico, is more variable and complex in terms of composition and stratigraphy than suevites observed at other craters. Detailed studies (microscope, electron microprobe, SEM, XRF) have been carried out on a noncontinuous set of samples from the drill hole Yucatàn 6 (Y6) located 50 km SW from the center of the impact structure. Three subunits can be distinguished in the suevite: the upper unit is a fine-grained carbonate-rich suevite breccia with few shocked basement clasts, mostly altered melt fragments, and formerly melted carbonate material; the middle suevite is a coarse-grained suevite with shocked basement clasts and altered silicate melt fragments; the lower suevite unit is composed of shocked basement and melt fragments and large evaporite clasts. The matrix of the suevite is not clastic but recrystallized and composed mainly of feldspar and pyroxene. The composition of the upper members of the suevite is dominated by the sedimentary cover of the Yucatàn target rock. With depth in well Y6, the amount of carbonate decreases and the proportion of evaporite and silicate basement rocks increases significantly. Even at the thin section scale, melt phases of different chemistry can be identified, showing that no widespread homogenization of the melt took place. The melt compositions also reflect the heterogeneity of the deep Yucatàn basement. Calcite with characteristic feathery texture indicates the existence of formerly pure carbonate melt. The proportion of carbonate to evaporite clasts is less than 5:1, except in the lower suevite where large evaporite clasts are present. This proportion constrains the amount of CO2 and SOx released by the impact event.
cited By 59

Three-dimensional crustal structure of Long Valley caldera, California, and evidence for the migration of CO2 under Mammoth Mountain - Foulger, G.R.; Julian, B.R.; Pitt, A.M.; Hill, D.P.; Shalev, E. (Journal of Geophysical Research: Solid Earth, 2003)
A temporary network of 69 three-component seismic stations captured a major seismic sequence in Long Valley caldera in 1997. We performed a tomographic inversion for crustal structure beneath a 28 km × 16 km area encompassing part of the resurgent dome, the south moat, and Mammoth Mountain. Resolution of crustal structure beneath the center of the study volume was good down to ∼3 km below sea level (∼5 km below the surface). Relatively high wave speeds are associated with the Bishop Tuff and lower wave speeds characterize debris in the surrounding moat. A low-Vp/Vs anomaly extending from near the surface to ∼1 km below sea level beneath Mammoth Mountain may represent a CO2 reservoir that is supplying CO2-rich springs, venting at the surface, and killing trees. We investigated temporal variations in structure beneath Mammoth Mountain by differencing our results with tomographic images obtained using data from 1989/1990. Significant changes in both Vp and Vs were consistent with the migration of CO2 into the upper 2 km or so beneath Mammoth Mountain and its depletion in peripheral volumes that correlate with surface venting areas. Repeat tomography is capable of detecting the migration of gas beneath active silicic volcanoes and may thus provide a useful volcano monitoring tool.
cited By 53
Keywords: caldera; crustal structure; seismic tomography; seismic velocity; velocity structure; California; Long Valley; Mammoth Mountain; United States

Trace element abundances of Mauna Kea basalt from phase 2 of the Hawaii Scientific Drilling Project: Petrogenetic implications of correlations with major element content and isotopic ratios - Huang, S.; Frey, F.A. (Geochemistry, Geophysics, Geosystems, 2003)
[1] The temporal geochemical variations defined by lavas erupted throughout the growth of a single volcano provide important information for understanding how the Hawaiian plume works. The Hawaii Scientific Drilling Project (HSDP) sampled the shield of Mauna Kea volcano to a depth of 3100 meters below sea level during Phase 2 of the HSDP. Incompatible element abundance ratios, such as La/Yb, Sm/ Yb, Nb/Zr, and Ti/Zr, in conjunction with SiO2 abundance and radiogenic isotopic ratios, especially He and Pb, in the reference sample suites of the Mauna Kea portion of cores from Phases 1 and 2 of the HSDP define three distinct geochemical groups. The upper 550 m of Mauna Kea lavas in the Phase 2 core include the Postshield Group with eruption ages of ̃200 ka to <370 ka. These lavas have relatively low SiO2 content, 3He/4He and 206Pb/204Pb, and they define a trend to relatively high La/Yb, Sm/Yb, and Nb/Zr. The eruption of these lavas coincides with migration of the Mauna Kea shield off the hot spot. As a result, extent of melting decreased, melt segregation occurred at greater depth, within the garnet stability field, and a geochemically distinct component associated with the periphery of the plume was sampled. Deeper in the Phase 2 core two other geochemical groups of lava are intercalated. One group has relatively low SiO2 abundance and high Nb/Zr Ti/Zr, 3He/4He and high 208Pb/204Pb at a given 206Pb/204Pb. These are distinctive geochemical characteristics of lavas erupted at Loihi seamount. Variations in incompatible element abundance ratios (e.g., Sm/Yb versus Nb/Zr and La/Yb versus Ti/Zr) define mixing trends between these low SiO2 lavas (Loihi-type) and lavas belonging to a high SiO2 group that are the dominant lava type in the shield part of the core (Kea-type). These two groups are presumed to reflect components intrinsic to the plume. Correlations of incompatible element abundance ratios, such as La/Nb, with radiogenic isotope ratios show that Hawaiian shields contain different proportions of geochemically distinctive components. The Koolau shield contains a recycled sedimentary component that is not present in the Mauna Kea shield. The anomalously high Ba/Th in Hawaiian lavas is inferred to be a source characteristic. Ba/Th is correlated with some radiogenic isotope ratios in Kilauea and Mauna Loa lavas, but there is no correlation in Mauna Kea lavas which range in Ba/Th by a factor of 2.6. Copyright 2003 by the American Geophysical Union.
cited By 75

Mid-holocene climate change in Lake Bosumtwi, Ghana - Russell, James; Talbot, Michael R.; Haskell, Brian J. (Quaternary Research, 2003)
Lake Bosumtwi is one of the most widely studied palaeoclimate archives in West Africa. Results from numerous AMS 14C dates of samples from four piston cores from Lake Bosumtwi show that an abrupt sedimentary transition from a mid-Holocene sapropel to calcareous laminated muds occurred at about 3200 cal yr B.P. High-resolution analyses of the nitrogen isotopic composition of organic matter across this transition confirm its abrupt nature, and suggest that the change may signal a step toward increased aridity and intensified surface winds that affected western equatorial Africa from Ghana to the Congo basin. Northern and Eastern Africa experienced a similar abrupt shift toward aridity during the late Holocene, but at about 5000 cal yr B.P., a difference in timing that illustrates the regional nature of climate changes during the Holocene and the importance of feedback mechanisms in regulating Holocene climate variability. Furthermore, an abrupt change at about 3000 cal yr B.P. occurs at several sites adjacent to the tropical and subtropical Atlantic, which may hint at major changes in the surface temperatures of the tropical Atlantic and/or Pacific at this time. © 2003 University of Washington. Published by Elsevier Inc. All rights reserved.
Keywords: Ghana; Lake Bosumtwi; aridity; Holocene; lacustrine deposit; nitrogen isotope; paleoclimate; paleolimnology

Modelling the thermal regime of permafrost and gas hydrate deposits to determine the impact of climate warming, mallik field area - Taylor, A.E. (Bulletin of the Geological Survey of Canada, 2003)
We apply a two-dimensional geothermal model to predict the permafrost and natural gas hydrate structure in theMallik field area, based on two paleoenvironmental scenarios deduced at otherwells in theMackenzieDelta area. ScenarioAindicated a subaerial history throughout theHolocene, and scenario B documented a several thousand year, subaqueous episode during theHolocene followed by recent subaerial exposure. The effects of these histories is limited largely to the 600mthick permafrost zone, with scenarioB predicting a substantial talik. The most defensible scenario can be resolved with ground temperatures or independent paleoenvironmental indicators. The effect of climatewarmingwill be apparent in awarming of the permafrost and, with marine transgression, creation of an underlying talik. Terrestrial methane hydrate deposits remain stable with increasing surface temperatures over several centuries, but the base of gas hydrate stability rises about 2 m after 300 years.
cited By 1

Structural properties of rift-related normal faults: The case study of the Gulf of Corinth, Greece - Micarelli, L.; Moretti, I.; Daniel, J.M. (Journal of Geodynamics, 2003)
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.
Keywords: Greece; Gulf of Corinth; deformation; faulting; neotectonics; rift zone; seismicity; tectonics

Microstructures, petrofabrics and seismic properties of ultra high-pressure eclogites from Sulu region, China: Implications for rheology of subducted continental crust and origin of mantle reflections - Ji, Shaocheng; Saruwatari, Kazuko; Mainprice, David; Wirth, Richard; Xu, Zhiqin; Xia, Bin (Tectonophysics, 2003)
Ultra high-pressure (UHP) eclogites from Sulu region (China) represent mafic components of the continental crust, which were first subducted to mantle depths greater than 100 km and then exhumed to the earth's surface. Detailed investigation of microstructures, chemical compositions, petrofabrics and seismic properties of the UHP eclogites can provide important information on the operating deformation mechanisms and rheology of subducted continental crust and on the origin of seismic reflections within the upper mantle. We present here results from field, optical and TEM observations, electron back-scattered diffraction (EBSD) measurements and numerical computations of the seismic properties of UHP eclogites collected from fresh surface outcrops at the drill site (Maobei, Donghai County, Jiangsu Province) of the Chinese Continental Scientific Drilling Program (CCSD). Two types of eclogites have been distinguished: Type-1 (coarse-grained) eclogites deformed by recovery-accommodated dislocation creep at the peak metamorphic conditions, and Type-2 (fine-grained) eclogites which are composed of reworked Type-1 materials during recrystallization-accommodated dislocation creep in shear zones which were active during the exhumation of the UHP metamorphic rocks. Both garnet and omphacite in these eclogites deformed plastically and the flow strength contrast between these two constituent minerals is apparently much less than an order of magnitude under the UHP metamorphic conditions. Plasticity of eclogites under UHP conditions can effectively facilitate channeled flow along the interplate shear zone. The preservation of the relict crustal materials within the continental lithosphere may produce regionally extensive, strong, seismic reflections in the upper mantle. This may explain the origin of mantle reflections observed in many areas of the world. © 2003 Elsevier B.V. All rights reserved.
Keywords: China; Sulu Belt; eclogite; mantle; microstructure; petrofabric; rheology; seismic reflection; subduction

Finding fault - Lee, A. (Engineer, 2003)
The project to determine the fault zone by San Andreas Fault Observatory at Depth (SAFOD) is discussed. The aim of the project is to drill straight into the heart of San Andreas Fault Zones and place sensors that will predict earthquake accurately. Engineers will be using SAFOD collected data to design roads and buildings to withstand the sort of earthquakes which they are going to expose.
cited By 0
Keywords: Earthquake faults; Drilling; Earthquakes; Investments; Project management; Seismology; Strategic planning; Seismic prospecting

Long continental records from Lake Baikal - Kashiwaya, Kenji (2003)

A late quaternary diatom record of tropical climatic history from Lake Titicaca (Peru and Bolivia) - Tapia, Pedro M.; Fritz, Sherilyn C.; Baker, Paul A.; Seltzer, Geoffrey O.; Dunbar, Robert B. (Palaeogeography, Palaeoclimatology, Palaeoecology, 2003)
A composite high-resolution diatom stratigraphy from three piston cores and one box-core in the deep sub-basin of Lake Titicaca reveals large moisture variations during the past 30 kyr in the Altiplano region. Diatom sequences indicate orbital and millennial-scale variability in water level and salinity. The pelagic freshwater diatom species Cyclotella andina and Cyclotella stelligera dominate Glacial-age sediments, suggesting that the lake was above its present outlet, Generally, wet conditions continued until 11 000 cal yr BP, as indicated by high percentages of freshwater planktonic diatoms. Large pulses of benthic diatom species between about 11 000 and 10 000 cal yr BP suggest brief intervals of large-amplitude declines in lake level. During the early Holocene (10 000-8500 cal yr BP), a freshwater diatom assemblage suggests overflowing conditions. Pelagic freshwater diatoms are replaced ca, 8500 cal yr BP by the salinity-indifferent species Cyclotella meneghiniana and by benthic taxa, indicating the beginning of lake regression. During the mid-Holocene (6000-3500 cal yr BP), the abundance of the saline taxon Chaetoceros muelleri, coupled with high abundances of epiphytic and epipelic diatoms, indicates maximum salinity and lowest lake levels in the entire 30 000 year record. Lake transgression began ca. 4000 cal yr BP, and the lake achieved modern levels by about 1500 cal yr BP. These water-level changes imply changes in effective moisture, most likely resulting from large precipitation changes. Precipitation was high throughout the Last Glacial Maximum (21 000-18 000 cal yr BP), likely due to an enhanced South American Summer Monsoon during peak summer insolation in the Southern Hemisphere. In contrast, the mid-Holocene transition was dryer than today in association with an austral summer insolation minimum and the subsequent weakening of the summer monsoon. © 2003 Elsevier Science B.V. All rights reserved.
Keywords: Bolivia; Lake Titicaca; Peru; Bacillariophyta; Chaetoceros; Chaetoceros muelleri; Chaetoceros muelleri; Cyclotella; Cyclotella andina; Cyclotella meneghiniana; Cyclotella stelligera; Cyclotella stelligera; biostratigraphy; diatom; lake level; Last Glacial Maximum; paleoclimate; Quaternary

A Look Inside the San Andreas fault at Parkfield Through Vertical Seismic Profiling - Chavarria, J.A.; Malin, P.; Catchings, R.D.; Shalev, E. (Science, 2003)
The San Andreas Fault Observatory at Depth pilot hole is located on the southwestern side of the Parkfield San Andreas fault. This observatory includes a vertical seismic profiling (VSP) array. VSP seismograms from nearby micro-earthquakes contain signals between the P and S waves. These signals may be P and S waves scattered by the local geologic structure. The collected scattering points form planar surfaces that we interpret as the San Andreas fault and four other secondary faults. The scattering process includes conversions between P and S waves, the strengths of which suggest large contrasts in material properties, possibly indicating the presence of cracks or fluids.
cited By 57
Keywords: Cracks; Earthquakes; Fluids; Scattering; Vertical seismic profiling (VSP); Seismology; geological structure; microearthquake; P-wave; S-wave; San Andreas Fault; vertical seismic profile; article; earthquake; electric resistance; geology; material state; priority journal; rock; sediment; United States; California; North America; Parkfield; United States

A vegetation and fire history of Lake Titicaca since the last glacial maximum - Paduano, Gina M.; Bush, Mark B.; Baker, Paul A.; Fritz, Sherilyn C.; Seltzer, Geoffrey O. (Palaeogeography, Palaeoclimatology, Palaeoecology, 2003)
Fine-resolution fossil pollen and charcoal analyses reconstruct a vegetation and fire history in the area surrounding Lake Titicaca (3810 m, Peru/Bolivia) since ca, 27 500 cal yr BP (hereafter BP). Time control was based on 26 accelerator mass spectrometer (AMS) radiocarbon dates. Seventeen AMS dates and 155 pollen and charcoal samples between ca. 17 500 BP and ca. 3100 BP allow a centennial-scale reconstruction of deglacial and early- to mid-Holocene events. Local and regional fire signals were based on the separation of two charcoal size fractions, ≥ 180 μm and 179-65 μm. Charcoal abundance correlated closely with the proportion of woody taxa present in the pollen spectra. Little or no pollen was detected in the sedimentary record prior to ca. 21 000 BP. Very cold climatic conditions prevailed, with temperatures suggested to be at least 5-8°C cooler than present. Increases in pollen concentration suggest initial warming at ca. 21 000 BP with a more significant transition toward deglaciation ca. 17 700 BP. Between 17 700 BP and 13 700 BP, puna brava is progressively replaced by puna and sub-puna elements. The most significant changes between the Pleistocene and the Holocene floras were largely complete by 13 700 BP, providing an effective onset of near-modern conditions markedly earlier than in other Andean records. Fire first occurs in the catchment at ca. 17 700 BP and becomes progressively more important as fuel loads increase. No evidence is found of a rapid cooling and warming coincident with the Younger Dryas chron. A dry event between ca. 9000 BP and 3100 BP, with a peak between 6000 and 4000 BP, is inferred from changes in the composition of aquatics, and the marsh community as pollen of Cyperaceae is replaced by Poaceae, Apiaceae, Plantago and the shrub Polylepis. Human disturbance of the landscape is evident in the pollen spectra after ca. 3100 BP with the appearance of weed species. © 2003 Elsevier Science B.V. All rights reserved.
Keywords: Bolivia; Lake Titicaca; Peru; Apiaceae; Apiales; Cyperaceae; Plantago; Plantago; Poaceae; Polylepis; Polylepis; deglaciation; fire history; Last Glacial Maximum; paleoclimate; pollen; Quaternary; vegetation history

Alteration of hyaloclastites in the HSDP 2 phase 1 drill core1. Description and paragenesis - Walton, A.W.; Schiffman, P. (Geochemistry, Geophysics, Geosystems, 2003)
[1] The core from the Hawaii Scientific Drilling Project 2 Phase 1 provides a unique opportunity for studying the low-temperature alteration processes affecting basalt in suboceanic-island environments. In hyaloclastites, which make up about one half of the lower 2 km of this core (the portion that accumulated below sea level), these processes have resulted in zones of incipient, smectitic, and palagonitic alteration. The alteration of sideromelane in these hyaloclastites has four distinct outcomes: dissolution, replacement by two different textural varieties of smectite (i.e., reddened and green grain-replacive), and conversion to palagonite. All samples show evidence of the incipient stage of alteration, suggesting that every sample passed through that zone. However, most samples that show palagonitic alteration do not also show evidence of smectitic alteration and vice versa, suggesting these two outcomes represent divergent paths of alteration. Incipient alteration (1080 to 1335 m depth) includes fracturing and mechanical reduction of porosity from 40-45% to about 20-30%; growth of one form of pore-lining smectite; dissolution of sideromelane; and formation of sideromelane-grain replacements consisting of Fe-hydroxide-strained smectite, titaniferous nodules, and tubules. DNA-specific stains and morphological features indicate that tubules are the result of microbial activity. Smectitic alteration (1405 to 1573 m) includes growth of a second variety of pore-lining smectite, pore-filling and grain-replacing smectite, and cements of phillipsite and Ca-silicate minerals. Palagonitic alteration (1573 m to the deepest samples) includes replacement of margins of shards with palagonite and growth of pore-filling chabazite. The porosity is reduced by cementation to less than 4% at 1573 m. Porosity does not decrease further down hole, nor does the thickness of palagonite rims on shards increase through the zone of palagonitic alteration. In these samples, palagonite is not an intermediate alteration product in the development of smectite. Rather, in hyaloclastites from the HSDP core, palagonite has formed after all observed smectites. Current downhole temperatures at the boundaries between the three alteration zones are in the range from 12° to 15°C, suggesting that geochemical thresholds or vital effects, not temperature conditions, control different outcomes of alteration. © 2003 by the American Geophysical Union.
cited By 74

Brief climate events in the sedimentary record of Lake Baikal between 130 and 70 kyr BP - Prokopenko, A.A.; Karabanov, E.B.; Kuz'min, M.I.; Williams, D.F.; Khursevich, G.K. (Geologiya i Geofizika, 2003)
The paper presents new biogenic silica (BiSi) and total organic carbon (TOC) data for the past 130 kyr from BDP-98 and BDP-96-2 cores, respectively, showing a distinct climate periodicity correlated with Late Pleistocene rhythms. The high-resolution sedimentary records from Lake Baikal allow approaching the problem of the length of the last interglacial and stability of its climate in the Northern Hemisphere. The Siberian archives for the interval of 70 to 130 kyr contain brief sub-Milankovitch millenial-scale excursions correlated with events in the high-resolution records of North Atlantic ice cores and European continental pollen sequences. The correlation of the mid-Eemian cooling about 122 kyr BP and the cold Montaigu event about 103 kyr BP with low production signals in the Baikal cores evidence for a climatic connection between the geographically distant North Atlantic, continental Europe, and continental Siberia regions. This connection was well pronounced during interglacials and interstadials and weak during the later glaciation. Rapid warm/cold transitions in the Baikal record, especially the Kazantsevo/Early Zyryanka transition, indicate that glaciation in Siberia began at 115-116 kyr BP, or 5-8 kyr earlier than in Europe and in the North Atlantic, as inferred from independent age models. The continuous climate record from Lake Baikal provides a solid stratigraphic background for detailed correlation of Siberian continental sections.
Keywords: Russian Federation; glacial-interglacial cycle; lacustrine deposit; paleoclimate; Quaternary; stratigraphic correlation

Characteristic magnetic behavior of subaerial and submarine lava units from the Hawaiian Scientific Drilling Project (HSDP-2) - Kontny, A.; Vahle, C.; De Wall, H. (Geochemistry, Geophysics, Geosystems, 2003)
This study presents rock magnetic properties and the magnetic mineralogy of subaerial and submarine lava flows of Mauna Loa and Mauna Kea volcanoes collected from the 3109 m deep HSDP-2 drill hole in Hawaii. Three different groups of magnetic behavior are recognized in the subaerial lava flows related to the degree of high temperature oxidation during extrusion. Group 1 shows homogenous titanomagnetite with low Xmt, low Curie temperatures (TC: 100°-200°C) and weak median demagnetizing fields ( < 20 mT). Further subdivision into 1a and 1b subgroups is based on the low temperature behavior of magnetic susceptibility (MS) and hysteresis loops, which indicate a contribution from ferrimagnetic Cr-Al spinel below ca.-160°C in the 1b-type samples. Group 2 samples, with exsolution lamellae of ilmenite in the titanomagnetites, have higher TC (480°-580°C) and higher coercive forces (20-40 mT). Group 3, the highest oxidation stage, is characterized by titanohematite-bearing assemblages with enhanced median demagnetizing fields (35-85 mT) and a significantly different low-temperature MS behavior. MS core logging shows a systematic variation occurs in the subaerial lava flows, directly related to the degree of high temperature oxidation and their flow morphology. Aa lava flows have higher mean MS than other lava flow types. Besides these factors, MS appears to be also affected by the magma composition of the various shield-building stages. Mauna Loa subaerial lava flows generally show lower mean susceptibilities (4.6 ± 3 × 10-3 SI) than subaerial Mauna Kea lava flows (9.8 ± 5 × 10-3 SI). As submarine lava flows show no group 3 assemblages no high temperature oxidation influenced these rocks. Some hyaloclastites and pillow breccias show low MS ( < 1 × 10-3 SI), small amounts of nearly pure magnetite (TC = 580°C) and high coercive forces up to 110 mT suggesting single domain and/or superparamagnetic behavior. The controlling mechanism of the magnetic properties in the submarine lava units is the cooling and quenching rate of lava flows, which creates large grain size variations in titanomagnetites of varying compositions. Hydrothermal alteration, as described from ocean floor or Icelandic basalts, is not an important process that influences the magnetic properties in the ocean island basalts from the HSDP-2 drill hole. © 2003 by the American Geophysical Union.
cited By 37

Earthquake locations and three-dimentional fault zone structure along the creeping section of the San Andreas fault near Parkfield, CA: Preparing for SAFOD - Thurber, C.; Roecker, S.; Roberts, K.; Gold, M.; Powell, L.; Rittger, K. (Geophysical Research Letters, 2003)
Arrival-time data from 453 local earthquakes and 6 explosions at Parkfield, CA, are inverted for earthquake locations and three-dimensional Vp and Vp/Vs structure. The structure is dominated by the velocity contrast across the SAF, with the southwest side about 20-25% faster, consistent with previous studies. Nearly all the earthquakes occur almost directly beneath the fault trace. We find high-Vp/Vs anomalies that correlate with low-resistivity features in a magnetotelluric model that are interpreted to represent fluids. We locate a magnitude 2 earthquake that is a potential target event for the final stage of SAFOD drilling, yielding a depth of 3.1 km below surface and an epicenter 100 m southwest of the fault trace. Nonlinear analyses indicate 95%-confidence relative and absolute uncertainties on the order of 500-700 m vertically and 200-300 m horizontally for this target earthquake.
cited By 91
Keywords: Data reduction; Drilling; Explosions; Geophysics; Arrival-time data; Earthquakes; earthquake epicenter; San Andreas Fault; seismicity; velocity structure; United States

Erratum: Paleointensity in Hawaiian Scientific Drilling Project Hole (HSDP2): Results from submarine basaltic glass (Geochemistry, Geophysics, Geosystems (2003) 4:5 (1042) DOI:10.1029/2001GC000276) - Tauxe, L.; Love, J.J. (Geochemistry, Geophysics, Geosystems, 2003)
cited By 0

Crosshole waveform tomography velocity and attenuation images of arctic gas hydrates - Gerhard Pratt, R.; Bauer, K.; Weber, M. (2003 SEG Annual Meeting, 2003)
Time-lapse crosshole data were acquired at the Mallik Field, Northwest Territories (Canada) as a part of a large-scale research project into the production potential of arctic gas hydrates. Waveform tomography was used to form high-resolution quantitative images of the velocity and attenuation factors. The hydrate-bearing sediments are characterized by an expected increase in velocity, and an unusual increase in attenuation. The layers are predominantly 1-D, with subtle lateral changes in velocity and attenuation. © 2003 SEG Annual Meeting. All rights reserved.
cited By 5
Keywords: Hydration; Petroleum prospecting; Tomography; Velocity; Attenuation factors; High resolution; Hydrate bearing sediments; Large-scale research; Quantitative images; Wave forms; Gas hydrates

Diatom turnover in the early Paleogene diatomite of the Sengiley section, middle Povolzhie, Russia: A response to the initial Eocene thermal maximum? - Oreshkina, Tatiana V.; Oberhaensli, Hedi (Causes and consequences of globally warm climates in the early Paleogene, 2003)
Full article available in PDF version.

Inferences on the hydrothermal system beneath the resurgent dome in Long Valley Caldera, east-central California, USA, from recent pumping tests and geochemical sampling - Farrar, C.D.; Sorey, M.L.; Roeloffs, E.; Galloway, D.L.; Howle, J.F.; Jacobson, R. (Journal of Volcanology and Geothermal Research, 2003)
Quaternary volcanic unrest has provided heat for episodic hydrothermal circulation in the Long Valley caldera, including the present-day hydrothermal system, which has been active over the past 40 kyr. The most recent period of crustal unrest in this region of east-central California began around 1980 and has included periods of intense seismicity and ground deformation. Uplift totaling more than 0.7 m has been centered on the caldera's resurgent dome, and is best modeled by a near-vertical ellipsoidal source centered at depths of 6-7 km. Modeling of both deformation and microgravity data now suggests that (1) there are two inflation sources beneath the caldera, a shallower source 7-10 km beneath the resurgent dome and a deeper source ∼15 km beneath the caldera's south moat and (2) the shallower source may contain components of magmatic brine and gas. The Long Valley Exploration Well (LVEW), completed in 1998 on the resurgent dome, penetrates to a depth of 3 km directly above this shallower source, but bottoms in a zone of 100°C fluid with zero vertical thermal gradient. Although these results preclude extrapolations of temperatures at depths below 3 km, other information obtained from flow tests and fluid sampling at this well indicates the presence of magmatic volatiles and fault-related permeability within the metamorphic basement rocks underlying the volcanic fill. In this paper, we present recently acquired data from LVEW and compare them with information from other drill holes and thermal springs in Long Valley to delineate the likely flow paths and fluid system properties under the resurgent dome. Additional information from mineralogical assemblages in core obtained from fracture zones in LVEW documents a previous period of more vigorous and energetic fluid circulation beneath the resurgent dome. Although this system apparently died off as a result of mineral deposition and cooling (and/or deepening) of magmatic heat sources, flow testing and tidal analyses of LVEW water level data show that relatively high permeability and strain sensitivity still exist in the steeply dipping principal fracture zone penetrated at a depth of 2.6 km. The hydraulic properties of this zone would allow a pressure change induced at distances of several kilometers below the well to be observable within a matter of days. This indicates that continuous fluid pressure monitoring in the well could provide direct evidence of future intrusions of magma or high-temperature fluids at depths of 5-7 km. © 2003 Elsevier B.V. All rights reserved.
cited By 14
Keywords: Geochemistry; Metamorphic rocks; Mineral resources; Seismology; Hydrothermal circulation; Volcanoes; caldera; fluid flow; geochemistry; hydraulic property; hydrology; hydrothermal system; volcanism; well testing; California; Long Valley Caldera; United States

Lithium isotope geochemistry of the Hawaiian plume: Results from the Hawaii scientific drilling project and koolau volcano - Chan, L.-H.; Frey, F.A. (Geochemistry, Geophysics, Geosystems, 2003)
[1] We determined lithium isotopic compositions of Mauna Loa and Mauna Kea basalts from the 3.1 km drill hole of the Hawaiian Scientific Drilling Project (HSDP); for comparison Li isotopic ratios were also determined for basalts from Koolau volcano. These two suites of samples define geochemical extremes in the range of Hawaiian shield lavas. The 400 Ka record of Mauna Kea in the HSDP core shows temporal fluctuations between low δ7Li (̃4% relative to the L-SVEC standard) and high δ7Li (5-6%), suggesting that the source components in the Hawaiian plume are heterogeneous in Li isotopic composition. Based on SiO2 content and isotopic ratios of He, Li, Nd, Hf and Pb, three geochemical groups are identified in Mauna Kea lavas. Mauna Kea basalts between 1900 and 2500 mbsl have relatively low δ7Li of about 4%. They are low SiO2 lavas distinguished by the highest 3He/4He and 208Pb/204Pb, and low 176Hf/ 177Hf and 143Nd/144Nd. Like basalt from Loihi seamount, this Mauna Kea group is considered to originate from the core of the plume. Above 1900 mbsl, high δ7Li lavas with high SiO 2 contents appear in both the submarine and subaerial sections. They are marked by low 3He/4He and high 176Hf/177Hf. The 7Li-rich signature of some samples (δ7Li up to 5.7) is indicative of recycled oceanic crust in the plume. This magma group defines the Kea component. The low SiO2 lavas in the subaerial section have low δ7Li (̃4%), 3He/4He and 208Pb/204Pb. Their δ7Li values overlap the range of δ7Li in unaltered mid-ocean ridge basalt (MORB) and are consistent with upper mantle material entrained by the plume or contamination of plume-derived magmas by the Pacific lithosphere. The δ7Li of Koolau lavas mostly fall within the range of 4.5 ± 0.3%. Exceptions are two samples that have δ7Li of 2-3%. The lightest isotopic values may indicate subducted Li that was isotopically fractionated during slab dehydration. In contrast to other isotopic systems, most Koolau samples, however, resemble Mauna Kea samples in Li isotopic composition. Mauna Loa samples have δ7Li values of 3.5 to 4.9%, within the range of the Koolau and Mauna Kea lavas. Based on these data, the Loa trend volcanoes and Kea trend volcanoes have largely overlapping Li isotopic compositions. In summary, the Hawaiian plume is not highly variable in Li isotopic composition; δ7Li is typically ̃4% with perturbations by subducted components to lower and higher ratios (2.5 to 5.7%). The overlap of most Hawaiian basalt and MORB in their range of Li isotopic ratios suggests minor influence of recycled oceanic crust in the plume and perhaps similar Li isotopic ratios in the upper and lower mantle. © 2003 by the American Geophysical Union.
cited By 118

Fluid flow in the resurgent dome of Long Valley Caldera: Implications from thermal data and deep electrical sounding - Pribnow, D.F.C.; Sch{utze, C.; Hurter, S.J.; Flechsig, C.; Sass, J.H. (Journal of Volcanology and Geothermal Research, 2003)
Temperatures of 100°C are measured at 3 km depth in a well located on the resurgent dome in the center of Long Valley Caldera, California, despite an assumed >800°C magma chamber at 6-8 km depth. Local downflow of cold meteoric water as a process for cooling the resurgent dome is ruled out by a Pecl'et-number analysis of temperature logs. These analyses reveal zones with fluid circulation at the upper and lower boundaries of the Bishop Tuff, and an upflow zone in the metasedimentary rocks. Vertical Darcy velocities range from 10 to 70 cm a-1. A 21-km-long geoelectrical profile across the caldera provides resistivity values to the order of 100 to >103 Ωm down to a depth of 6 km, as well as variations of self-potential. Interpretation of the electrical data with respect to hydrothermal fluid movement confirms that there is no downflow beneath the resurgent dome. To explain the unexpectedly low temperatures in the resurgent dome, we challenge the common view that the caldera as a whole is a regime of high temperatures and the resurgent dome is a local cold anomaly. Instead, we suggest that the caldera was cooled to normal thermal conditions by vigorous hydrothermal activity in the past, and that a present-day hot water flow system is responsible for local hot anomalies, such as Hot Creek and the area of the Casa Diablo geothermal power plant. The source of hot water has been associated with recent shallow intrusions into the West Moat. The focus of planning for future power plants should be to locate this present-day flow system instead of relying on heat from the old magma chamber. © 2003 Elsevier B.V. All rights reserved.
cited By 34
Keywords: Cooling; High temperature effects; Sediment transport; Water; Magma chambers; Geothermal fields; caldera; electrical resistivity; fluid flow; geochemistry; hydrothermal fluid; thermal regime; California; Long Valley Caldera; United States

Late Quaternary clay mineral record in Central Lake Baikal (Academician Ridge, Siberia) - Fagel, N.; Boski, T.; Likhoshway, L.; Oberhaensli, H. (Palaeogeography, Palaeoclimatology, Palaeoecology, 2003)
We investigated the mineralogical composition of two cores recovered on the Academician Ridge (Central Lake Baikal, Siberia). Sedimentological features show that the cores are unaffected by turbidity currents. However, hemipelagic deposition is not continuous, but intermittently disturbed by syn- or post-sediment reworking (e.g., bioturbation, slumps, faulting). Such modes of deposition are consistent with the complex uplift history of the ridge. Bulk mineralogy suggests that terrigenous sediment supplies are constant through glacial/interglacial stages, and diluted by diatom-rich intervals related to warmer interglacial stages. The core stratigraphy is based on the correlation of the diatom zonation and opal abundance with the marine oxygen isotope reference curve SPECMAP. The ∼8-m cores partly recover the last four interglacial/glacial cycles, i.e., since oxygen isotope stage 8. We test the use of clay minerals as a proxy for paleoclimatic reconstruction. The clays are more weathered during the diatom-rich intervals in agreement with warmer climate conditions. However, the mean clay composition does not change significantly through glacial/interglacial stages. This observation implies that, in the Academician Ridge sediments, a simple smectite/illite ratio (S/I) does not alone provide a reliable indicator of climatic variation. It reflects the complex clay assemblages, especially the smectite group, delivered to Central Lake Baikal. Smectites include primarily illite-smectite mixed layers, made of a mixture of montmorillonite and beidellite. According to their behavior after cation saturation, the illite-smectite mixed layers are primarily transformed smectites, with some neoformed smectites intermittently observed. In addition, Al-smectites occur in minor proportions. We conclude that the S/I ratio has a climatic significance only if it evolves in parallel with the weathering stage of the clays and is confirmed by a change in the composition of the smectites. © 2002 Elsevier Science B.V. All rights reserved.
Keywords: Russian Federation; Bacillariophyta; clay mineral; illite; lacustrine deposit; paleoclimate; proxy climate record; Quaternary; smectite

Investigating a 65-Ma-Old smoking gun: Deep drilling of the chicxulub impact structure - Dressler, B.O.; Sharpton, V.L.; Morgan, J.; Buffler, R.; Moran, D.; Smit, J.; St{affler, D.; Urrutia, J. (Eos, 2003)
cited By 64

Late Quaternary lake-level changes constrained by radiocarbon and stable isotope studies on sediment cores from Lake Titicaca, South America - Rowe, Harold D.; Guilderson, Thomas P.; Dunbar, Robert B.; Southon, John R.; Seltzer, Geoffrey O.; Mucciarone, David A.; Fritz, Sherilyn C.; Baker, Paul A. (Global and Planetary Change, 2003)
We present and compare AMS-14C geochronologies for sediment cores recovered from Lake Titicaca, South America. Radiocarbon dates from three core sites constrain the timing of late Quaternary paleoenvironmental changes in the Central Andes and highlight the site-specific factors that limit the radiocarbon geochronometer. With the exception of mid-Holocene sediments, all cores are generally devoid of macrophyte fragments, thus bulk organic fractions are used to build core chronologies. Comparisons of radiocarbon results for chemically defined fractions (bulk decalcified, humate, humin) suggest that ages derived from all fractions are generally coherent in the post-13,500 yr BP time interval. In the pre-13,500 yr BP time interval, ages derived from humate extracts are significantly younger (300-7000 years) than ages from paired humin residues. Gross age incoherencies between paired humate and humin sub-fractions in pre-13,500 yr BP sediments from all core sites probably reflect the net downward migration of humates. Ages derived from bulk decalcified fractions at our shallow water (90 m) and deep water (230 m) core sites consistently fall between ages derived from humate and humin sub-fractions in the pre-13,500 yr BP interval, reflecting that the bulk decalcified fraction is predominantly a mixture of humate and humin sub-fractions. Bulk decalcified ages from the pre-13,500 yr BP interval at our intermediate depth core site (150 m) are consistently older than humate (youngest) and humin sub-fractions. This uniform, reproducible pattern can be explained by the mobilization of a relatively older organic sub-fraction during and after the re-acidification step following the alkaline treatment of the bulk sediment. The inferred existence of this 'alkali-mobile, acid-soluble' sub-fraction implies a different depositional/post-depositional history that is potentially associated with a difference in source material. While internally consistent geochronologies can be developed for the Lake Titicaca sequence using different organic fractions, mobile organic sub-fractions and fractions containing mobile sub-fractions should generally be avoided in geochronology studies. Consequently, we believe humin and/or bulk decalcified ages provide the most consistent chronologies for the post-13,500 yr BP interval, and humin ages provide the most representative ages for sedimentation prior to 13,500 yr BP interval. Using the age model derived from the deep water core site and a previously published isotope-based lake-level reconstruction, we present a qualitative record of lake level in the context of several ice-core records from the western hemisphere. We find the latest Pleistocene lake-level response to changing insolation began during or just prior to the Bølling/Allerød period. Using the isotope-based lake-level reconstruction, we also find the 85-m drop in lake level that occurred during the mid-Holocene was synchronous with an increase in the variability of ice-core δ18O from a nearby icecap, but was not reflected in any of the polar ice-core records recovered from the interior of Antarctica and Greenland. © 2003 Published by Elsevier B.V.
Keywords: Lake Titicaca; South America; geochronology; lake level; paleolimnology; radiocarbon dating; sediment core; stable isotope

High-resolution MIS 11 record from the continental sedimentary archive of Lake Baikal, Siberia - Karabanov, Eugene B.; Prokopenko, Alexander A.; Williams, Douglas F.; Khursevich, Galina K.; Kuzmin, Mikhail I.; Bezrukova, Elena V.; Gvozdkov, Alexander N. (Geophysical Monograph Series, 2003)

Hawaiian lava flows in the third dimension: Identification and interpretation of pahoehoe and ̀àa distribution in the KP-1 and SOH-4 cores - Katz, M.G.; Cashman, K.V. (Geochemistry, Geophysics, Geosystems, 2003)
Hawaiian lava flows are classified as pahoehoe or ̀àa by their surface morphology. As surface morphology reflects flow emplacement conditions, the surface distribution of morphologic flow types has been used to study the evolution and eruptive history of basaltic volcanoes. We extend this analysis to the third dimension by determining the distribution of flow types in two deep drill cores, the Scientific Observation Hole-4 (SOH-4) core, drilled near Kilauea's East Rift Zone (ERZ), and the pilot hole (Kahi Puka-1 (KP-1)) for the Hawaiian Scientific Drilling Project (HSDP), drilled through distal flows from Mauna Loa and Mauna Kea. Flows are classified using both internal structures and groundmass textures, with the latter useful when identification based on mesoscopic flow features (e.g., surface morphology and vesicle content and distribution) is ambiguous. We then examine the temporal distribution of pahoehoe and ̀àa flows in proximal (SOH-4) and distal (KP-1) settings. Sequence analysis shows that the two flow types are not randomly distributed in either core but instead are strongly clustered. The proximal SOH-4 core is dominated by thin pahoehoe flows (̃60% by volume), consistent with the common occurrence of surface-fed pahoehoe flows in near-vent settings. The distal KP-1 core has a high proportion of ̀àa (̃58% by volume), although the proportion of pahoehoe and̀àa varies dramatically throughout the Mauna Kea sequence. Thick inflated pahoehoe flows dominate when the drill site was near sea level, consistent with the numerous inflated pahoehoe fields on the current coastal plains of Kilauea and Mauna Loa. ̀Àa flows are abundant when the site was far above sea level. As slope increases from the coastal plains to Mauna Kea's flank, this correlation may reflect the combined effect of long transport distances and increased slopes on flow emplacement. These results demonstrate that flow type and thickness variations in cores provide valuable information about both vent location and local site environment. Observed variations in flow type within the KP-1 core raise interesting questions about feedback between volcano evolution and flow morphology and suggest that flow type is an important variable in models of volcano growth and related models for lava flow hazard assessment. © 2003 by the American Geophysical Union.
cited By 40

Hawaiian hot spot dynamics as inferred from the Hf and Pb isotope evolution of Mauna Kea volcano - Janne, Blichert-Toft; Weis, Dominique; Maerschalk, Claude; Agranier, Arnaud; Albarède, Francis (Geochemistry, Geophysics, Geosystems, 2003)
The present work reports multiple collector inductively coupled plasma mass spectrometry (MC-ICPMS) measurements of the isotopic compositions of Hf and Pb in the first 3 km of the deep core retrieved by the Hawaii Scientific Drilling Project. The measurements cover all the samples from the standard geochemical reference set, glasses from the deep hole, and replicates from the pilot hole. Both Hf and Pb are less radiogenic in Mauna Loa compared to Mauna Kea. The transition between Mauna Kea and Mauna Loa lavas in the deep core is progressive for eHf and 208Pb/204Pb, but a sharp discontinuity is observed for 208Pb*/206Pb*. There is no correlation between the alkalinity of the samples and isotopic composition. In detail, the Hf isotope compositions of samples from the pilot hole are not all identical to those of the HSDP-2 core for samples retrieved from a similar depth, suggesting that steep topography existed at the time of emplacement or that a different eruptive sequence was recorded. The strong correlation between 208Pb*/206Pb* and 3He/ 4He (He data from M. D. Kurz et al. (Rapid helium isotopic variability in Mauna Kea shield lavas from the Hawaiian Scientific Drilling Project, submitted to Geochemistry Geophysics Geosystems, 2002)) requires the episodic incorporation of a component that resembles the basalts erupted by either Kilauea or the Loihi eruptive centers (this component is referred to as K/L). The data suggest that some 500 kyr ago, Mauna Kea was tapping a mantle source similar to that tapped by Kilauea today. Isotopic variability of Pb and He cannot be accounted for by radiogenic ingrowth in a closed system, but requires the mixing of mantle source components with distinct outgassing histories. The time series of isotopic and concentration data in Mauna Kea samples spanning about 350,000 years of age indicate the recurrence of geochemical patterns in the melting column. Ignoring the most recent alkalic samples, we find that the dominant fluctuations of eHf and 207Pb/204Pb correspond to a period of 50,000 years. For La/ Yb, Zr/Nb, 87Sr/ 86Sr, 206Pb/204Pb, 207Pb/ 206Pb, and 208Pb/206Pb, a dominant period of ca. 18,000 years is obtained. Once provision is made for the existence of harmonics, the consistency between the isotopic spectrum of the pilot hole and the HDSP-2 core is very good. The input of the K/L component does not seem to be periodic. We use these recurrence intervals in conjunction with the upwelling rate deduced from buoyancy flux and seismic evidence of the maximum dimension of scatterers to constrain the radius of the Hawaiian plume conduit to be in the range of 10-50 km and the upwelling velocity to be in the range of 0.13-3 m/yr. Plausible vertical length scales of heterogeneities in the conduit are 6.5-160 km. © 2003 by the American Geophysical Union.

Geology and geochemistry of shallow drill cores from the Bosumtwi impact structure, Ghana - Boamah, Daniel; Koeberl, Christian (Meteoritics and Planetary Science, 2003)
The 1.07 Ma well-preserved Bosumtwi impact structure in Ghana (10.5 km in diameter) formed in 2 Ga-old metamorphosed and crystalline rocks of the Birimian system. The interior of the structure is largely filled by the 8 km diameter Lake Bosumtwi, and the crater rim and region in the environs of the crater is covered by tropical rainforest, making geological studies rather difficult and restricted to road cuts and streams. In early 1999, we undertook a shallow drilling program to the north of the crater rim to determine the extent of the ejecta blanket around the crater and to obtain subsurface core samples for mineralogical, petrological, and geochemical studies of ejecta of the Bosumtwi impact structure. A variety of impactite lithologies are present, consisting of impact glass-rich suevite and several types of breccia: lithic breccia of single rock type, often grading into unbrecciated rock, with the rocks being shattered more or less in situ without much relative displacement (autochthonous?), and lithic polymict breccia that apparently do not contain any glassy material (allochtonous?). The suevite cores show that melt inclusions are present throughout the whole length of the cores in the form of vesicular glasses with no significant change of abundance with depth. Twenty samples from the 7 drill cores and 4 samples from recent road cuts in the structure were studied for their geochemical characteristics to accumulate a database for impact lithologies and their erosion products present at the Bosumtwi crater. Major and trace element analyses yielded compositions similar to those of the target rocks in the area (graywacke-phyllite, shale, and granite). Graywacke-phyllite and granite dikes seem to be important contributors to the compositions of the suevite and the road cut samples (fragmentary matrix), with a minor contribution of Pepiakese granite. The results also provide information about the thickness of the fallout suevite in the northern part of the Bosumtwi structure, which was determined to be ≤15 m and to occupy an area of ∼1.5 km2. Present suevite distribution is likely to be caused by differential erosion and does not reflect the initial areal extent of the continuous Bosumtwi ejecta deposits. Our studies allow a comparison with the extent of the suevite at the Ries, another well-preserved impact structure.
Keywords: Bosumtwi Impact Structure; Ghana; breccia; ejecta; geochemistry; geological structure; impact structure; tektite

Preservation of primary volcanic textures in the ultrahigh-pressure terrain of Dabie Shan - Oberh{ansli, R.; Martinotti, G.; Schmid, R.; Liu, X. (Geology, 2002)
Delicate primary volcanic features such as ash layers, volcaniclastic breccias, and pillow lavas with amygdaloidal rims have been identified in a terrigenous volcanosedimentary sequence forming part of the ultrahigh-pressure unit of the Dabie Shan, China. The presence of coesite relies in crosscutting dikes confirms an ultrahigh-pressure evolution for the entire sequence. Evaluation of the probable reaction history for zeolite compositions as supposed for the amygdaloidal rims reveals a scarcity of reactions at very high pressures. Such special conditions of low reactivity and fluid- conserving reactions, combined with a lack of directed stress, have allowed the remarkable preservation of these primary volcanic features despite deep subduction.
Keywords: China; High pressure effects; Levees; Zeolites; Pillow lava; pillow lava; preservation; ultrahigh pressure metamorphism; volcanic feature; volcaniclastic deposit; Volcanoes

The Corinth Rift Laboratory: Monitoring of active faults - Moretti, I.; Delhomme, J.P.; Cornet, F.; Bernard, P.; Schmidt-Hattenberger, C.; Borm, G. (First Break, 2002)
In their aim to understand the hydraulic behaviour of faults and fractures, and their changes with stress variation, geoscientists are still poorly armed. On the one hand, data indicate that the rheology of faulted strata plays a major role: shale may create clay smearing (Lehner & Pilaar 1997), whereas, at temperatures over 80/100 °C, pressure solution processes quickly seal fractures in sandstones after rupture (Morentti et al. 2000; Labaume & Moretti 2001). The hydraulic behaviour of discontinuities also appears to be stress-dependent (Sibson 1994), and may therefore change during the depletion of oil and gas fields. We are still unable to quantify these changes and indicate eventualthresholds. On the other hand, fluid pressureinfluences the friction on the fault planes, and therefore changes the rate of displacement. A large number of seismologists consider that the low average friction coefficient in large fault zones is due to high fluid pressure (Evans 1992 and many others). The permanent and instantaneous hydraulic behaviour of faults and fractures under a stress/strain regime obviously differs due to diagenetic processes, but the rate of the fluid/rock interactions which could be related to the build up of high pressures in the faults themselvesis still a subject of debate. In order to clarify these issued, a complete dataset is necessary. European academicand private laboratories (as well as someoil companies) have decided to pool their efforts to collect such a dataset by creating the Corinth Rift Laboratory (CRL), with the help of the EC. The goal is to investigate fault mechanics and their relation to fluid flow and earthquakes by continuous monitoring of strain, seismicity, fluid pressure and geochemistry - at the surface and at various depths in boreholes intersecting active faults. Due to the volume of data collected and the necessity of disctributing these data to various centres, research in data management is also being carried out. Around the world, two other ambitious projects have been set up with the same goals but in different geological contexts: one to monitor the San Andreas fault, which is a strike-slip fault outcropping in granite, and a second to drill the subduction zone in Japan. In our case, we will drill extensional faults through sedimentary rocks; mainly limestones.
Keywords: active fault; hydraulic property; pressure solution; rheology; stress change

Temperature Memory Survey Results and Future Plan for the Recently Erupted Unzen Volcano - Saito, S.; Henfling, J. (Transactions - Geothermal Resources Council, 2002)
The International Scientific Drilling Project to drill into the recently erupted magma conduit was initiated in 1999, at the Unzen Volcano, Japan. Since then, two flank core holes and a pilot hole, with a total length of 2.5 km, were drilled successfully. Borehole temperatures were measured using existing Sandia CTDL (Core Tube Data Logger) while coring. The conduit hole, with a very high angle and with about a 1700m-drilled length, is planned to be drilled beginning in January 2003. To identify conduit position and to assure drilling safety, temperature measurements are planned during conduit drilling operations. New memory tools, with 250 °C temperature rating, and possibly 500 °C with MR dewar, are planned to be designed and fabricated to satisfy the project requirements.
cited By 1
Keywords: Boreholes; Project management; Underground temperature measurement; Volcanoes; Memory tools; Rock drilling

Studies of sedimentary facies, stratigraphy, and deformation structures of the Chelungpu fault zone on cores from drilled wells in Fengyuan and Nantou, Central Taiwan - Huang, S.-T.; Wu, J.-C.; Hung, J.-H.; Tanaka, H. (Terrestrial, Atmospheric and Oceanic Sciences, 2002)
Stratigraphy, sedimentary facies, and deformation characteristics of drill cores from Fengyuan and Noutou wells reveal important attributes for the westmost portion of active Chelungpu thrust fault and contrasting deformation mechanisms between the north and south ends. Stratigraphy of the Fengyuan well (including BH-1 and BH-1A boreholes) is composed of three major units, including the upper Miocene to Pliocene Kueichulin Formation (455.4-224.7 m), the Pliocene Chinshui Shale (224.7-3.9 m) and recent terrace deposits (3.9-0 m). The Kueichulin Formation comprises three upward coarsening, tide-dominated delta parasequences with sandstone and sandstone-Shale alternations. The Chinshui Shale is dominated by shallow marine facies with siltstone, mudstone and fine-grained sandstone. Shallow marine facies are occasionally intercalated with tide-dominated delta deposits. Terrace deposits are characterized by paleosol, yellowish mud, mottled leaching soil and thin pebble layers. The Chi-Chi earthquake slip zone is located at a transgressive deposit, which is also the sequence boundary between the Kueichulin Formation and the Chinshui Shale. Other two major brecciated shear zones are also the parasequence boundaries within the Kueichulin Formation. Stratigraphic sequence of the Nantou well (CLF-2) is composed of the Pleistocene Toukoshan Formation (211.9-177 m) in the footwall, and the Chinshui Shale (177-8.7 m) and terrace deposits (8.7-0 m) in the hangingwall. The Toukoshan Formation is characterized by alternation of conglomerates and yellowish fine-grained deposits with drifted pebbles, an indication of braided fluvial deposits. The Chinshui Shale comprises alternating shallow marine and tidal deposits. The shallow marine face is dominated by mudstone, siltstone and fine-grained sandstone with moderate to high degree of bioturbation. Terrace deposits are characterized by yellowish gray mud, pebble layer, and mottled paleosol. Overall, shear zones in the Nantou well is characterized by foliated gouge or breccia as opposed to breccia or gouge of random fabrics in the Fengyuan well.
cited By 19
Keywords: Chi-Chi earthquake 1999; deformation; fault zone; sequence stratigraphy; Tertiary; thrust fault; (Central); Taiwan

Structural mapping of the 1999 Chi-Chi earthquake fault, Taiwan by seismic reflection methods - Wang, C.-Y.; Li, C.-L.; Su, F.-C.; Leu, M.-T.; Wu, M.-S.; Lai, S.-H.; Chern, C.-C. (Terrestrial, Atmospheric and Oceanic Sciences, 2002)
Several seismic reflection surveys were conducted to investigate the seismogenic structure of the 1999 Chi-Chi earthquake (Mw=7.6) in central Taiwan. Two 40 km-long seismic profiles that crossed the area near the epicenter were acquired using the deep reflection method with a targeting depth of 10 km, to search for the decollement boundary. One of the obtained sections shows a clear reflection event that dips to the east by 40° until reaching a depth of 8 km where the earthquake's source was located. This slant event is unambiguously related to the thrusting Chelungpu fault surface. The abundant eastward dipping reflectors on the deep reflection sections faithfully describe thrusting features predicted by the earthquake faulting model. Besides these deep reflections, we also used many shallow seismic reflection lines to delineate the structures in the northern portion of the fault zone, where large ruptures (about 10 m) occurred both on the surface and underground. The 3D structure of the fault surface can be deduced using this cost-effective approach. Although the depth imaged may be limited (e.g., 3 km), shallow seismic data still provides reliable information for the study of large ruptures, and to make better plans for deep wells that might be drilled in this area in the future.
cited By 39
Keywords: Chi-Chi earthquake 1999; fault; geological mapping; seismic reflection; (Central); Taiwan

Sonic waveform attenuation in gas hydrate-bearing sediments from the Mallik 2L-38 research well, Mackenzie Delta, Canada - Guerin, G.; Goldberg, D. (Journal of Geophysical Research: Solid Earth, 2002)
The Mallik 2L-38 research well was drilled to 1150 m under the Mackenzie Delta, Canada, and penetrated a subpermafrost interval where methane hydrate occupies up to 80% of the pore space. A suite of high-quality downhole logs was acquired to measure in situ the physical properties of these hydrate-bearing sediments. Similar to other hydrate deposits, resistivity and compressional and shear sonic velocity data increase with higher hydrate saturation owing to electrical insulation of the pore space and stiffening of the sediment framework. In addition, sonic waveforms show strong amplitude losses of both compressional and shear waves in intervals where methane hydrate is observed. We use monopole and dipole waveforms to estimate compressional and shear attenuation. Comparing with hydrate saturation values derived from the resistivity log, we observe a linear increase in both attenuation measurements with increasing hydrate saturation, which is not intuitive for stiffening sediments. Numerical modeling of the waveforms allows us to reproduce the recorded waveforms and illustrate these results. We also use a model for wave propagation in frozen porous media to explain qualitatively the loss of sonic waveform amplitude in hydrate-bearing sediments. We suggest that this model can be improved and extended, allowing hydrate saturation to be quantified from attenuation measurements in similar environments and providing new insight into how hydrate and its sediment host interact.
cited By 139
Keywords: acoustic logging; gas hydrate; porous medium; wave propagation; well logging; Canada; Mackenzie Delta

Shallow reflection seismics aiding geological drilling into the Chelungpu fault after the 1999 Chi-Chi earthquake, Taiwan - Wang, C.-Y.; Tanaka, H.; Chow, J.; Chen, C.-C.; Hong, J.-H. (Terrestrial, Atmospheric and Oceanic Sciences, 2002)
Two shallow holes (∼300m) were drilled to uncover cores to study the properties of the Chelungpu fault, which was activated during the 1999 Chi-Chi earthquake (Mw=7.6), Taiwan. Before drilling, we collected seismic reflection data near the wells to aid the drilling processes. The depths predicted by the seismic reflection sections proved to be very close to the drilling results. These seismic sections also provided details of underground 2D structures, which are of help in clarifying the relationship of the well with the neighboring geology. Besides this, we also present several seismic sections describing the undisturbed structures on the Chelungpu fault's footwall side opposite the violated hanging-wall side. A detachment type of movement is suggested to explain this extraordinary phenomenon. Finally, a combination of seismic and electric methods was implemented to explore the near-surface structure of the Sanyi fault, which is believed to be the counterpart of the Chelungpu fault but at a deeper location. The results show that the Sanyi fault is old and has ceased its movement, perhaps not having been involved in the Chi-Chi earthquake's action.
cited By 14
Keywords: drilling; earthquake; fault; seismic reflection; Chichi; Taiwan

Seismic investigation of the Lake Bosumtwi impact crater: Preliminary results - Karp, Tobias; Milkereit, Bernd; Janle, Peter; Danuor, Sylvester K.; Pohl, Jean; Berckhemer, Hans; Scholz, Christopher A. (Planetary and Space Science, 2002)
The Lake Bosumtwi impact crater in Ghana, West Africa, has a diameter of 10.5 km and is one of the youngest (1.07 Ma) well-preserved large craters on Earth. It has a total dynamic range of topography of more than 400 m, and it is the source crater of tektites and microtektites of the Ivory Coast strewn field. The crater was excavated in early Proterozoic rocks. According to its size, the Bosumtwi impact crater should be a complex impact structure, with a central peak. Multichannel seismic (MCS) reflection and wide angle data, using Ocean-Bottom-Hydrophones (OBHs), were acquired in order to investigate the structure's subsurface, image the presumed central uplift and determine the thickness of impact-related formations and the post-impact sediments. An integrated interpretation of the seismic data sets, and modelling and inversion of the OBH data yield an initial 2D velocity-depth model, which shows indications for a central peak feature. Due to the relatively low seismic velocity (3.0 kms-1) of the corresponding layer, the top of the uplifted structure is interpreted to consist of allochthonous breccia. The central peak has a width of ∼1.8 km and a maximum height of 120 m above the top of the breccia away from the center. Fracturing may be responsible for the relatively low velocity of 3.8 kms-1 in the crater floor. The post-impact sediments covering the crater structure are 180-300 m thick. The apparent crater depth, defined as the difference between the original target surface and the top of the breccia layer, is ∼550 m and thereby slightly deeper than some other larger complex impact structures on Earth. The results indicate that the Lake Bosumtwi impact structure provides an interesting setting for scientific drilling of a young large impact crater and will be supplemented by complimentary recent geophysical (potential field) and possibly future drilling studies. © 2002 Elsevier Science Ltd. All rights reserved.
Keywords: Drilling; Earth (planet); Geophysics; Hydrophones; Rocks; Topography; Seismic prospecting

Researchers propose earthquake observatory inside San Andreas fault - Harris, C.A. (Civil Engineering, 2002)
Researchers from universities around the country and from the United States Geological Survey (USGS), headquartered in Reston, Virginia, are planning to dig a pilot hole inside California's San Andreas Fault with hopes of eventually developing a larger, adjacent hole that will serve as an earthquake observatory. The observatory, to be called the San Andreas Fault Observatory at Depth (SAFOD), will be designed to determine the physical and chemical processes at work in an active fault zone.
cited By 0
Keywords: Earthquakes; Geological surveys; Observatories; Earthquake observatory; Civil engineering; active fault; borehole; earthquake; observatory; United States

Relationship of gas hydrate concentration to porosity and reflection amplitude in a research well, Mackenzie Delta, Canada - Jin, Y.K.; Lee, M.W.; Collett, T.S. (Marine and Petroleum Geology, 2002)
Well logs acquired at the Mallik 2L-38 gas hydrate research well. Mackenzie Delta, Canada, reveal a distinct trend showing that the resistivity of gas-hydrate-bearing sediments increases with increases in density porosities. This trend, opposite to the general trend of decrease in resistivity with porosity, implies that gas hydrates are more concentrated in the higher porosity. Using the Mallik 2L-38 well data, a proportional gas hydrate concentration (PGHC) model, which states that the gas hydrate concentration in the sediment's pore space is linearly proportional to porosity, is proposed for the general habitat of gas hydrate in sediments. Anomalous data (less than 6% of the total data) outside the dominant observed trend can be explained by local geological characteristics. The anomalous data analysis indicates that highly concentrated gas-hydrate-bearing layers would be expected where sediments have high proportions of gravel and coarse sand. Using the parameters in the PGHC model determined from resistivity-porosity logs, it is possible to qualitatively predict the degree of reflection amplitude variations in seismic profiles. Moderate-to-strong reflections are expected for the Mallik 2L-38 well. © 2002 Elsevier Science Ltd. All rights reserved.
cited By 14
Keywords: Pore spaces; Gas hydrates; Porosity; Reflection; Sand; Sediments; Seismic prospecting; Well logging; Geology; amplitude; gas hydrate; porosity; seismic data; seismic reflection; well logging; Canada

Episodic thermal perturbations associated with groundwater flow: An example from Kilauea Volcano, Hawaii - Hurwitz, Shaul; Ingebritsen, Steven E.; Sorey, Michael L. (Journal of Geophysical Research: Solid Earth, 2002)
Temperature measurements in deep drill holes on volcano summits or upper flanks allow a quantitative analysis of groundwater induced heat transport within the edifice. We present a new temperature-depth profile from a deep well on the summit of Kilauea Volcano, Hawaii, and analyze it in conjunction with a temperature profile measured 26 years earlier. We propose two groundwater flow models to interpret the complex temperature profiles. The first is a modified confined lateral flow model (CLFM) with a continuous flux of hydrothermal fluid. In the second, transient flow model (TFM), slow conductive cooling follows a brief, advective heating event. We carry out numerical simulations to examine the timescales associated with each of the models. Results for both models are sensitive to the initial conditions, and with realistic initial conditions it takes between 750 and 1000 simulation years for either model to match the measured temperature profiles. With somewhat hotter initial conditions, results are consistent with onset of a hydrothermal plume ∼550 years ago, coincident with initiation of caldera subsidence. We show that the TFM is consistent with other data from hydrothermal systems and laboratory experiments and perhaps is more appropriate for this highly dynamic environment. The TFM implies that volcano-hydrothermal systems may be dominated by episodic events and that thermal perturbations may persist for several thousand years after hydrothermal flow has ceased.
Keywords: Hawaii; Kilauea Volcano; United States; groundwater flow; heat transfer; hydrothermal system; perturbation; transient flow; volcano; well

Origin and evolution of the western Snake River Plain: Implications from stratigraphy, faulting, and the geochemistry of basalts near Mountain Home, Idaho - Shervais, John W; Shroff, Gaurav; Vetter, Scott K; Matthews, Scott; Hanan, Barry B; McGee, James J; Bonnichsen, B; White, CM; McCurry, M (Tectonic and Magmatic Evolution of the Snake River Plain Volcanic Province, 2002)

Drilling program investigates permafrost gas hydrates - Dallimore, S.R.; Collett, T.S.; Weber, M.; Uchida, T. (Eos, 2002)
cited By 7

Biot-Gassmann theory for velocities of gas hydrate-bearing sediments - Lee, M.W. (Geophysics, 2002)
Elevated elastic velocities are a distinct physical property of gas hydrate-bearing sediments. A number of velocity models and equations (e.g., pore-filling model, cementation model, effective medium theories, weighted equations, and time-average equations) have been used to describe this effect. In particular, the weighted equation and effective medium theory predict reasonably well the elastic properties of unconsolidated gas hydrate-bearing sediments. A weakness of the weighted equation is its use of the empirical relationship of the time-average equation as one element of the equation. One drawback of the effective medium theory is its prediction of unreasonably higher shear-wave velocity at high porosities, so that the predicted velocity ratio does not agree well with the observed velocity ratio. To overcome these weaknesses, a method is proposed, based on Biot-Gassmann theories and assuming the formation velocity ratio (shear to compressional velocity) of an unconsolidated sediment is related to the velocity ratio of the matrix material of the formation and its porosity. Using the Biot coefficient calculated from either the weighted equation or from the effective medium theory, the proposed method accurately predicts the elastic properties of unconsolidated sediments with or without gas hydrate concentration. This method was applied to the observed velocities at the Mallik 2L-39 well, Mackenzie Delta, Canada.
cited By 119
Keywords: Concentration (process); Elasticity; Hydrates; Porosity; Sediments; Acoustic wave velocity; Gas hydrates; Gases; Hydration; Seismic waves; Seismology; Shear waves; Wave propagation; Elastic velocities; Compressional velocities; Effective medium theories; Empirical relationships; Gas hydrate bearing sediments; Gas hydrate concentrations; Shear wave velocity; Solid structures; Unconsolidated sediment; Geophysics; Shear flow; Biot theory; elastic wave; gas hydrate; P-wave; S-wave; velocity

Occurrence of metastable rocks in deeply subducted continental crust from the Dabie Mountains, central China - Shuwen, Dong; Oberh{ansli, R.; Schmid, R.; Xiaochun, Liu; Jiafu, Tang; Huaimin, XUE (Episodes, 2002)
A recent study reveals that metamorphic volcanic and sedimentary rocks have survived deep subduction of continental crust in the Dabie Mountains, central China. These low-grade (LG) rocks are intercalated within the ultrahigh-pressure (UHP) metamorphic terrane and well preserve primary volcanic breccia structure and sedimentary bedding. The presence of coherent field relations between LG and UHP rocks suggests that LG rocks are not of exotic origin and therefore suffered from UHP metamorphism. They, however, did not record the UHP event likely due to slow reaction kinetics, and only experienced epidote-amphibolite to greenschist facies metamorphism and deformation at the retrograde stage of UHP rocks. The protolith age of c. 790 Ma and metamorphic age of 232.2 Ma obtained from LG rocks, which agree well with those from UHP rocks, also suggest that both LG and UHP rocks shared the same evolutionary history since Late-Proterozoic. Except for the lack of deformation and fluid during UHP metamorphism, the rapid subduction, short stay at mantle depths and rapid exhumation are necessary for the preservation of primary assemblage and fabric in LG rocks within deeply subducted continental crust.
Keywords: China; Dabie Shan; continental crust; metamorphic facies; metamorphic rock; subduction

Conjugate thrust faulting associated with the 1999 Chi-Chi, Taiwan, earthquake sequence - Chen, K.-C.; Huang, B.-S.; Wang, J.-H.; Yen, H.-Y. (Geophysical Research Letters, 2002)
The geometrical structure of the responsible faults of the 20 September 1999 Chi-Chi, Taiwan, earthquake (ML = 7.3, Mw = 7.6) and its aftershocks can be clearly depicted by well-located hypocenters and focal mechanisms of large aftershocks. The mainshock and two large aftershocks with ML = 6.8 were characterized by thrust faulting along a N-S striking fault plane dipping to the east. The underground structure of the Chelungpu fault, which is probably merging with the decollement beneath the Western Foothills, can be clearly associated with the seismicity pattern and the focal mechanisms of the three largest events. A group of deeper aftershocks including two moderate events (ML = 6.3 and 6.0, respectively) were located to the southeast of the mainshock along a fault plane dipping steeply to the west down to a depth of about 30 km. Our results suggest that the spatial pattern of the aftershocks in the southern part of the source area can be interpreted by a conjugate-fault system. This conjugate-fault system is comprised of the gently east-dipping Chelungpu fault and a steeply west-dipping deeper fault zone. Copyright 2002 by the American Geophysical Union.
cited By 34
Keywords: Earthquakes; Geophysics; Underground structures; Chi-Chi; Taiwan; Conjugate faults; Earthquake sequences; Focal mechanism; Geometrical structure; Seismicity pattern; Spatial patterns; Thrust faulting; Faulting

Chelungpu fault drilling could resolve seismological issues - Mori, J.; Ito, H.; Wang, C.-Y. (Eos, 2002)
cited By 13

High-resolution seismic velocities and shallow structure of the San Andreas fault zone at Middle Mountain, Parkfield, California - Catchings, R.D.; Rymer, M.J.; Goldman, M.R.; Hole, J.A.; Huggins, R.; Lippus, C. (Bulletin of the Seismological Society of America, 2002)
A 5-km-long, high-resolution seismic imaging survey across the San Andreas fault (SAF) zone and the proposed San Andreas Fault Observatory at Depth (SAFOD) drill site near Parkfield, California, shows that velocities vary both laterally and vertically. Velocities range from <1.0 km/sec near the surface to as much as 4.8 km/sec at 750-m depth. The lowest velocities (<1.0 to ∼3.0 km/sec) correspond to unconsolidated sediment, mudstone, and sandstone in the near surface, and the higher velocities (>4.0 km/sec) probably correspond to granitic rock of the Salinian block, which is exposed a few kilometers southwest of the SAF. The depth to the top of probable granitic rock varies laterally along the seismic profile but is about 600 m below the surface at the proposed SAFOD site. We observe a prominent, lateral low-velocity zone (LVZ) beneath and southwest of the surface trace of the SAF. The LVZ is about 1.5 km wide at 300-m depth but tapers to about 600 m wide at 750-m depth. At the maximum depth of the velocity model (750 m), the LVZ is centered approximately 400 m southwest of the surface trace of the SAF. Similar velocities and velocity gradients are observed at comparable depths on both sides of the LVZ, suggesting that the LVZ is anomalous relative to rocks on either side of it. Velocities within the LVZ are lower than those of San Andreas fault gouge, and the LVZ is also anomalous with respect to gravity, magnetic, and resistivity measurements. Because of its proximity to the surface trace of the SAF, it is tempting to suggest that the LVZ represents a zone of fractured crystalline rocks at depth. However, the LVZ instead probably represents a tectonic sliver of sedimentary rock that now rests adjacent to or encompasses the SAF. Such a sliver of sedimentary rock implies fault strands on both sides and possibly within the sliver, suggesting a zone of fault strands at least 1.5 km wide at a depth of 300 m, tapering to about 600 m wide at 750-m depth. Fluids within the sedimentary sliver are probably responsible for observed low-resistivity values.
cited By 61
Keywords: Granite; Imaging techniques; Landforms; Sandstone; Sediments; Surveying; Velocity measurement; Seismic imaging surveys; Seismology; crustal structure; fault zone; San Andreas Fault; seismic velocity; seismicity; United States

Initial science report of shallow drilling, penetrating into the Chelungpu fault zone, Taiwan - Tanaka, H.; Wang, C.-Y.; Chen, W.-M.; Sakaguchi, A.; Ujiie, K.; Ito, H.; Ando, M. (Terrestrial, Atmospheric and Oceanic Sciences, 2002)
The Chelungpu fault, a reverse fault with left lateral component dipping moderately to the east, was activated by the Chi-Chi earthquake (Mw = 7.6) in 21 September, 1999 with maximum vertical and lateral offsets of 5.6 m and 9.8 m. Characteristics of earthquake and related phenomena are contrasting between northern and southern regions along the Chelungpu fault. The northern region has (1) larger displacements (4 to 9 m), (2) low frequency seismic waves with higher velocity of slip surface, and (3) less disastrous except the most northern area compared to those in the southern region. Drilling into the Chelungpu fault was thus conducted at two locations, northern (Fengyuan) and southern (Nantou) sites, and successfully completed in March 2001. The project was motivated to explore the fundamental controlling factors of the mode of slip motion at northern and southern regions through analysis of intrafault materials. Meso- and microstructural examinations and measurements of static/ dynamic physical properties have been conducted for each drill core. The ongoing analyses have shown interesting results: (1) fault zone architecture is totally different between the northern and southern fault zones. The rocks are mainly composed of random fabric fault breccia with extremely thin fault gouge in the northern core, whereas the foliated fault breccia is dominantly associated with ultracataclasite and pseudotachylite in the southern core, (2) possible fault zones activated by the Chi-Chi earthquake can be listed up by combining geological, geophysical logging and reflection seismic data, which are 225 m and 330 fracture zones in the core from northern well and 177 m and 180 m fracture zones in the core from southern well, (3) water contents of the core of the 225 m rupture zone in the northern well attains up to 45 vol.%, and (4) some temperature rises were detected at 330 m fracture zone in the northern well and 180 m fracture zone in the southern well by temperature logging, which could be attributed to residual heat generated during the Chi-Chi earthquake or postseismic influx of hydrothermal fluid into the fault zones.
cited By 61
Keywords: breccia; Chi-Chi earthquake 1999; dip-slip fault; drilling; fault zone; microstructure; reverse fault; Chelungpu Fault; Taiwan

Late Pliocene vegetation and climate of the Lake Baikal region, southern East Siberia, reconstructed from palynological data - Demske, Dieter; Mohr, Barbara; Oberh{ansli, Hedi (Palaeogeography, Palaeoclimatology, Palaeoecology, 2002)
Palynomorphs from a palaeomagnetically dated Late Pliocene sediment core (3.6-2.35 Ma) reflect highly variable climatic conditions and repeated expansion of steppe and boreo-alpine vegetation. Mixed coniferous forests (Picea, Tsuga, Pinus) with associated broadleaved taxa (Quercus, Corylus, Ulmus, Tilia) were affected by dry and cold intervals already between 3.48 and 3.39 Ma. Peak records of non-arboreal pollen types and spores, including subarctic-boreo-alpine Selaginella selaginoides, evidence advances of open vegetation, which can be correlated to glacial marine oxygen isotope stages. A cooling trend occurred from 3.15 Ma to 2.5 Ma, documented by cold-dry intervals and increased fluctuations of Picea and Pinus. Around 3-2.9 Ma the palaeoclimate variation pattern shifted towards cold-dry and warm-moist oscillations. Cooling strengthened after 2.89 Ma and severely restricted Tsuga development. Cool and oligotrophic limnic conditions are documented by abundant Gonyaulax dinoflagellate cysts in the interval 2.65-2.57 Ma. Due to decreasing precipitation extensive areas with open steppe and rock-steppe vegetation became permanently established after 2.62 Ma. Dry conditions became a dominant environmental factor in the Baikal region, coeval in time with the Red Clay-Loess shift in northern China. This climatic transition can be explained as a consequence of major intensification of Northern Hemisphere Glaciation around 2.75 Ma evidenced by marine records of ice-rafted debris. © 2002 Elsevier Science B.V. All rights reserved.
Keywords: Lake Baikal; Russian Federation; Siberia; Bacteria (microorganisms); Corylus; Corylus; Dinophyceae; Gonyaulax; Gonyaulax; Malvaceae; Picea; Picea; Pinus; Quercus; Quercus; Selaginella; Selaginella selaginoides; Selaginella selaginoides; Tilia; Tilia; Tsuga; Tsuga; Ulmus; Ulmus; lacustrine deposit; paleoclimate; paleoenvironment; palynology; Pliocene; vegetation history

Late-Quaternary lowstands of Lake Titicaca: Evidence from high-resolution seismic data - D'Agostino, Karin; Seltzer, Geoffrey; Baker, Paul; Fritz, Sherilyn; Dunbar, Robert (Palaeogeography, Palaeoclimatology, Palaeoecology, 2002)
Approximately 600 km of high-resolution seismic reflection data were collected to investigate the late-Quaternary stratigraphic development of Lake Titicaca. The focus of this report is on two seismic sequence boundaries, which are interpreted as erosional surfaces formed at times of low lake level. The younger erosional surface occurs as much as 90 m below the present lake level and up to 8 m below the present sediment-water interface. This erosional surface is interpreted to be coeval with a well-documented early- to mid-Holocene lowstand, dated between ∼ 8000 and 3600 cal yr BP. An earlier and previously unknown erosional surface occurs at a sub-bottom depth of approximately 30 m, and as much as 240 m below the present lake level, which implies a major late-Pleistocene lowstand of Lake Titicaca. By extrapolation of sedimentation rates from the upper ∼ 14 m of sediment, we estimate the age of this older lowstand at > 90000 cal yr BP. Both lowstands of Lake Titicaca indicated by the seismic data are likely to have been a response to climatic change in the region. © 2002 Elsevier Science B.V. All rights reserved.
Keywords: Bolivia; Peru; lake level; paleoclimate; Quaternary; seismic data; seismic reflection; seismic stratigraphy

Mapping the northern portion of the Chelungpu fault, Taiwan by shallow reflection seismics - Wang, C.-Y.; Li, C.-L.; Yen, H.-Y. (Geophysical Research Letters, 2002)
The Chelungpu fault was activated by the 1999 Chi-Chi earthquake (Mw = 7.6), Taiwan. This fault exhibited extraordinarily large surface ruptures (up to 9.8 m) as well as underground fault slippages (up to 12 m) during the earthquake. These large displacements were concentrated along the northern portion of the fault, 40 km north of the epicenter. To prepare data for the future drilling of deep wells in this area, many shallow seismic reflection surveys were conducted to investigate the sites. An approximate 3D structure of the fault surface can be deduced by this cost-effective approach. Although the depth penetration may be limited (e.g., 3 km), the method still provides reliable information to study large ruptures, and to better plan future deep wells.
cited By 20
Keywords: Cost effectiveness; Seismology; Structure (composition); Surveys; Fault slippages; Earthquakes; borehole; earthquake rupture; fault slip; seismic reflection; seismic survey; Chelungpu Fault; Taiwan

Late Pliocene sedimentation in Lake Baikal: Implications for climatic and tectonic change in SE Siberia - Noe-Nygaard, Nanna; Heiberg, Erik Otto (Palaeogeography, Palaeoclimatology, Palaeoecology, 2001)
Within the framework of the Baikal Drilling Project (BDP), a 192 m long sediment core (BDP-96-1) was recovered from the Academician Ridge, a submerged topographic high between the North and Central Basins of Lake Baikal. Sedimentological, clay mineralogical and geochemical investigations were carried out on the core interval between 90 and 124 m depth, corresponding to ca. 2.4-3.4 Ma. The aim was to reconstruct the climatic and tectonic history of the continental region during the intensification of Northern Hemisphere glaciation in Late Pliocene time. A major climate change occurred in the Lake Baikal area at about 2.65 Ma. Enhanced physical weathering in the catchment, mirrored in the illite to smectite ratio, and temporarily reduced bioproduction in the lake, reflected by the diatom abundance, evidence a change towards a colder and more arid climate, probably associated with an intensification of the Siberian High. In addition, the coincident onset of distinct fluctuations in these parameters and in the Zr/Al ratio suggests the beginning of the Late Cenozoic high amplitude climate cycles at about 2.65 Ma. Fluctuations in the Zr/Al ratio are traced back to changes in the aeolian input, with high values in warmer, more humid phases due to a weaker Siberian High. Assuming that the sand content in the sediment reflects tectonic pulses, the Lake Baikal area was tectonically active during the entire investigated period, but in particular around 2.65 Ma. Tectonic movements have likely led to a gradual catchment change since about 3.15 Ma from the western towards the eastern lake surroundings, as indicated in the geochemistry and clay mineralogy of the sediments. The strong coincidence between tectonic and climatic changes in the Baikal area hints at the Himalayan uplift being one of the triggers for the Northern Hemisphere Glaciation. © 2001 Elsevier Science B.V. All rights reserved.
Keywords: Russian Federation; Bacillariophyta; geochemistry; lacustrine deposit; paleoclimate; Pliocene; sedimentology; tectonic evolution; weathering

Thermal properties of bottom sediments of Lake Baikal (according to data on temperature field recovery in underwater boreholes) - Duchkov, A.D.; Lee, T.-C.; Morozov, S.G. (Geologiya i Geofizika, 2001)
Thermal conductivity of rocks is a key parameter in heat flow determination. Thermal conductivity of the Baikal bottom sediments in cores from first underwater boreholes (BDP-93 and BDP-96) was measured by a needle probe and a thermal-conductivity comparator, and the two methods showed a difference of up to 30-50% in the values of both thermal conductivity and heat flow. In this study we used a new method implying evaluation of thermal conductivity of sediments through interpretation (inversion modeling) of temperature field recovery in boreholes after the cessation of drilling. The modeling parameters are steady-state temperature and thermal conductivity of bottom sediments at a depth of temperature monitoring. As a result, it was concluded that the needle probe measurements are reliable, and the thermal-conductivity comparator cannot be used to study unconsolidated sediments with water contents above 40%. The obtained estimates of steady-state temperature and geothermal gradient confirmed that the heat flow is relatively low (about 50 mW/m2) in the Bugul'deika saddle (BDP-93) and high (78 mW/m2, an updated value) in the axial part of the Akademichesky Ridge (BDP-96). The new method allowed us to estimate, for the first time, the thermal-diffusivity coefficient and the specific heat capacity of the Baikal sediments. Thus, the numerical analysis of temperature monitoring data permits estimation of all geothermal parameters of sediments.

The sponge fauna of lake baikal in the late pliocene (according to studies of core samples from deep borehole BDP-96-1) - Weinberg, E.V. (Geologiya i Geofizika, 2001)
Spicules have been investigated in Late Pliocene core samples from the borehole BDP-96-1. The samples have been dated paleomagnetically. A total of 42 spicule types have been discovered, of which 15 are found in modern Baikal sponges and 27 are not. Analysis of the range of species and quantitative indices of the spicules in bottom sediments has revealed three stages of sponge-fauna development: 3.1-2.9, 2.9-2.5, and 2.5-2.1 Ma BP. The sponge fauna of the first stage is indicative of warm-water conditions. Most of thermophilic and fossil species disappeared from the second stage, which points to a dramatic cooling. At the third stage, regeneration of the sponge fauna began. However, it did not reach the efflorescence of the first stage. The results are in agreement with the palynological and diatom analysis of the core samples from the investigated depth range.

The southwestern edge of the North Baikal basin: Geologic structure and correlation with cenozoic sections of Ol'khon and BDP-96 and BDP-98 cores - Khlystov, O.M.; Mats, V.D.; De Batist, M. (Geologiya i Geofizika, 2001)
The paper presents new details of the structure of the basement and sedimentary cover of the southwestern edge of the North Baikal basin. The basement structure involves smaller-scale structures (Maloe More basin, Zama basin, and Zunduk-Zama horst), which, in turn, comprise third-order features. The presence of several en-echelon NE striking sub-basins within the Maloe More and Zama basins indicates that the tectonic framework of the region formed by listric and dextral faulting. Seismostratigraphy data and analysis of uppermost bottom sediments from BDP-96 and BDP-98 cores revealed three seismic layers in the sediment section, which are separated by discontinuities and can be correlated with deposits exposed on land in the neighboring Ol'khon region. Unit A' (X) correlates with the Lower-Middle Oligocene Ular'yar Formation, unit A, with the Lower-Middle Miocene Tagai Formation, and unit B, with the Upper Miocene-Pliocene Sasa Formation and with Quaternary deposits. The upper section of the Maloe More fill includes a separate seismic horizon correlatable with a layer of widely distributed fine-grained sand. Unit B, comprises three deposition centers within the limits of the Maloe More basin, i.e., large lakes existed in the region prior to the final stage of transgression from the North Baikal basin. Thus, the southwestern edge of the Baikal basin has an intricate structure and underwent a complex evolution.

The new BDP-98 600-m drill core from Lake Baikal: A key late Cenozoic sedimentary section in continental Asia - Antipin, V.; Afonina, T.; Badalov, O.; Bezrukova, E.; Bukharov, A.; Bychinsky, V.; Dmitriev, A.A.; Dorofeeva, R.; Duchkov, A.; Esipko, O.; Fileva, T.; Gelety, V.; et al. (Quaternary International, 2001)
The new 600-m drill core BDP-98 from the Academician Ridge of Lake Baikal recovered a continuous sedimentary record of the past 10 Ma. The entire section is represented by lacustrine sediments, which gradually change from distal deltaic facies at the bottom of the section to fine undisturbed hemipelagic sediments of the upper 300-m interval. The entire 10-Ma lacustrine section contains abundant diatoms, thus allowing extension of Plio-Pleistocene diatom and biogenic silica records into the Miocene. Above the Matuyama/Gauss paleomagnetic reversal boundary, the BDP-98 record contains clearly delineated glacial/interglacial lithologic cycles. Below this boundary the diatom signal is quite different: average diatom contents are higher and variations are of lower amplitude. Although most likely paleoclimatic in origin, these variation presumably reflect past changes in the moisture regime of southeast Siberia under conditions of warm subtropical climate during the Miocene and Early-Middle Pliocene. The continuous BDP-98 drill core, which covers the hiati present in the composite continental sections of the Baikal region, is a key section for reconstructing the Neogene-Quaternary climatic evolution of continental Asia. The BDP-98 section also places several important time constraints on the rifting history of Lake Baikal by providing reliable correlation of lithological and physical properties of the drill core sediments with calculated positions of the acoustic reflection boundaries interpreted from multichannel seismic studies. The lithologic composition indicates that, on the stable block of Academician Ridge where the BDP-96 and BDP-98 drill sites are located, acoustic reflection boundaries are not associated with major erosional events, but instead result from changes in sediment density and composition. Several lithologic indices further suggest that significant changes have occurred in the physics and chemistry of Lake Baikal waters, affecting the carbonate equilibrium and oxygen regime of Baikal. © 2001 Published by Elsevier Science Ltd.
Keywords: Asia; Russian Federation; Cenozoic; lacustrine deposit; sediment core

The link between tectonic and paleoclimatic events at 2.8-2.5 Ma BP in the Lake Baikal region - Prokopenko, Alexander A.; Karabanov, Eugene B.; Williams, Douglas F.; Kuzmin, Mikhail I.; Khursevich, Galina K.; Gvozdkov, Alexander A. (Quaternary International, 2001)
The records of diatom abundance, clay fraction content and magnetic susceptibility from Lake Baikal drill core BDP-96-1 reflect the climate-driven Plio-Pleistocene evolution of the Lake Baikal sedimentation system. In addition to the dramatic variations in the proxy records in response to the Northern Hemisphere ice ages, the Baikal record also indicates climatic deterioration from 2.8 to 2.5 Ma BP with evidence for an early Siberian glaciation around the Matuyama/Gauss paleomagnetic reversal boundary. The drill core data also allow correlation of this early glacial interval with the basin-wide seismic sequence boundary B10, which marks the unconformity produced by the active neotectonic phase in the Baikal rift zone. At the BDP-96 drill site, however, the strong B10 acoustic reflection was produced not by an erosional boundary, but instead by deposition of glacial clay layers. The paleomagnetic age scale of BDP-96-1 constrains the upper age of the Neobaikalian uplift/subsidence phase in the Baikal-Sayan region at ca. 2.5 Ma BP. The coincident timing of the paleoclimatic and tectonic events recorded in Lake Baikal sediments suggests a close causal link between regional tectonics and climate and indicate that the late Pliocene uplift contributed to the initiation of the early glaciation in southeast Siberia between 2.8 and 2.6 Ma BP by creating an elevated terrain with lower snowline, favorable for alpine glaciation, and by changing the heat balance of the region. © 2001 Elsevier Science Ltd and INQUA. All rights reserved.
Keywords: Russian Federation; lacustrine deposit; neotectonics; paleoclimate; paleomagnetism; sediment core

Paleoclimate record in bottom sediments of lake Baikal, from magnetic susceptibility data - Krainov, M.A.; Kravchinskii, V.A.; Peck, J.A.; Sakai, H.; King, J.W.; Kuz'min, M.I. (Geologiya i Geofizika, 2001)
Two boreholes on the Akademichesky Ridge, 100 and 200 m deep (BDP-96-1 and BDP-96-2), were drilled and cored continuously as part of the Baikal Drilling Project. Results of low-frequency magnetic susceptibility measurements of the two cores were correlated with the SPECMAP oceanic oxygen isotope curve, and a composite section was compiled. The SPECMAP curve and the magnetic susceptibility of the BDP-96 cores showed a good fit. Frequency analysis of time variations in magnetic susceptibility showed a periodicity of the paleoclimate signal with intervals of 23, 35, 41, 52, 68, 85, 106, and 164 ka. The periods of 23, 41, 52, and 106 ka correspond to cycles of precession, inclination of the Earth's axis, an eccentricity harmonic overlapped with inclination, and the Earth's orbit eccentricity itself, respectively. The periods of 35, 68, and 164 ka, distinguished for the first time in magnetic susceptibility studies but noted earlier in biogenic silica analysis, reflect different eccentricity harmonics, and the period of 68 ka bears an effect of precession. The period of 85 ka has never been revealed before in the climate record and is most likely related to regional periodicity rather than to any astronomic cycles.

Multiwave XRF-SR determination of U and Th in bottom sediments of Lake Baikal: Brunhes paleoclimatic chronology - Fedorin, M.A.; Gol'dberg, E.L.; Bobrov, V.A.; Khlystov, O.M.; Grachev, M.A. (Geologiya i Geofizika, 2001)
The paper presents a new method of U and Th determination in bottom sediments implying synchrotron radiation with monochromatic beams at different energies, registration, and iterative joint processing of the resultant XRF (X-ray fluorescent) spectra series. The validity of the new method was proved by comparison of U and Th determinations by XRF-SR in different sediment samples with earlier INAA and ICP-MS results for the same samples. Profiles of U and Th were measured at the sampling rate of 2 ka in a drilling core (BDP-96-2) of bottom sediments from Lake Baikal deposited 40 to 780 ka BP. Oscillations of U contents and U/Th ratios record global climate change throughout the Brunhes epoch (780 ka BP), and the response of these warm proxies is similar to that observed earlier in shorter cores spanning the last two interglacials (220-0 ka BP).

Mineral inclusions in zircons of para- and orthogneiss from pre-pilot drillhole CCSD-PP1, Chinese Continental Scientific Drilling Project - Liu, Fulai; Xu, Zhiqin; Katayama, Ikuo; Yang, Jingsui; Maruyama, Shigenori; Liou, J.G. (Lithos, 2001)
The pre-pilot drillhole CCSD-PPI, Chinese Continental Scientific Drilling Project (CCSD), with depth of 432 m, is located in the Donghai area in the southwestern Sulu terrane. The core samples are mainly comprised of paragneiss, orthogneiss and ultramafic rock with minor intercalated layers of eclogite and phengite-bearing kyanite quartzite. All analyzed paragneiss and orthogneiss samples were overprinted on amphibolite facies retrograde metamorphism. Coesite and coesite-bearing ultrahighpressure (UHP) mineral assemblages were identified by Raman spectroscopy and electron microprobe analysis as inclusions in zircons separated from paragneiss, eclogite and phengite-bearing kyanite quatzite samples. In the paragneiss samples, UHP mineral inclusion assemblages mainly consist of Coe + Omp+ Grt+ Phe, Coe + Jd + Phe + Ap preserved in the mantles (M) and rims (R) of zircons. These UHP mineral inclusion assemblages yield temperatures of 814-852 °C and pressures of ≥28 kbar, presenting the P-T condition of UHP peak metamorphism of these country rocks. According to the mineral inclusions and cathodoluminescence images of zircons, the orthogneisses can be divided into two types: UHP (OGI) and non-UHP (OG2). In OGI orthogneisses, low-pressure mineral inclusion assemblage, mainly consisting of Qtz + Phe +Ab +Ksp + Ap, were identified in zircon cores (C), while coesite or coesite-bearing UHP mineral inclusions were identified in the mantles (M) and rims (R) of the same zircons. These features suggest that the OG1 orthogneisses, together with the paragneisses, phengitebearing kyanite quartzite and eclogite experienced widespread UHP metamorphism in the Sulu terrane. However, in the zircons of OG2 orthogneiss samples, no UHP mineral inclusions were found. Inclusions mainly comprised Qtz + Phe + Ap and were identified in cores (C), mantles (M) and rims (R) of OG2 zircons; the cathdoluminescence images of all analyzed zircons showed clear zonings from cores to rims. These features indicate that the OG2 orthogneisses in pre-pilot drillhole CCSD-PP1 did not experience UHP metamorphism. Therefore, we should not rule out the possibility that some orthogneisses in Sulu terrane might represent relatively low-pressure granitic intrusives emplaced after the UHP event. © 2001 Elsevier Science B.V. All rights reserved.
Keywords: China; drilling; mineralogy; orthogneiss; paragenesis; zircon

Late quaternary climate and hydrology of tropical South America inferred from an isotopic and chemical model of Lake Titicaca, Bolivia and Peru - Cross, Scott L.; Baker, Paul A.; Seltzer, Geoffrey O.; Fritz, Sherilyn C.; Dunbar, Robert B. (Quaternary Research, 2001)
A simple mass balance model provides insight into the hydrologic, isotopic, and chemical responses of Lake Titicaca to past climatic changes. Latest Pleistocene climate of the Altiplano is assumed to have been 20% wetter and 5°C colder than today, based on previous modeling. Our simulation of lacustrine change since 15,000 cal yr B.P. is forced by these modeled climate changes. The latest Pleistocene Lake Titicaca was deep, fresh, and overflowing. The latest Pleistocene riverine discharge from the lake was about 8 times greater than the modern average, sufficient to allow the expansion of the great paleolake Tauca on the central Altiplano. The lake δ18O value averaged about - 13‰ SMOW (the modern value is about -4.2‰). The early Holocene decrease in precipitation caused Lake Titicaca to fall below its outlet and contributed to a rapid desiccation of paleolake Tauca. Continued evaporation caused the 100-m drop in lake level, but only a slight (1-2‰) increase (relative to modern) in δ18O of early Holocene lake waters. This Holocene lowstand level of nearly 100 m was most likely produced by a precipitation decrease, relative to modern, of about 40%. The lake was saline as recently as 2000 cal yr B.P. The timing of these hydrologic changes is in general agreement with calculated changes of insolation forcing of the South American summer monsoon. © 2001 University of Washington.
Keywords: Bolivia; Lake Titicaca; Peru; geochemistry; lake level; paleoclimate; paleohydrology; paleolimnology; Quaternary

Biostratigraphic significance of new fossil species of the diatom genera Stephanodiscuts and Cyclotella from Upper Cenozoic deposits of Lake Baikal, Siberia - Khursevich, G.K.; Karabanov, E.B.; Prokopenko, A.A.; Williams, D.F.; Kuzmin, M.I.; Fedenya, S.A. (Micropaleontology, 2001)
Three new extinct taxa of the genus Stephanodiscus Ehrenberg (S. williamsii sp., nov., S. princeps sp. nov., S. yukonensis var. antiquus var. nov.) and four new extinct species and two new extinct varieties of the genus Cyclotella (K{utzing) Br'ebisson (C. iris var. insueta var. nov., C. tempereiformica sp. nov., C. distincta sp. nov., C. comtaeformica sp. nov., C. comtaeformica var. spinata var. nov. and C. praeminuta sp. nov.) are described from Upper Cenozoic lacustrine sediments of Lake Baikal (boreholes BDP-96-1 and BDP-96-2). The narrow biostratigraphic ranges of the new taxa warrant their use as index-fossil species for defining a diatom zonation, for accurate correlation and age control of Lake Baikal sediments. The comparative morphological analysis of the newly described Cyclotella taxa allowed us to trace the evolution of morphological features in these diatoms through time. The short time intervals during which the Stephanodiscus taxa existed in ancient Baikal imply that these species were unable to adapt to rapidly changing paleoecological and paleolimnological conditions caused by abrupt Pleistocene climatic fluctuations.
Keywords: Russian Federation; biostratigraphy; Cenozoic; diatom; lacustrine deposit; new species; paleoclimate

Lake Titicaca: An archive of South American paleoclimate - Baker, Paul A.; Fritz, Sherilyn C.; Seltzer, Geoffrey O. (Geotimes, 2001)

Geochemical constraints of the eclogite and granulite facies metamorphism as recognized in the Raobazhai complex from North Dabie Shan, China - Xiao, Y.L.; Hoefs, J.; Van Den Kerkhof, A.M.; Li, S.G. (Journal of Metamorphic Geology, 2001)
A combined study of major and trace elements, fluid inclusions and oxygen isotopes has been carried out on garnet pyroxenite from the Raobazhai complex in the North Dabie Terrane (NDT). Well-preserved compositional zoning with Na decreasing and Ca and Mg increasing from the core to rim of pyroxene in the garnet pyroxenite indicates eclogite facies metamorphism at the peak metamorphic stage and subsequent granulite facies metamorphism during uplift. A P-T path with substantial heating (from c. 750 to 900 °C) after the maximum pressure reveals a different uplift history compared with most other eclogites in the South Dabie Terrane (SDT). Fluid inclusion data can be correlated with the metamorphic grade: The fluid regime during the peak metamorphism (eclogite facies) was dominated by N2-bearing NaCl-rich solutions, whereas it changed into CO2-dominated fluids during the granulite facies retrograde metamorphism. At a late retrograde metamorphic stage, probably after amphibolite facies metamorphism, some external low-salinity fluids were involved. In situ UV-laser oxygen isotope analysis was undertaken on a 7 mm garnet, and impure pyroxene, amphibole and plagioclase. The nearly homogenous oxygen isotopic composition (δ18OVSMOW=c. 6.7‰) in the garnet porphyroblast indicates closed fluid system conditions during garnet growth. However, isotopic fractionations between retrograde phases (amphibole and plagioclase) and garnet show an oxygen isotopic disequilibrium, indicating retrograde fluid-rock interactions. Unusual MORB-like rare earth element (REE) patterns for whole rock of the garnet pyroxenite contrast with most ultra-high-pressure (UHP) eclogites in the Dabie-Sulu area. However, the age-corrected initial εNd(t) is - 2.9, which indicates that the protolith of the garnet pyroxenite was derived from an enriched mantle rather than from a MORB source. Combined with the present data of oxygen isotopic compositions and the characteristics N2 content in the fluid inclusions, we suggest that the protolith of the garnet pyroxenite from Raobazhai formed in an enriched mantle fragment, which has been exposed to the surface prior to the Triassic metamorphism.
Keywords: China; eclogite; geochemistry; granulite; metamorphism

Elastic properties of gas hydrate-bearing sediments - Lee, M.W.; Collett, T.S. (Geophysics, 2001)
Downhole-measured compressional- and shear-wave velocities acquired in the Mallik 2L-38 gas hydrate research well, northwestern Canada, reveal that the dominant effect of gas hydrate on the elastic properties of gas hydrate-bearing sediments is as a pore-filling constituent. As opposed to high elastic velocities predicted from a cementation theory, whereby a small amount of gas hydrate in the pore space significantly increases the elastic velocities, the velocity increase from gas hydrate saturation in the sediment pore space is small. Both the effective medium theory and a weighted equation predict a slight increase of velocities from gas hydrate concentration, similar to the field-observed velocities; however, the weighted equation more accurately describes the compressional- and shear-wave velocities of gas hydrate-bearing sediments. A decrease of Poisson's ratio with an increase in the gas hydrate concentration is similar to a decrease of Poisson's ratio with a decrease in the sediment porosity. Poisson's ratios greater than 0.33 for gas hydrate-bearing sediments imply the unconsolidated nature of gas hydrate-bearing sediments at this well site. The seismic characteristics of gas hydrate-bearing sediments at this site can be used to compare and evaluate other gas hydrate-bearing sediments in the Arctic.
cited By 82
Keywords: Elasticity; Hydrates; Pore size; Sediments; Velocity; Gases; Hydration; Poisson ratio; Shear flow; Shear waves; Hydrate-bearing sediments; Compressional; Effective medium theories; Elastic properties; Gas hydrate bearing sediments; Gas hydrate concentrations; Gas hydrate saturations; Sediment porosities; Shear wave velocity; Geophysics; Gas hydrates; elastic property; gas hydrate; P-wave; S-wave; sediment; seismic velocity

Eclogite-facies quartz veins within metabasites of the Dabie Shan (Eastern China): Pressure-temperature-time-deformation path, composition of the fluid phase and fluid flow during exhumation of high-pressure rocks - Franz, Leander; Romer, Rolf L.; Klemd, Reiner; Schmid, Robert; Oberh{ansli, Roland; Wagner, Thomas; Shuwen, Dong (Contributions to Mineralogy and Petrology, 2001)
Metabasites in the high-pressure unit of the southern Dabie Shan (eastern China) contain quartz veins with high-pressure mineral assemblages. Two veins with the parageneses quartz-paragonite-ankerite-rutile and quartz-kyanite-talc-zoisite-rutile-calcite (pseudomorph after aragonite) as well as quartz-kyanite-paragonite- garnet-omphacite were investigated in detail. Host rocks of these veins are garnet amphibolites with eclogitic relics and quartz eclogites, respectively. The oldest phase of deformation (DI) is recorded in the eclogite, which displays a schistosity with shape alignment of high-pressure minerals such as omphacite, phengite and garnet. An estimate of the metamorphic P-T conditions yields 19-21 kbar at 570-620 °C for the eclogites. Fluid inclusion studies reveal a primary, low-salinity aqueous fluid phase responsible for the formation of the quartz veins. This is in correspondence with calculations of phase equilibrium curves on minerals of the vein paragenesis, which yield P-T conditions of 19.4 kbar and 591 °C in the presence of an aqueous fluid phase. Geochronology using the U-Pb system of rutile gives evidence for a Triassic minimum age of 207-221 Ma for the vein formation, which fits in the geotectonic framework of the orogen. This rather wide age range arises from the heterogeneous nature of the initial lead isotopic composition, reflecting the heterogeneous sources of the lead and the fluid phase. Part of the fluid was probably generated by prograde dehydration reactions in the country rocks of the veins, whereas strong evidence is also given for the derivation from an older basement, which also underwent subduction. The veins may have formed by extensive hydraulic fracturing or by volume reduction during eclogitisation of the crust. Subsequent to the formation of the veins, great parts of the high-pressure rocks experienced an intensive, syn-tectonic (D2) amphibolite facies metamorphism at 8.5-10.5 kbar and 590-645 °C, indicating isothermal decompression during exhumation. In the course of this exhumation, pseudosecondary CO2 inclusions were trapped in quartz of the veins. The last recorded stage of the retrograde overprint took place under static, greenschist facies conditions (450-480 °C) in the stability field of kyanite. The shape of the metamorphic P-T path for this stage is reflected by the fluid phase, which either re-equilibrated during this event or penetrated the rock on fractures. These findings show that fluid flow during subduction-related high-pressure metamorphism and subsequent uplift is rather complex; the fluid phase may originate from different sources and may interact with country rocks in the form of a chromatographic column transforming mineral assemblages, and also changing its own character.
Keywords: China; deformation; eclogite; exhumation; fluid flow; metamorphism; P-T conditions; P-T-t path

Differences in glacial and interglacial clay mineral associations of Baikal sediments from BDP-93-2 and BDP-96 cores - Solotchina, E.P.; Prokopenko, A.A.; Kuz'min, M.I.; Vasilevskii, A.N.; Shul'zhenko, S.G. (Geologiya i Geofizika, 2001)
By comparing the compositions of glacial and interglacial intervals of the Late Pliocene, Pleistocene, and Holocene from different parts of Lake Baikal, we demonstrate the use of the Baikal clay mineral associations as indicators of past changes of environment and climate. The methodical part of this work discusses the optimal sample preparation techniques and develops the previously suggested method for structural modeling of X-ray diffraction patterns as the most suitable for studying the Baikal sediments. The clay mineral associations in the Selenga delta area and at the Akademichesky Ridge are very similar, thus indicating a minor influence of local sediment sources on their formation. The similarities revealed by this comparison indicate uniformity of the Baikal catchment basin in terms of the processes of erosion, weathering, and supply of sediments. Despite the conditions of continuous hemipelagic sedimentation at the sites where deep drilling took place, both drill cores reveal a strong relation between the composition of clay mineral associations and paleoclimatic conditions. Intervals if glacial sediments during both the Late Pleistocene and the Late Pliocene are similar and are characterized by dramatic enrichment in well-crystallized mica (muscovite) and plagioclase, indicating intensification of physical weathering under cold climatic conditions. Interglacial intervals are enriched in fine illite, which suggests that it is a secondary mineral, the product of chemical weathering. In addition, the content of smectite layers in mixe layer illite-smectite increases during interglacials, which is also likely to imply a warmer and more humid climate. The lower interval of the BDP-96 core with the age over 4.5 Ma is characterized by an anomalous composition: high content of illite-smectite with a high concentration of smectite component, the presence of chlorite-smectite, and low contents of illite and muscovite. This specific mineral association has formed under climatic conditions much warmer than at present.

Detailed diatom biostratigraphy of Baikal sediments during the brunhes chron and climatic factors of species formation - Khursevich, G.K.; Karabanov, E.B.; Prokopenko, A.A.; Williams, D.F.; Kuz'min, M.I.; Fedenya, S.A.; Gvozdkov, A.N.; Kerber, E.V. (Geologiya i Geofizika, 2001)
The Baikal bottom sediments (borehole BDP-96-2) are stratigraphically characterized in detail (with a 500 year resolution) within the Brunhes Chron. The sediment section 35 m thick shows a distribution of diatoms (25 species and varieties), sponge spicules, and golden-algae cysts. Thirty-one local diatom zones are recognized in the section, providing a detailed stratigraphic division of the Baikal sediments. The distribution of diatom frustules and the diatom zones are correlated with the marine isotope stratigraphy. Climate played an important role in the formation of species of plankton diatoms in the lake. More than 21 new species of diatoms appear and then disappear in the section. Diatom species extinguished when glaciations began, and new species appeared during interglaciations. This suggests that the evolution of plankton diatoms in Baikal is under climatic control.

Density of basalt core from Hilo drill hole, Hawaii - Moore, James G (Journal of Volcanology and Geothermal Research, 2001)
Density measurements of 1600 samples of core from 889 to 3097 m depth below sea level in the Hawaii Scientific Drilling Program hole near Hilo, Hawaii show marked differences between the basaltic rock types and help define stratigraphy in the hole. Water-saturated densities of subaerial lava flows (occurring above 1079 m depth) have the broadest range because of the large density variation within a single lava flow. Water-saturated densities commonly range from 2.0 to 3.0 with an average of 2.55 ± 0.24 g/cc. Dikes and sills range from 2.8 to 3.1 g/cc). Densities of hyaloclastite commonly range from 2.3 to 2.7, with an overall average of about 2.5 g/cc. The low-density of most hyaloclastite is due primarily to palagonitization of abundant glass and presence of secondary minerals in the interstices between fragments. Four principal zones of pillow lava, separated by hyaloclastite, occur in the drill core. The shallowest (1983-2136 m) is paradoxically the densest, averaging 3.01 ± 0.10 g/cc. The second (2234-2470 m) is decidedly the lightest, averaging 2.67 ± 0.13 g/cc. The third (2640-2790 m) and fourth (2918-bottom at 3097 m) are high, averaging 2.89 ± 0.17 and 2.97 ± 0.08 g/cc, respectively. The first pillow zone includes degassed pillows i.e. lava erupted on land that flowed into the sea. These pillows are poor in vesicles, because the subaerial, one-atmosphere vesicles were compressed when the flow descended to deeper water and higher pressure. The second (low-density, non-degassed) pillow zone is the most vesicle-rich, apparently because it was erupted subaqueously at a shallow depth. The higher densities of the third and fourth zones result from a low vesicularity of only a few percent and an olivine content averaging more than 5% for the third zone and about 10% for the fourth zone. The uppermost hyaloclastite extending about 400 m below the bottom of the subaerial basalt is poorly cemented and absorbs up to 6 wt% of water when immersed. Progressing downward the hyaloclastite absorbs less water and becomes better cemented. This change is apparently due to palagonitization of glass and addition of secondary minerals in the deeper older hyaloclastite, a process favored by the increase of temperature with depth. The cementation is largely complete at 1800 m depth where the temperature attains about 20°C. The zone of freshest, uncemented hyaloclastite represents the weakest rock in the drill hole and is a likely level for tectonic or landslide disruption. © 2001 Published by Elsevier Science B.V.
Keywords: United States; Landslides; Stratigraphy; Thermal effects; Volcanic rocks; basalt; density; hyaloclastite; pillow lava; stratigraphy; Hyaloclastite; Volcanoes

Dating of deep-water sediments of Lake Baikal from the ratios of thorium and uranium isotopes in various fractions of their authigenic part - Sandimirov, I.V.; Kosov, A.A.; Vladimirova, T.A. (Geologiya i Geofizika, 2001)
We propose a method for dating Baikal deep-water sediments from the ratios of radioactive isotopes of Th and U in various fractions of the authigenic part of the sediments. We have determined the Th-U age of six horizons of the borehole BDP-96-2 and the average parameters of gravitational consolidation of the sediments. The extrapolated Th-U ages show a good correlation with paleomagnetic data throughout the core of BDP-96-2.

Crustal structure of the eastern Dabie Shan interpreted from deep reflection and shallow tomographic data - Schmid, R.; Ryberg, T.; Ratschbacher, L.; Schulze, A.; Franz, L.; Oberh{ansli, R.; Dong, S. (Tectonophysics, 2001)
A 20-km-long seismic line characterises the crustal reflection pattern of the easternmost Dabie Shan, the archetypal ultra-high-pressure (UHP) orogen of eastern China. The weak- to non-reflective upper crust (5 s two-way travel time (TWT); ∼ 15 km depth) is interpreted to comprise UHP rocks thrust over lithologically similar but non-UHP crust. The tectonic contact, although not imaged as a distinct reflector, is probably outlined by the rather abrupt change to diffuse but strong reflectivity within the mid to lower crust. Thus, the seismic pattern of the upper crust implies that mafic, oceanic crust does not constitute a significant proportion. The middle to lower crust (5-10 s TWT; ∼15-33 km depth) probably represents cratonal Yangtze basement, unaffected by the UHP metamorphism. The prominent lowermost reflectors (10-12 s TWT; ∼33-40 km depth) are interpreted to trace the Moho, excluding the presence of a crustal root inherited from the UHP orogeny. A tomographic P-wave velocity model for the uppermost crust (<700 m) traces shallowly W-dipping sedimentary rocks east and UHP gneisses west of the Cenozoic Tan Lu fault which is imaged to dip steeply eastward. The UHP rocks exhibit little lateral and vertical velocity variations (<10%), reflecting grossly homogeneous, gneissic lithology. Hundred-metre-scale velocity variations, however, may trace distinct large-scale structures, e.g. folds, known from outcrops and maps. © 2001 Elsevier Science B.V. All rights reserved.
Keywords: China; crustal structure; deep seismic sounding; seismic tomography; ultrahigh pressure metamorphism

A high-resolution record of east siberian paleoclimates in the early and middle pleistocene by palynological studies of baikal sediments from the deep borehole BDP-96-1 - Bezrukova, E.V.; Letunova, P.P. (Geologiya i Geofizika, 2001)
Two boreholes, BDP-96-1 (200 m thick) and BDP-96-2 (100 m thick), drilled at the top of the underwater Akademichesky Ridge at 53°41′48″N and 108°21′06″, gave two parallel cores which were dated paleomagnetically. The correlation of the paleomagnetic data with the global magnetic scale has shown the age of the sediments exposed at a depth of 200 m to be 5 Ma. The average sedimentation rate was constant, about 4 cm/ka. Earlier palynological studies of core samples from BDP-96-1 were performed at 2 m intervals, which corresponds to the period of ∼45-50 ka. In this paper, we present results of a detailed palynological analysis of the upper 30 m of the core performed at 20 cm intervals (4-5 ka). The age of the investigated core from BDP-96-1 covers most of the Brunhes epoch. Taking into account the lost upper 630 cm, it matches the time range from 170 to 780 ka BP. Palynological analysis of the sediments from the range under discussion revealed 13 epochs in the development of the regional flora structure: seven epochs of predominance of forest plant formations and six epochs of its significant degradation. The epochs are correlated with the stages of changes in the volume of global ice, recorded in the marine oxygen isotope scale. It is difficult to determine the extents of the coolings, because spores and pollen are scarce in the sediments of these epochs. However, it is apparent that forests did not disappear from East Siberia during the coolings. All main arboreal species now growing there persisted during Pleistocene glaciations. The structure of the vegetative cover underwent profound changes. The areas occupied by forests decreased. Larch and spruce north-taiga forests were predominant under the humid cold conditions of the beginning of interglacials and the end of glaciations; they were then replaced by fir and cedar pine forests. Larch-pine and cedar pine middle-taiga forests expanded under the moderately warm and relatively dry climate of the mid-interglacials. Obviously, this was accompanied by a wide spread of steppe vegetation. A comparison of the frequency and habit of changes in vegetation type in the study region 170-780 ka BP with those in various regions of the Northern Hemisphere demonstrates that the East Siberian vegetation responded to global climatic changes synchronously with the vegetations of other regions.

Kinematics of exhumation of high- and ultrahigh-pressure rocks in the Hong'an and Tongbai Shan of the Qinling-Dabie collisional orogen, eastern China - Webb, Laura E.; Ratschbacher, Lothar; Hacker, Bradley R.; Dong, Shuwen (Memoir of the Geological Society of America, 2001)
The Hong'an region offers an unique opportunity to investigate the tectonics of the continental collision event preserved in high-pressure (P) and ultrahigh-P metamorphic rocks in the Qinling-Dabie orogen of eastern China. Here, the extensive Cretaceous tectonic and thermal overprint observed in the Dabie Shan is weak. Normal-sense shear along the north-dipping Huwan detachment zone at the northern edge of the Hong'an block occurred ca. 235 Ma. This detachment facilitated the bulk of the exhumation of the high- and ultrahigh-P rocks as a penetratively deformed slab. The high- and ultrahigh-P rocks are exposed in a warped extensional footwall within which kinematic indicators in the high- and ultrahigh-P units show approximately top-to-north shear. Deformation was accompanied by retrograde metamorphism at amphibolite to greenschist facies conditions. Locally, younger northeast-southwest subhorizontal extension is recorded in ductile to brittle fabrics and the timing of deformation is defined by white mica recrystallization ca. 195 Ma. An Early Cretaceous dextral shear zone along the southwest boundary of the Tongbai Shan was synchronous with plutonism and normal to sinistral-oblique slip along the Xiaotian-Mozitang fault, which forms the northern boundary of the Dabie Shan. Coeval dextral and sinistral shear zones along the southwestern and northern margins of these blocks would have caused eastward lateral extrusion of the Tongbai, Hong'an, and Dabie Shan, perhaps driven by collision of the Lhasa block with Eurasia.
Keywords: China; Eurasia; Tongbai Mountains; Deformation; Kinematics; Mica; Tectonics; Continental collisions; Early Cretaceous; High pressure; Kinematic indicators; Lateral extrusion; Qinling-Dabie orogen; Sinistral shear; Ultrahigh pressure rocks; collision; Cretaceous; detachment fold; fault zone; metamorphic rock; phosphate rock; recrystallization; tectonics; Metamorphic rocks

Comparison of elastic velocity models for gas-hydrate-bearing sediments - Lee, M.W.; Collett, T.S. (Geophysical Monograph Series, 2000)
One of the distinct physical properties of gas-hydrate-bearing sediments is elevated seismic velocities. A number of velocity models and equations have been presented to describe the effect of gas hydrate on the seismic velocities; e.g., pore-filling model, cementation model, effective medium theory, a weighted equation, and time-average equation. The data set from Mallik 2L-38 gas hydrate research well drilled in northern Canada provided us a unique opportunity to test the velocity models for gas-hydrate-bearing sediments. Velocities predicted from an effective medium theory and those from a weighted equation are compared with observed well log velocities. In the case where there is no gas hydrate in the pore space, P-wave velocities predicted from the effective medium theory are lower than those from the weighted equation when porosity is less than about 30% and higher when porosity is higher than about 30%. For S-waves, effective medium theory predicts generally higher velocities than those from the weighted equation. Both theories predict similar increases in P- and S-wave velocities when gas hydrate occupies the pore space. Even though gas hydrate concentration in the pore space is not known accurately, analyses using both P- and S-wave velocities and their ratios enable us to test the validity of velocity models. Considering only P-wave velocities, there is not much difference between the effective medium and weighted equation. However, considering both P- and S-wave velocities and their ratios, the weighted equation is preferred to the effective medium theory in predicting elastic wave velocities for gas-hydrate-bearing sediments at the Mallik 2L-38 well. © 2001 by the American Geophysical Union.
cited By 18

High precision lead isotope systematics of lavas from the Hawaiian Scientific Drilling Project - Abouchami, W.; Galer, S.J.G.; Hofmann, A.W. (Chemical Geology, 2000)
We report Pb isotopic compositions for 35 samples of the volcanoes Mauna Loa and Mauna Kea from the Hawaiian Scientific Drilling Project (HSDP-1) core at Hilo. These data were obtained with an external precision of 100 ppm (2σ(ext.)) on the ratios 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb by using a Pb triple spike to correct for instrumental mass fractionation. The Pb isotopic compositions in the lower section (1200 to 280 m) of the core sample 200 to 400 ka-old Mauna Kea lavas, and display two well-defined linear arrays in 207Pb/204Pb-206Pb/204Pb and 208Pb/204Pb-206Pb/204Pb isotope spaces. There is a suggestion that Mauna Loa (0 to 280 m depth) also displays such linear array(s). However, analysis of the Mauna Loa samples is complicated by residual contamination and/or sample heterogeneity. While these latter data exhibit a satisfactory array in 208Pb/204Pb vs. 206Pb/204Pb, there still remains scatter in 207Pb/204Pb-206Pb/204Pb space, making it difficult to assess the true Pb isotope systematics of Mauna Loa. The presence of two linear Pb isotopic arrays in Mauna Kea can be interpreted as either reflecting two parallel isochrons or in terms of binary mixing. If interpreted as isochrons, the 207Pb/204Pb-206Pb/204Pb systematics correspond to an age of 1.9 Ga. Comparison of measured Th/U ratios in the lavas and those inferred from Pb isotope systematics strongly suggest that the Pb isotopic arrays reflect binary mixing, and this bears directly on the question of how many distinct components are present in the Hawaiian plume. Most of the new Mauna Kea data lie well outside the mixing-component triangle defined in the literature by the 'Kea', 'Loihi', and 'Koolau' components. On the basis of the relationships between Pb isotope ratios in 3D and a principal component analysis of the Mauna Kea Pb isotope dataset, we show here that a three-component mixing model can in principle explain both mixing lines. However, such an explanation requires a highly specific set of mixing conditions in order to produce parallel arrays in Pb isotope space (2D and 3D). Therefore, our preferred interpretation is that the two arrays reflect binary mixing, with four discrete source components involved in the generation of the Kea lavas. Comparison of the Pb isotope characteristics of these lavas with those of East Pacific Rise (EPR) MORB glasses further suggests that EPR-type Pacific lithosphere does not contribute to the source of Kea lavas. The position of samples along the mixing lines does not correlate with stratigraphic height in the core, and therefore the age of the lavas. Rather, it appears as though the relative proportions of the endmembers are controlled by the spatial configuration of these endmembers, and by melting and transport processes in the source itself. The stratigraphic fluctuations of Pb and Sr isotopes contrast with the monotonic decrease of ε(Nd) and ε(Hf) values as a function of age. This may in part be explained by differences in analytical precision of isotope measurements relative to the total range of values observed. This analytical resolution is far higher for Pb than for the other radiogenic isotopes. Alternatively, the observed fluctuation may be caused by the mobility of lead (as well as Rb and/or Sr) during the ancient differentiation process that created the differences in parent-daughter ratios. (C) 2000 Elsevier Science B.V. All rights reserved.
Keywords: United States; geochemistry; hot spot; lava; lead isotope

Fluid history of UHP metamorphism in Dabie Shan, China: A fluid inclusion and oxygen isotope study on the coesite-bearing eclogite from Bixiling - Xiao, Yilin; Hoefs, Jochen; Van Den Kerkhof, Alfons M.; Fiebig, Jens; Zheng, Yongfei (Contributions to Mineralogy and Petrology, 2000)
The coesite-bearing eclogites and associated ultramafic rocks of Bixiling form the largest metamorphic complex in the Dabie-Sulu ultrahigh-pressure (UHP) metamorphic belt. They mainly consist of 'fresh' eclogite, kyanite-rich eclogite, retrograded eclogite and garnet peridotite. Fluid inclusion and oxygen isotope studies have been carried out on the different types of eclogite and peridotite in order to identify the role of fluids during the metamorphic evolution culminating in UHP metamorphism. Five types of fluid inclusions were distinguished based on textural criteria and fluid compositions: (1) primary Ca-rich brines in quartz blebs in kyanite; (2) primary NaCl-dominated high-salinity inclusions in omphacite and kyanite; (3) primary NaCl-dominated medium- to high-salinity inclusions in matrix quartz; (4) carbonic inclusions in omphacite and matrix quartz; (5) secondary low-salinity aqueous (or pure water) inclusions in matrix quartz. The Ca-rich fluid inclusions in quartz blebs in kyanite represent the earliest recognizable fluids (prograde metamorphism) as they largely escaped late re-equilibration. Fluid inclusions in omphacite and kyanite may have been trapped during peak metamorphic conditions, whereas low-salinity aqueous inclusions in matrix quartz were trapped during the latest stage of uplift. UV-laser oxygen isotope measurements show that garnet and clinopyroxene from the 'fresh' eclogite, kyanite-rich eclogite and garnet peridotite have narrow δ18O values ranging from 3.0 to 3.9‰. In contrast, garnet and omphacite in retrograded eclogite have δ18O values of -1.8 to -1.2 and of -1.1 to -0.6‰, respectively. The difference in oxygen isotope composition is interpreted to result from partial oxygen isotope exchange between the UHP complex and retrograde fluids during late exhumation. Fluids derived from the surrounding gneiss were probably responsible for the low-salinity solutions found in secondary fluid inclusions and the lowering of the δ18O values of the retrograded eclogite.
Keywords: Bixiling; China; Dabie Shan; coesite; eclogite; fluid composition; fluid inclusion; isotopic composition; oxygen; ultrahigh pressure metamorphism

Dissociation of natural gas hydrates observed by X-ray CT scanner - Mikami, J.; Masuda, Y.; Uchida, T.; Satoh, T.; Takeda, H. (Annals of the New York Academy of Sciences, 2000)
Core samples containing pore-spare gas hydrate within granular sands were collected from 913.76 m of the research well named JAPEX/JNOC/GSC Mallik 2L-38. X-ray CT images of the core were acquired while warming from -18 to 4°C, and subsequently during stepped decreases of 0.1 MPa in the chamber pressure below the methane hydrate equilibrium pressure. Discharged gas flows and sample temperatures were monitored continuously. Changes in CT values indicated that gas hydrate dissociated simultaneously both on the exposed surfaces and within the pore spaces of the sample in response to pressure changes. This suggested that pressure reductions were effectively transmitted through the sample most likely because the samples contained some amount of fluids. The result of gas flow measurements indicated that a larger pressure drawdown caused a higher dissociation rate.
cited By 21
Keywords: methane; natural gas; computer assisted tomography; conference paper; dissociation; gas; gas flow; pressure; sand; temperature; X ray analysis

A new estimate of the Holocene lowstand level of Lake Titicaca, central Andes, and implications for tropical palaeohydrology - Cross, Scott L.; Baker, Paul A.; Seltzer, Geoffrey O.; Fritz, Sherilyn C.; Dunbar, Robert B. (Holocene, 2000)
New evidence from piston cores and high-resolution seismic reflection data shows that water levels in Lake Titicaca were as much as 100 m below the present level during the early to mid-Holocene (between >6 and 3.8 14C kyr BP). Climatological and modelling studies indicate that Lake Titicaca rainfall depends on convective activity in upwind Amazonia; the lake-level data therefore suggest a drier Amazon Basin during this time. This view is bolstered by an excellent match between the Titicaca lake-level curve and decreased methane concentrations in Greenland ice, previously ascribed to drying of low-latitude wetlands (Blunier et al., 1995). The postglacial history of Lake Titicaca fits a global pattern of lake-level change in the tropics, characterized by opposite phasing between the Southern and Northern Hemispheres. This pattern is most likely the result of orbital controls over the intensity of summer insolation.
Keywords: Bolivia; Lake Titicaca; Holocene; lake level; lowstand; paleohydrology

Comparison of biogenic silica, Br and Nd distributions in the sediments of Lake Baikal as proxies of changing paleoclimates of the last 480 kyr - Phedorin, M.A.; Goldberg, E.L.; Grachev, M.A.; Levina, O.L.; Khlystov, O.M.; Dolbnya, I.P. (Nuclear Instruments and Methods in Physics Research, Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 2000)
The paper presents a comparison of the profiles of biogenic silica (BiSi), Br and Nd in the first 20 m of the BDP-96-2 core of sediments of Lake Baikal obtained on top of the underwater Akademichesky Ridge. The concentrations of the two elements mentioned were measured by X-ray fluorescent analysis with synchrotron radiation (SR-XFA) at 10 cm intervals. Br correlates positively, and Nd negatively with BiSi. Spectral analysis of the profiles revealed the 100, 41, 23 and 19 kyr major external forcing periods. We studied the elemental composition of extant diatom algae of Lake Baikal in order to find out whether any elements are bound by them specifically. SR-XFA revealed, among others that Br is accumulated by extant diatom algae of Lake Baikal, presumably due to incorporation into their organic matter. Br can be regarded as a new proxy of the paleo-productivity of Lake Baikal. The paper also considers arguments suggesting that the first 2 m of sediment are missing in the BDP-96-2 core.
Keywords: Bromine; Geochemistry; Lakes; Neodymium; Organic compounds; Sediments; Silica; Spectrum analysis; Synchrotron radiation; Biogenic silica; Lake Baikal; Paleoclimate indicators; Sedimentary geochemistry; X ray fluorescence analysis; X ray analysis

Climate-induced fluctuations of 10Be concentration in Lake Baikal sediments - Horiuchi, K.; Kobayashi, K.; Oda, T.; Nakamura, T.; Fujimura, C.; Matsuzaki, H.; Shibata, Y. (Nuclear Instruments and Methods in Physics Research, Section B: Beam Interactions with Materials and Atoms, 2000)
Sedimentary 10Be records covering the last 150 kyr were obtained from three cores collected at the Academician Ridge (BDP-96/hole2 core and VER96/st.3 core) and at the Buguldeika Saddle (BDP-93/hole2 core) in Lake Baikal. The 10Be concentrations of the three cores varied between 0.5 × 109 and 1.5 × 109 atoms/g, and coincidently dropped at the stratigraphic intervals of marine oxygen isotope stages (MIS) 2, 4, 5d and 6. The depositional fluxes of 10Be, on the other hand, generally rose in those stages having an increase in the dry bulk densities and sediment accumulation rates. These results are consistent with previous work (Horiuchi et al., 1999), suggesting that the dilution effects of low-10Beconcentration particles principally controlled the fluctuations of the 10Be concentrations of Lake Baikal sediments. Low-10Be-concentration particles have been intensively produced by mechanical weathering and physical erosion under the cold and dry climatic conditions during the peak glaciation period, and have been directly brought from the source areas into the lake as a result of the thin vegetative cover of the watershed. © 2000 Elsevier Science B.V. All rights reserved.
Keywords: Beryllium; Climate change; Erosion; Glacial geology; Lakes; Oxygen; Radioisotopes; Sediments; Stratigraphy; Watersheds; Weathering; Accelerated mass spectrometry; Marine oxygen isotope stages; Marine sediments; Mass spectrometry

Clay minerals in bottom sediments of Lake Baikal as indicators of climate change - Kuz'min, M.I.; Solotchina, E.P.; Vasilevskii, A.N.; Stolpovskaya, V.N.; Karabanov, E.B.; Geletii, V.F.; Bychinskii, V.A.; Anoshin, G.N.; Shul'zhenko, S.G. (Geologiya i Geofizika, 2000)
The mineral assemblage of Pliocene - Pleistocene bottom sediments of Lake Baikal, exposed by the deep-sea BDP-96-2 (Baikal Drilling Project) has been studied by the methods of XRD and IR spectroscopy. Complex XRD patterns of clay minerals have been modeled using an advanced algorithm based on calculation of interference function from one-dimensional disordered crystals of finite thickness. The modeling implies fitting of theoretical to observed patterns and optimization procedure by methods of nonlinear programming. It has been shown that major clay minerals in the sediments that formed for the past 2.5 Ma during cold stages are illite-illite-smectites with predominantly illite layers, and chlorite; the principal clay minerals in the sediments that formed during warm periods are smectite, illite-smectites with high contents of smectite layers, and chlorite-smectites. These data allow unravelling the climate history of Lake Baikal from biogenic silica record.

A new record of Holocene climate change from the bottom sediments of Lake Baikal - Karabanov, E.B.; Prokopenko, A.A.; Williams, D.F.; Khursevich, G.K. (Palaeogeography, Palaeoclimatology, Palaeoecology, 2000)

JAPEX/JNOC/GSC Mallik 2L-38 gas hydrate research well, Mackenzie Delta, N.W.T.: overview of field operations - Ohara, T.; Dallimore, S.R.; Fercho, E. (SPE Proceedings - Gas Technology Symposium, 2000)
The Mallik 2L-38 well was drilled in February and March, 1998 to a depth of 1150m at a site located in the Mackenzie Delta, N. W. T., Canada. Undertaken as a collaborative agreement between the Japan National Oil Corporation and the Geological Survey of Canada, the well was conducted as a research and development project with engineering goals to evaluate various technologies for drilling and coring gas-hydrate-bearing strata. The Mallik site was chosen as it had favorable logistics and was though to contain a thick interval of gas hydrates between 897 and 1110 m depth. Drilling operation included a surface hole (with 8 coring runs) to 687m for installation of a 340mm surface casing, and a main hole (with 16 coring runs) to the target depth of 1150m. The drilling system utilized a KCl/polymer drilling mud that was cooled to 2°C using a plate type heat exchanger. Drilltreat, a chemical mud additive was used in the main hole to stabilize the hydrate within drill cuttings and formation sediments. Drilling operations were conducted without any serious hole problems, accidents, or mishaps. However, delays were caused by adverse weather and mechanical problems, causing adjustments in the overall program. Coring in the main hole was particularly successful allowing evaluation of four different core barrels. Gas-hydrate-bearing core was collected in a variety of sediments between 886 and 952m. The excellent condition of the core samples, controlled gas hydrate dissociation within the mud column, and near-gauge hole, confirmed that the combination of chilled mud with Drilltreat performed extremity well.
cited By 8
Keywords: Additives; Gas hydrates; Heat exchangers; Mud logging; Natural gas well drilling; Oil field development; Organic polymers; Petroleum industry; Petroleum prospecting; Potassium compounds; Sediments; Societies and institutions; Chemical mud additive; Coring; Coring gas hydrate bearing strata; Polymer drilling mud; Potassium chloride drilling mud; Natural gas fields

High-Si phengite, mineral chemistry and P-T evolution of ultra-high-pressure eclogites and calc-silicates from the Dabie Shan, eastern China - Schmid, Robert; Franz, Leander; Oberhnsli, Roland; Dong, Shuwen (Geological Journal, 2000)
A suite of coesite-eclogites and associated calc-silicate rocks from the ultra-high-pressure (UHP) belt in the Dabie Shan (eastern China) was investigated petrologically. Field relations and the presence of UHP minerals such as coesite, omphacite and high-Si phengite in the eclogites and the enclosing calc-silicates testify to a common metamorphic evolution for these two lithologies. Except for one sample, all bear phengite with unusually high silica contents (Si up to 3.7 per formula unit). Phengite occupies various textural positions indicating that different metamorphic stages are reflected by these white micas, which correlate with distinct mineral zonation patterns. Using the latest thermobarometric calibrations for eclogite-facies rocks, maximum pressure-temperature (P-T) conditions of 40-48 kbar at < 750°C were estimated for the peak-metamorphic mineral assemblages. These P-T conditions were calculated for both eclogitic garnet porphyroblasts with diffusion-controlled zoning as well as garnet porphyroblasts with prograde growth zonation patterns. Most samples were affected by a strong retrograde overprint mainly under eclogite- and amphibolite-facies conditions. Thermobarometry using mineral sets from different textural positions reveals cooling and decompression of the UHP rocks down to < 20 kbar at < 600°C for the bulk of the samples. Decompression and heating indicated by a few samples is interpreted to result from mineral chemical disequilibrium or late thermal influence. These new data show that subduction of continental crust in the Dabie Shan was deeper than previously thought, and also that some cooling and decompression took place at upper-mantle depths. Copyright © 2000 John Wiley and Sons, Ltd.
Keywords: China; Dabie Shan; eclogite; metamorphism; P-T conditions; ultrahigh pressure metamorphism

Recycled oceanic crust observed in 'ghost plagioclase' within the source of Mauna Loa lavas - Sobolev, A.V.; Hofmann, A.W.; Nikogosian, I.K. (Nature, 2000)
The hypothesis that mantle plumes contain recycled oceanic crust is now widely accepted. Some specific source components of the Hawaiian plume have been inferred to represent recycled oceanic basalts, pelagic sediments or oceanic gabbros. Bulk lava compositions, however, retain the specific trace- element fingerprint of the original crustal component in only a highly attenuated form. Here we report the discovery of exotic, strontium-enriched melt inclusions in Mauna Loa olivines. Their complete trace-element patterns strongly resemble those of layered gabbros found in ophiolites, which are characterized by cumulus plagioclase with very high strontium abundances. The major-element compositions of these melts indicate that their composition cannot be the result of the assimilation of present-day oceanic crust through which the melts have travelled. Instead, the gabbro has been transformed into a (high-pressure) eclogite by subduction and recycling, and this eclogite has then been incorporated into the Hawaiian mantle plume. The trace-element signature of the original plagioclase is present only as a 'ghost' signature, which permits specific identification of the recycled rock type. The 'ghost plagioclase' trace-element signature demonstrates that the former gabbro can retain much of its original chemical identity through the convective cycle without completely mixing with other portions of the former oceanic crust.
cited By 300
Keywords: basalt; mantle plume; melt inclusion; oceanic crust; petrogenesis; petrology; recycling; strontium; article; geography; geology; natural science; oceanic regions; priority journal; United States; volcano; United States

Mapping the Hawaiian plume conduit with converted seismic waves - Li, X.; Kind, R.; Priestloy, K.; Sobolev, S.V.; Tilmann, F.; Yuar, X.; Weber, M. (Nature, 2000)
The volcanic edifice of the Hawaiian islands and seamounts, as well as the surrounding area of shallow sea floor known as the Hawaiian swell, are believed to result from the passage of the oceanic lithosphere over a mantle hotspot. Although geochemical and gravity observations indicate the existence of a mantle thermal plume beneath Hawaii, no direct seismic evidence for such a plume in the upper mantle has yet been found. Here we present an analysis of compressional-to-shear (P-to-S) converted seismic phases, recorded on seismograph stations on the Hawaiian islands, that indicate a zone of very low shear-wave velocity ( < 4 km s-1) starting at 130-140 km depth beneath the central part of the island of Hawaii and extending deeper into the upper mantle. We also find that the upper-mantle transition zone (410-660 km depth) appears to be thinned by up to 40-50 km to the south-southwest of the island of Hawaii. We interpret these observations as localized effects of the Hawaiian plume conduit in the asthenosphere and mantle transition zone with excess temperature of 300°C. Large variations in the transition-zone thickness suggest a lower-mantle origin of the Hawaiian plume similar to the Iceland plume, but our results indicate a 100°C higher temperature for the Hawaiian plume.
cited By 172
Keywords: crustal structure; hot spot; mantle plume; seismic velocity; air temperature; article; atmosphere; geography; plume; priority journal; sea; shear rate; United States; volcano; Pacific Ocean

Multi-wavelength synchrotron radiation XRF determination of U and Th in sedimentary cores from Lake Baikal - Phedorin, Michael A.; Goldberg, Evgeny L.; Bobrov, Vladislav A.; Khlystov, Oleg M.; Grachev, Michael A. (Geostandards Newsletter, 2000)
A new technique for the determination of uranium and thorium in lacustrine sediments is based on non-destructive synchrotron radiation X-ray fluorescence analysis (SR-XRF) of sediment samples using monochromatic beams of different energies as excitation sources, with the recording and subsequent iterative processing of the X-ray fluorescence spectra. The technique has a multielement capability and enables fast, simultaneous analysis of a few tens of elements. The proposed SR-XRF technique was tested against INAA and ICP-MS methods and showed a number of advantages with a generally good correlation of results by the three methods. Uranium and Th profiles have been measured at a time resolution of 2 kyr in a drill core (BDP-96) from Lake Baikal bottom sediments deposited between 780 and 40 kyr BP. During this time span, peaks in the U and U/Th concentration marked warm climates. Oscillations of U and U/Th in Baikal bottom sediments during the Brunhes chron reflect climate-induced global change in the volume of polar ice, controlled by long-term cyclicity of the Earth's orbital parameters. The response of these warm periods is similar to that observed earlier in shorter cores that span the two last interglacials (220-0 kyr BP).

Numerical experiments for seismic tomography using the Singular Value Decomposition (SVD) technique - Nakamura, M.; Hayashida, T.; Takenaka, H.; Shimizu, H.; Suzuki, S. (Science Reports of the Kyushu University, Department of Earth and Planetary Sciences, 2000)
The purpose of this study is to learn how to consider setting the borehole and arrangement of the measuring instruments for seismic tomography. For this purpose we made an underground structure model of Unzen Volcano based on the results of previous studies, and simulated the seismic tomography identifying this model with the true structure because one of some plans in Scientific drilling project at Unzen Volcano is seismic tomography by using a borehole approaching conduit We located sources and receivers along the borehole and at the surface, and calculated synthetic seismograms at the receiver positions by a 2-D finite difference method. We measured the first P arrival time in the synthetic seismograms, and constructed the observational equation for seismic tomography. We investigated the solution of this equation by the Singular Value Decomposition (SVD) technique. We found we can consider borehole location and arrangement of sources and receivers through such a simulation by Singular values and eigenvalue that got in this way.
cited By 0
Keywords: array; crosshole seismic method; seismic tomography

Occurrences of natural gas hydrates beneath the permafrost zone in Mackenzie Delta. Visual and X-ray CT imagery - Uchida, T.; Dallimore, S.; Mikami, J. (Annals of the New York Academy of Sciences, 2000)
The JAPEX/JNOC/GSC Mallik 2L-38 research well was drilled to a depth of 1150 m beneath the permafrost zone in the Mackenzie Delta, N.W.T., Canada, early in 1998. A large amount of natural gas hydrates were successfully retrieved from a variety of sandy and gravel sediments. Over 110 m of gas hydrate-bearing sediments were found to be distributed between 897 m and 1100 m deep. Approximately 37 meters of core were recovered in this interval with most of the recovered gas hydrates being less than 2 mm in size occurring mainly in intergranular porosity of silty to clean massive sand and conglomerate (granule to pebble). Typically, hydrate-bearing strata were between 10 cm and more than one meter thick with an estimated porosity of 25 to 35%. The largest form of hydrate was about 2 cm in diameter, occurring as clasts and intergranular porosity within granular sands. Occurrences of natural gas hydrate have been observed visually at the drill site and in core samples preserved in pressurized storage vessels utilizing an X-ray CT scanner technique. Quantitative assessments of gas hydrate concentrations in core samples have been made based on pressure response of dissociation vessels and direct volumetric measurements. Six types of gas hydrate have been recognized: (1) pore-space hydrate, (2) platy hydrate, (3) layered/massive hydrate, (4) disseminated hydrate, (5) nodule hydrate, and (6) vein/dyke hydrate. The X-ray CT images proved useful for characterizing macroscopic forms of gas hydrate. Finer grained occurrences were more difficult to study, however the distribution of gas hydrates and granular grains ran be recognized. The occurrences of natural gas hydrates in the Mallik well are compared to the previous natural gas hydrate core samples obtained from ODP/DSDP programs and other field studies.
cited By 34
Keywords: methane; natural gas; Canada; computer assisted tomography; conference paper; gas; gas analysis; oil industry; porosity; sand; sediment; X ray analysis

Overlapping volcanoes: The origin of Hilo Ridge, Hawaii - Holcomb, Robin T.; Nelson, Bruce K.; Reiners, Peter W.; Sawyer, Nuni-Lyn (Geology, 2000)
The submarine Hilo Ridge has been interpreted as a part of Mauna Kea volcano, but is crossed at 1100 m depth by a submerged shoreline terrace composed of basalts that are isotopically distinct from those of Mauna Kea and similar to those of Kohala volcano. This terrace evidently is a product of Kohala instead of Mauna Kea. Almost all of Hilo Ridge below the terrace therefore must predate the principal growth of Mauna Kea, which has superficially isolated the ridge from its Kohala source by overlapping its proximal segment. The Mauna Kea section penetrated by the Hawaii Scientific Drilling Project is predicted to be thinner than expected previously, owing to the overlap. Similar overlaps are suspected among other volcanoes and may cause significant changes in the understanding of Hawaiian volcanism.
Keywords: Hawaii [(ISL) Hawaiian Islands]; Hawaii [United States]; Hawaiian Islands; Hilo; Kohala; Mauna Kea; United States; Isotopes; Hilo Ridge; Isotope ratios; Kohala; Mauna Kea; Scientific drilling; isotopic ratio; shoreline; submarine; terrace; volcanism; volcano; Volcanoes

Relation between gas hydrate and physical properties at the Mallik 2L-38 research well in the Mackenzie delta - Winters, W.J.; Dallimore, S.R.; Collett, T.S.; Jenner, K.A.; Katsube, J.T.; Cranston, R.E.; Wright, J.F.; Nixon, F.M.; Uchida, T. (Annals of the New York Academy of Sciences, 2000)
As part of an interdisciplinary field program, a 1150-m deep well was drilled in the Canadian Arctic to determine, among other goals, the location, characteristics, and properties of gas hydrate. Numerous physical properties of the host sediment were measured in the laboratory and are presented in relation to the lithology and quantity of in situ gas hydrate. Profiles of measured and derived properties presented from that investigation include: sediment wet bulk density, water content, porosity, grain density, salinity, gas hydrate content (percent occupancy of non-sediment grain void space), grain size, porosity, and post-recovery core temperature. The greatest concentration of gas hydrate is located within sand and gravel deposits between 897 and 922 m. Silty sediment between 926 and 952 m contained substantially less, or no, gas hydrate perhaps because of smaller pore size.
cited By 9
Keywords: methane; natural gas; Arctic; Canada; conference paper; gas; gas analysis; porosity; river; salinity; sand; sediment; temperature; water content

Results from a TEM survey for gas hydrate delineation in permafrost - Hyde, C.; Dallimore, S.; Hunter, J.; Douma, M.; Good, R.; Burns, R. (SEG Technical Program Expanded Abstracts, 2000)
In conjunction with the 1998 Mallik 2L-38 gas-hydrate research well program (Dallimore et al, 1999), two time-domain electromagnetic sounding profiles were surveyed in an attempt to delineate a known gas-hydrate zone at depth. The research well is located on Richard's Island in the Mackenzie Delta, Northwest Territories. Electrical resistivity logging at the Mallik site in 1972 and 1998 defined a resistive zone from 900 m to 1100 m coincident with a gas-hydrate zone lying within unfrozen and electrically conductive sediments. A resistive layer, interpreted as the gas-hydrate zone was detected by the TEM survey at all soundings except for those overlain by thin permafrost and a thick unfrozen zone. This thin permafrost/thick unfrozen zone assemblage resulted in greater than expected conductivity-thickness product and limited the depth investigation of the survey. In addition, late-time negative transients interpreted as induced polarization effects, obscured the clear detection of gas hydrates. © 2000 Society of Exploration Geophysicists.
cited By 0
Keywords: Electric logging; Gases; Hydration; Permafrost; Petroleum prospecting; Surveys; Electrically conductive; Hydrate zones; Induced polarization; Resistive zones; TEM surveys; Time domain electromagnetics; Gas hydrates

Sedimentation processes and new age constraints on rifting stages in Lake Baikal: Results of deep-water drilling - Kuzmin, M.I.; Karabanov, E.B.; Prokopenko, A.A.; Gelety, V.F.; Antipin, V.S.; Williams, D.F.; Gvozdkov, A.N. (International Journal of Earth Sciences, 2000)
With this paper we present a first attempt to combine the direct results on lithology, composition and age dating in the boreholes BDP-93, BDP-96 and BDP-97 with geological and seismic data from the areas where those sections were drilled. The sedimentary environments represented by the BDP boreholes are markedly different and possess characteristic lithological features. The results of the deep drilling provide the essential means for testing numerous age models used in geological reconstructions of the Lake Baikal rifting dynamics. Neither the basin-wide unconformity interpreted from seismic data, nor the interpreted change from shallow-water to deep-water facies at the boundary of the seismic stratigraphic complexes were found in the BDP-96 boreholes on Academician Ridge. Also, lithology does not support the proposed reconstructions of intense lake level fluctuations and transgressions during the Pliocene at Academician Ridge. The continuous deep-water hemipelagic sedimentation at Academician Ridge has existed for the past 5 Ma. The beginning of an intense rifting phase of the Neobaikalian sub-stage and related drastic changes in sedimentation processes were interpreted on seismic sections as the basin-wide unconformity B10. Different age estimates for this boundary ranged from Late Pliocene (3.5 Ma) to Plio-Pleistocene boundary. As shown by BDP-96 borehole, B10 is associated with a lithological change from diatomaceous ooze to dense silty clay and not with an erosional contact. The new age for this boundary in BDP-96 is approximately 2.5 Ma. This new age constraint suggests that the upper sedimentary strata of Northern Baikal (1.5-1.7 km thick) have formed during the past 2.5 Ma with average sedimentation rates of 60-70 cm/ka. The BDP-93 boreholes at Buguldeika suggest that uplift in Primorsky Range took place prior to 1.07-1.31 Ma, a date which exceeds the age of previous geological models.
Keywords: Russian Federation; depositional environment; paleoclimate; paleoenvironment; Pliocene; Quaternary; rifting; sedimentation rate

The application of Raman spectroscopy to the study of gas hydrates - Tulk, C.A.; Ripmeester, J.A.; Klug, D.D. (Annals of the New York Academy of Sciences, 2000)
Raman spectroscopy is reviewed with particular emphasis placed on its application to gas hydrates. Experimental examples discussed include studies of the totally symmetric C-H stretching vibration v1 (A1) of methane in both synthetic and natural hydrate samples (from the JAPEX/JNOC/GSC Mallik 2L-38 research well); a comparison of the coupled O-H vibrations of water in the host lattice of CH4 hydrate and ice I(h) at low temperature; and local structural details of the relaxation of the hydrogen-bonded water on crystallization to structure II hydrate of amorphous tetrahydrofuran (THF) aqueous solutions. This paper is intended to be an introduction to Raman spectroscopy with specific examples from research at the National Research Council of Canada, and is aimed at those who wish to apply the technique as a tool to investigate gas hydrates.
cited By 86
Keywords: methane; water; aqueous solution; conference paper; crystallization; gas; gas analysis; Raman spectrometry

The Parkfield, California earthquake experiment: An update in 2000 - Roeloffs, E. (Current Science, 2000)
The US Geological Survey, in cooperation with other institutions, continues to monitor the San Andreas Fault (SAF) near Parkfield, California, hoping to capture high resolution records of continuous deformation before, during and after a magnitude 6 earthquake, as well as the details of its rupture initiation and strong ground motion. Despite the failure of the prediction that the next M 6 Parkfield earthquake would occur before 1993, Parkfield still has a higher known probability (1 to 10% per year) than anywhere else in the US of a M 6 or greater earthquake. Parkfield instrumentation is still largely in place, although there have been losses due to attrition as well as improvements made possible by new technology. Most Parkfield data sets are now available via the Internet, and all others may be obtained upon request from individual investigators. Detailed seismic monitoring has shown that events with identical seismograms, recurring in exactly the same locations, account for a high proportion of the background seismicity at Parkfield. Geophysical studies have revealed that fault zone seismic and electrical properties are consistent with high fluid content. The rate of interseismic slip on the SAF changed significantly in late 1992 or early 1993, during a period of relatively high seismic activity. The strain-rate change, measured by borehole tensor strainmeters and the two-colour electronic distance-measuring network, was also manifested as shortened recurrence intervals of repeating microearthquakes. Whether or not the accelerated deformation turns out to be an intermediate-term precursor to the next M 6 Parkfield earthquake, documenting the variation of interseismic strain rates with time has important implications for fault dynamics and seismic hazard estimation. Two possible instances of pre-earthquake signals have been recorded at Parkfield: water-level and strain changes over a period of three days prior to the nearby 1985 Mw 6.1 Kettleman Hills, California, earthquake and anomalous electromagnetic signals prior to the M 5 earthquake near Parkfield on 20 December 1994. Future work planned at Parkfield includes a National Science Foundation proposal to construct an SAF Observatory at Depth (SAFOD), as part of the Earthscope initiative. The Observatory will consist of a 4-km-deep borehole to penetrate the SAF and a shallow microearthquake cluster on Middle Mountain, directly above the hypocenter of the 1966 Parkfield earthquake.
cited By 28

UNAM Scientific shallow-drilling program of the Chicxulub impact crater - Rebolledo-Vleyra, M.; Urrutia-Fucugauchi, J.; Mar'in, L.E.; Trejo-Garc'ia, A.; Sharpton, V.L.; Soler-Arechalde, A.M. (International Geology Review, 2000)
A scientific drilling program is being carried out by the National Autonomous University of Mexico (UNAM) at the southern sector of the Chicxulub impact crater in the Yucatan Peninsula, Mexico. Eight boreholes, ranging in depth from 60 m to 702 m, with a total of 2.62 km of continuos core, were recovered. A high recovery rate of up to 99% (overall average recovery rate for the eight boreholes is 87%) allows us to investigate in detail the stratigraphy of the impact lithologies and the Tertiary carbonate sequence. Three of the boreholes (UNAM-5, UNAM-6, and UNAM-7, with core recovery rates from 89 to 99%) sampled impact breccias that were classified in two units-an upper breccia sequence rich in basement clasts, impact glass, and fragments of melt (suevite-like breccia) and a lower breccia sequence rich in limestone, dolomite, and evaporite clasts (bunte-like breccia). Depths of contact between the Tertiary carbonate sequence and the impact breccias are 332.0 m in UNAM-5, 222.2 m in UNAM-7, and 282.8 m in UNAM-6, giving the depth to the K/T boundary. In UNAM-7, the contact between the upper and the lower breccias is at 348.4 m, which yields a thickness of 126.2 m for the suevite-like breccia. The rest of the boreholes sampled part of the Tertiary carbonate sequence (∼200 m thick), composed mainly of limestones, dolomitized carbonates, and calcarenite, with some fossiliferous horizons.
cited By 50

8 - Forest-desert alternation history revealed by the pollen-record in lake baikal over the past 5 million years - Kawamuro, K.; Shichi, K.; Hase, Y.; Iwauchi, A.; Minoura, K.; Oda, T.; Takahara, H.; Sakai, H.; Morita, Y.; Miyoshi, N.; Kuzmin, M.I. (Lake Baikal, 2000)

9 - Vegetation history of the southeastern and eastern coasts of Lake Baikal from bog sediments since the last interstade - Takahara, H.; Krivonogov, S.K.; Bezrukova, E.V.; Miyoshi, N.; Morita, Y.; Nakamura, T.; Hase, Y.; Shinomiya, Y; Kawamuro, K. (Lake Baikal, 2000)

1 - Baikal drilling project - Kuzumin, M.I.; Williams, D.F.; Kawai, T. (Lake Baikal, 2000)

7 - TEM analysis of smectite-illite mixed-layer minerals of core BDP 96 hole 1: Preliminary results - M{uller, J.; Kasbohm, J.; Oberh{ansli, H.; Melles, M.; Hubberten, H.W. (Lake Baikal, 2000)

20 - Study of the elemental composition of suspended particles in large continental lakes (Baikal and Khubsgul) - Potyomkina, G.; Potyomkin, L. (Lake Baikal, 2000)

6 - Palaeoclimatic changes from 3.6 to 2.2 ma b.p. derived from palynological studies on lake baikal sediments - Demske, D.; Mohr, B.; Oberh{ansli, H. (Lake Baikal, 2000)

10 - Estimation of paleoenvironmental changes in the Eurasian continental interior during the past 5 million years inferred from organic components in the BDP96 1 - Matsumoto, G.I.; Kosaku, S.; Takamatsu, N.; Akagi, T.; Kawai, T.; Ambe, Y. (Lake Baikal, 2000)

11 - Paleoenvironmental changes in the Eurasian continent interior inferred from chemical elements in sediment cores (BDP96/1, BDP96/2) from Lake Baikal - Takamatsu, N.; Matsumoto, I.G.; Kato, N.; Kawai, T. (Lake Baikal, 2000)

12 - A new preparation method for qualitative and quantitative analysis of fossil sponge spicules by light microscopy - Eckert, C.; Veinberg, E.V.; Kienel, U.; Oberh{ansli, H. (Lake Baikal, 2000)

13 - Evolution of freshwater centric diatoms within the Baikal rift zone during the late Cenozoic - Khursevich, G.K.; Karabanov, E.B.; Williams, D.F.; Kuzmin, M.I.; Prokopenko, A.A. (Lake Baikal, 2000)

14 - Elemental composition of short sediment cores and ferromanganese concretions from Lake Baikal - Takamatsu, T.; Kawai, T.; Nishikawa, M. (Lake Baikal, 2000)

15 - Mercury distribution in the bottom and stream sediments of Lake Baikal,water reservoirs of the Angara river cascade, and adjacent drainage basins - Koval, P.V.; Kalmychkov, G.V.; Geletyi, V.F.; Andrulaitis, L.D. (Lake Baikal, 2000)

17 - Remote sensing methods in studies of Lake Baikal environment - Semovski, S.V. (Lake Baikal, 2000)

18 - Environmental impact on the dynamics of Lake Baikal phytoplankton taxonomic groups: Modelling attempt - Semovski, S.V. (Lake Baikal, 2000)

19 - Nonlinear stability near the temperature of maximum density and thermobaric instability in lake baikal during summer stratification - Granin, N.G.; Gnatovsky, R. Yu.; Kay, A.; Galkin, L.M. (Lake Baikal, 2000)

2 - Changes in lake baikal water levels and runoff direction in the quaternary period - Mats, V.D.; Fujii, S.; Mashiko, K.; Osipov, E.Yu.; Yefrimova, I.M.; Klimansky, A.V. (Lake Baikal, 2000)

16 - Correlations between geochemical features of recent bottom and stream sediments in the Baikal geoecological polygon - Koval, P.V.; Gvozdkov, A.N.; Romanov, V.A. (Lake Baikal, 2000)

21 - Atmospheric and riverine input of nutrients and organic matter into lake baikal - Sorokovikova, L.M.; Khodzher, T.V.; Sinyukovich, V.N.; Golobokova, L.P.; Bashenkhaeva, N.D.; Netsvetaeva, O.G. (Lake Baikal, 2000)

29 - The importance of habitat stability for the prevalence of sexual reproduction - Martens, K.; Sch{on, I. (Lake Baikal, 2000)

(untitled) - Karabanov, E.B.; Kuzmin, M.I.; Prokopenko, A.A.; Williams, D.F.; Khursevich, G.K.; Bezrukova, E.V.; Kerber, E.V.; N.

22 - Comparison of persistent organochlorine pollutant behavior in the food webs of lakes baikal and superior - Kucklick, J.R.; Baker, J.E (Lake Baikal, 2000)

4 - Paleoclimatic signals printed in lake baikal sediments - Kashiwaya, K.; Tanaka, A.; Sakai, H.; Kawai, T. (Lake Baikal, 2000)

3 - Paleomagnetic and rock-magnetic studies on lake baikal sediments -BDP96 borehole at academician ridge- - Sakai, H.; Nomura, S.; Horii, M.; Kashiwaya, K.; Tanaka, A.; Kawai, T.; Kravchinsky, V.; Peck, J. (Lake Baikal, 2000)

28 - Morphometric comparison of skulls of seals of the subgenus Pusa - Amano, M.; Koyama, Y.; Petrov, E.A.; Hayano, A.; Miyazaki, N. (Lake Baikal, 2000)

27 - Myological peculiarities of the comephoridae: An endemic fish taxon in Lake Baikal (Pisces: Teleostei) - Yabe, M.; Sideleva, V.G. (Lake Baikal, 2000)

26 - Genetic differentiation of gammarid (Eulimnogammarus cyaneus) populations in relation to past environmental changes in lake baikal - Mashiko, K.; Kamaltynov, R.; Morino, H.; Sherbakov, D.Yu. (Lake Baikal, 2000)

25 - Contamination of the ecosystems of lake baikal by persistent organochlorines - Nakata, H.; Tanabe, S.; Iwata, H.; Amano, M.; Miyazaki, N.; Petrov, E.A.; Tatsukawa, R. (Lake Baikal, 2000)

24 - Some speculations on the possibility of changes in deep-water renewal in Lake Baikal and their consequences - Kipfer, R.; Peeters, F. (Lake Baikal, 2000)

23 - Carbon and nitrogen isotope studies of the pelagic ecosystem and environmental fluctuations of lake baikal - Ogawa, N.O.; Yoshii, K.; Melnik, N.G.; Bondarenko, N.A.; Timoshkin, O.A.; Smirnova-Zalumi, N.S.; Smirnov, V.V.; Wada, E. (Lake Baikal, 2000)

Petrophysical studies on rocks from the Dabie ultrahigh-pressure (UHP) metamorphic belt, Central China: implications for the composition and delamination of the lower crust - Kern, Hartmut; Gao, Shan; Jin, Zhengmin; Popp, Till; Jin, Shuyan (Tectonophysics, 1999)
The Dabie –Sulu ultrahigh-pressure (UHP) metamorphic belt of Central China represents a zone in which upper and lower continental crust has been subducted and then rapidly exhumed from the mantle back into the crust. About 30 UHP rock samples representing major lithologies were collected from surface exposures and their physical properties were measured in the laboratory at pressures up to 600 MPa (room temperature) and temperatures up to 600°C (600 MPa confining pressure). The experimentally determined data include compressional (Vp) and shear wave velocities (Vs), velocity anisotropy (shear wave splitting), density, and intrinsic pressure and temperature derivatives of Vp and Vs. Using a regional geotherm, velocity –depth profiles were calculated for the different lithologies spanning compositions from felsic through intermediate to mafic, and metamorphic grades up to granulite and eclogite facies. The velocity data, in particular the Poisson ratio (Vp/Vs ratio), were used to interpret lithologically the seismic models evaluated from seismic refraction data. The seismic velocity profiles reveal a four-layer structure (upper, middle, upper-lower and lowermost crust) with an average thickness of 34 km. From our results, we infer that a mixture of about 90% felsic gneiss with variable amounts of high Vp amphibolite/gabbro constitute the middle crust. Intermediate granulite and mafic granulite fit both the Vp and Poisson ratio of the upper-lower and lowermost crust, respectively. From the combined Vp and Poisson ratio (Vp/Vs) data, we conclude that eclogite is not a major constituent of the present Dabie deep crust. Combining the laboratory-derived in-situ seismic data (Vp and Poisson ratio) with refraction seismic data, along with evidence from geological and geochemical investigations, suggests that lower crustal delamination played an important role in the modification of the East China crust.
Keywords: UHP rocks; seismic properties (V and V); densities; crustal lithology; delamination

Production potential of the Mallik field reservoir - Khairkhah, D. (Bulletin of the Geological Survey of Canada, 1999)
Immense volumes of naturally occurring gas hydrate in different parts of the world, onshore and offshore, have encouraged the belief that gas hydrate in the next century may become a viable energy resource. Various issues need to be resolved to convert gas hydrate from an energy resource to an energy reserve of real commercial value. The production capability of a gas hydrate reservoir and the gas production technique that could be utilized should be addressed through geological and petrophysical studies, well-production tests and reservoir simulation. To make the simulation of practical value, the controlling mechanisms of fluid flow, kinetics, and heat transfer should be incorporated in the model. The Mallik gas hydrate accumulation in the Mackenzie Delta has exhibited promising potential to be considered a gas reserve through the assessments made of the Mallik L-38 and 2L-38 wells. The data available from both wells and the results of production tests in JAPEX/JNOC/GSC Mallik L-38 gas hydrate research well accommodate basic requirements for comprehensive modelling of the reservoir and production of gas from the in situ gas hydrate through various methods.
cited By 6

Post-eruptive volcanic dome evolution as revealed by deformation and microgravity observations at Usu volcano (Hokkaido, Japan) - Jousset, Philippe; Okada, Hiromu (Journal of Volcanology and Geothermal Research, 1999)
Usu volcano (Hokkaido, Japan) is a dacitic volcano, known for its high production rate of lava domes and crypto-domes. It is thus a good target to study processes of volcanic dome evolution (upheaval and/or relaxation). We carried out repeated GPS and microgravity surveys on the three most recent domes of Mt. Usu (1910: Meiji Shinzan; 1943-1945: Showa-Shinzan and 1977-1982: Usu-Shinzan). The repeat period was 1 to 2 months and extended from October 1996 to June 1997. We also compare new data with results from former studies. More than 20 years after the start of Usu-Shinzan dome growth, there is still subsidence at a maximum rate of about 7 to 8 cm/year. The reasons for this subsidence are discussed. Repeated gravity surveys revealed an increase of gravity on the domes (about 60 ± 10 microgal/year for Usu-Shinzan, about 15 microgal at Showa-Shinzan and 10 to 20 microgal for Meiji-shinzan); this gravity increase exceeds that expected due to subsidence. We discuss and interpret the excess gravity change in terms of a density increase in the edifice, caused by a combination of processes (contraction of the edifice, water level change, devesiculisation, cooling and magma intrusion). Quantification of these processes at Usu volcano may help to understand the processes of evolution at domes on other volcanoes such as Merapi (Indonesia), Unzen (Japan) or Montserrat (West Indies).
Keywords: Japan; crustal deformation; gravity anomaly; lava dome; volcanic eruption

Pore-water geochemistry, JAPEX/JNOC/GSC Mallik 2L-38 gas hydrate research well - Cranston, R.E. (Bulletin of the Geological Survey of Canada, 1999)
A pore-water research program was designed to measure dissolved components in interstitial water from sediment core samples collected during the JAPEX/JNOC/GSC Mallik 2L-38 gas hydrate research-well project. Pore waters from the gas-hydrate-bearing samples had an average salinity of 8 ppt compared to 34 ppt for non-gas-hydrate-bearing samples. The difference in salinities suggests that 80-90% of the pore space in the gas-hydrate-bearing sediment was filled with gas hydrate, which dissociated during recovery. Potassium concentration was also measured in pore water, to estimate the amount of drill-mud contamination in pore-water samples, since the drill mud contained brine solution made from potassium chloride. On average, pore-water salinities were estimated to be enhanced by 2 ppt due to drill-mud contamination.
cited By 20

Physical properties of sediments from the JAPEX/JNOC/GSC Mallik 2L-38 gas hydrate research well - Winters, W.J. (Bulletin of the Geological Survey of Canada, 1999)
A 1150 m deep gas hydrate research well was drilled in the Canadian Arctic in February and March 1998 to investigate the interaction between the presence of gas hydrate and the natural conditions presented by the host sediments. Profiles of the following measured and derived properties are presented from that investigation: water content, sediment wet bulk density, grain size, porosity, gas hydrate quantity, and salinity. These data indicate that the greatest concentration of gas hydrate is located within sand and gravel deposits between 897 m and 922 m. American Society for Testing and Materials 1997: Standard test method for specific gravity of soil solids by gas pycnometer D 5550-94; in American Society for Testing and Materials, Annual Book of ASTM Standards, v. 04.09, Soil and Rock, West Conshohocken, Pennsylvania, p. 380-383.
cited By 34

Overview of science program, JAPEX/JNOC/GSC Mallik 2L-38 gas hydrate research well - Dallimore, S.R. (Bulletin of the Geological Survey of Canada, 1999)
The JAPEX/JNOC/GSC Mallik 2L-38 gas hydrate research well was drilled in February and March, 1998, in the Mackenzie Delta, Northwest Territories, Canada, to a depth of 1150 m. The scientific program was conducted through a collaborative agreement between the Japan National Oil Corporation and the Geological Survey of Canada with key participation by the Japan Petroleum Exploration Company and the United States Geological Survey. A primary objective of the well was to undertake a comprehensive scientific research program to study an arctic gas hydrate accumulation. Field research conducted as part of the Mallik 2L-38 program included collection of permafrost and gas-hydrate-bearing core samples, downhole geophysical logging, and a vertical seismic profile survey. Laboratory and modelling studies undertaken during the field program, and subsequently as part of a post-field research program, documented the sedimentology, biostratigraphy, physical/petrophysical properties, pore-water and gas geochemistry, geophysics, and reservoir characteristics of the Mallik field gas hydrate accumulation.
cited By 25

Petrophysical environment of sediments hosting gas hydrate, JAPEX/JNOC/GSC Mallik 2L-38 gas hydrate research well - Katsube, T.J. (Bulletin of the Geological Survey of Canada, 1999)
A petrophysical study has been performed on mudstone and sandstone samples from depths of 880-950 m to determine the petrophysical controls on gas hydrate distribution in the sedimentary sequence at the J APEX/JNOC/GSC Mallik 2L-38 well site, Northwest Territories, Canada. Within the cored interval of the Mallik 2L-38 well gas hydrate is hosted in two sandstone horizons with overlying and underlying mudstone horizons, with minor gas hydrate concentrations within some mudstone formations. Results indicate that, although the interbedded mudstone units have relatively high porosities (24-30%) and are at relatively shallow depths, they have a well developed framework-supported texture, probably due to high silt and sand content (56-78 weight per cent), and a maximum burial depth greater than present. Regardless of this, the minor matrix content (13-25 weight per cent) controls the fluid transport characteristics, resulting in extremely low mudstone permeability sections (2×10-21 m to 2×10-19 m2). There are indications that these low permeabilities and the storage pore sizes contribute to the gas hydrate distribution.
cited By 13

Petrology of the 1991-1995 eruption at Unzen: Effusion pulsation and groundmass crystallization - Nakada, Setsuya; Motomura, Yoshinobu (Journal of Volcanology and Geothermal Research, 1999)
Effusive eruption of dacite magma (2.1 X 108 m3) during 1991-1995 formed a lava dome at the summit of Unzen Volcano, Japan. The effusion rate was highest at the beginning, 4.0 X 105 m3/day (4.6 m3/s), and decreased roughly with time, to almost zero before this pattern was repeated with a second pulse of magma supply. The whole-rock chemistry of lavas shows significant variation attributable to variations in phenocryst abundance; the more mafic, the more abundant the phenocrysts. The pattern of chemical variation with time shows some difference from that of the effusion rate. All phenocrysts in dacite (plagioclase, hornblende, biotite, quartz and magnetite) show evidence of disequilibrium with melt. Although a glomerophyric aggregation of phenocrysts suggests coexistence with each other, phenocrysts are isotopically heterogeneous from species to species. The calculated initial melt composition was rhyodacite, and was nearly constant throughout the activity. In contrast, the bulk phenocryst population is andesite. A model explaining the textures and the isotopic heterogeneity is the capture of diorite fragments (or xenocrysts) by parental rhyodacite magma. It is suggested that, when effusion rate was high, less viscous crystal-poor magma exited from the reservoir. Groundmass glass and plagioclase microlite rims show temporal chemical variations correlating with the effusion rate; the higher the effusion rate, the more evolved the compositions. Groundmass crystallinity increased with decreasing effusion rate; from 33% to 50%. Textures in dome lavas suggest that groundmass crystallization had been mostly completed when magma reached the conduit top. The Fe-Ti oxide temperature (880-780°C) was low when the crystallinity was high. Micropumice erupted before dome growth provided a sample recording magmatic foam in the conduit. Porosity of dome lavas was lower at lower effusion rates. Collapse of foam magma and simultaneous escape of volatiles through the conduit top were probably responsible for the accompanying low-frequency earthquakes. Phenocrysts were broken and the breakdown rims on hornblende phenocrysts were torn off during collapse and successive compaction. When effusion waned, degassing and the resultant crystallization proceeded more completely, so that the magma became too viscous to flow in the conduit top and behaved as a plug, resulting in a temporary halt of effusion. In turn, groundmass crystallization in magma below the plug increased excess pressure in the upper parts of conduit due to slow cooling. The plug was scavenged when rising excess pressure overcame its effective strength. Then, the second pulse of magma supply began. Strong endogenous growth and extrusion of a lava spine in the later stage probably occurred for the same reason.
Keywords: Japan; dacite; petrology; volcanic eruption

Petrology and Fe-Ti oxide reequilibration of the 1991 Mount Unzen mixed magma - Venezky, Dina Y.; Rutherford, Malcolm J. (Journal of Volcanology and Geothermal Research, 1999)
A dacitic magma (64.5 wt.% SiO2), a mixture of phenocryst-rich rhyodacite and an aphyric mafic magma, was erupted during the recent 1991-1995 Mount Unzen eruptive cycle. The experimental and analytical results of this study reveal additional details about conditions in the premixing and postmixing magmas, and the nature of the mixing process. The preeruption rhyodacitic magma was at a temperature of 790 ± 20°C according to Fe-Ti oxide phenocryst cores, and at a depth of 6 to 7 km (160 MPa) according to Al-in-hornblende geobarometry. The mafic magma that mixed with the rhyodacite is found as andesitic (54 to 62 wt.% SiO2) enclaves in the erupted magma and was essentially aphyric when intruded. Phase equilibria indicate that an aphyric andesite at 160 MPa is >1030°C (H2O-saturated) and possibly as high as 1130°C (2 wt.% H2O). The composition of the rhyodacite which was mixed with the andesite is estimated to lie between 67 and 69 wt.% SiO2. Using these compositions and temperatures, the temperature of the Unzen magma after mixing is estimated to be at least 850°to 870°C. The groundmass Fe-Ti oxide microphenocrysts and those in pargasite-bearing reaction zones around biotite phenocrysts both give 890 ± 20°C temperatures; the oxide-oxide contacts give temperatures of 910 ± 20°C. The 900 ± 30°C postmixing temperatures are consistent with phase-equilibria experiments which show that the magma was not above 930°C at 160 MPa. Our Fe-Ti oxide reequilibration experiments suggest that the mixing of the two magmas began within a few weeks of the eruption, which is a shorter time than is calculated using available diffusion data. There is also evidence that some mixing took place much closer to the time of extrusion based on the presence of unrimmed biotite phenocrysts in the magma.
Keywords: Japan; dacite; geobarometry; magma chemistry; petrology; volcanic eruption

Palynology, age, and paleoenvironmental interpretations from the JAPEX/JNOC/GSC Mallik 2L-38 gas hydrate research well - White, J.M. (Bulletin of the Geological Survey of Canada, 1999)
The JAPEX/JNOC/GSC Mallik 2L-38 gas hydrate research well was drilled in 1998 to investigate the geological, geochemical, geophysical, and engineering properties of a gas hydrate accumulation previously identified in the Mallik L-38 well. Palynological analysis of core and cuttings from 670 m to 1150 m (TD) are reported here. Detailed quantitative analysis has been done on the 886-952 m cored interval that hosts the main gas hydrate accumulation. The pollen and spore evidence suggests the following biostratigraphic subdivisions for the 670-1150 m succession: 670-785 m, Late Miocene or older; 775-897 m, within the range of Early Miocene to Late Eocene; 897-930 m, probably Late Eocene; 930-995 m, Late Eocene; and 995-1151m within the range Early to Middle Eocene. Below 930 m the rocks are best assigned to the Richards and upper Taglu sequences. A dominantly continental succession is indicated, with a marginal marine and/or estuarine episode between about 945 m and 948 m, in the Late Eocene. The dinoflagellates in this interval are considered to be indigenous to the sampled rock. There is evidence of two episodes of edaphic-climatic dryness in the Late Eocene and probable Late Eocene.
cited By 3
Keywords: Dinophyceae

Overview of the 1990-1995 eruption at Unzen Volcano - Nakada, Setsuya; Shimizu, Hiroshi; Ohta, Kazuya (Journal of Volcanology and Geothermal Research, 1999)
Following 198 years of dormancy, a small phreatic eruption started at the summit of Unzen Volcano (Mt. Fugen) in November 1990. A swarm of volcano-tectonic (VT) earthquakes had begun below the western flank of the volcano a year before this eruption, and isolated tremor occurred below the summit shortly before it. The focus of VT events had migrated eastward to the summit and became shallower. Following a period of phreatic activity, phreatomagmatic eruptions began in February 1991, became larger with time, and developed into a dacite dome eruption in May 1991 that lasted approximately 4 years. The emergence of the dome followed inflation, demagnetization and a swarm of high-frequency (HF) earthquakes in the crater area. After the dome appeared, activity of the VT earthquakes and the summit HF events was replaced largely by low-frequency (LF) earthquakes. Magma was discharged nearly continuously through the period of dome growth, and the rate decreased roughly with time. The lava dome grew in an unstable form on the shoulder of Mt. Fugen, with repeating partial collapses. The growth was exogenous when the lava effusion rate was high, and endogenous when low. A total of 13 lobes grew as a result of exogenous growth. Vigorous swarms of LF earthquakes occurred just prior to each lobe extrusion. Endogenous growth was accompanied by strong deformation of the crater floor and HF and LF earthquakes. By repeated exogenous and endogenous growth, a large dome was formed over the crater. Pyroclastic flows frequently descended to the northeast, east, and southeast, and their deposits extensively covered the eastern slope and flank of Mt. Fugen. Major pyroclastic flows took place when the lava effusion rate was high. Small vulcanian explosions were limited in the initial stage of dome growth. One of them occurred following collapse of the dome. The total volume of magma erupted was 2.1 x 108 m3 (dense-rock-equivalent); about a half of this volume remained as a lava dome at the summit (1.2 km long, 0.8 km wide and 230-540 m high). The eruption finished with extrusion of a spin endogenous dome top. Several monitoring results convinced us that the eruption had come to an end: the minimal levels of both seismicity and rockfalls, no discharge of magma, the minimal SO2 flux, and cessation of subsidence of the western flank of the volcano. The dome started slow deformation and cooling after the halt of magma effusion in February 1995.
Keywords: Japan; earthquake; lava dome; pyroclastic flow; volcanic eruption

Regional gas hydrate occurrences, permafrost conditions, and Cenozoic geology, Mackenzie Delta area - Dallimore, S.R. (Bulletin of the Geological Survey of Canada, 1999)
The occurrence of natural gas hydrate within Cenozoic sediments of the Mackenzie Delta-Beaufort Sea region has been well documented. In preparation for the JAPEX/JNOC/GSC Mallik 2L-38 gas hydrate research well, a detailed evaluation of terrestrial gas hydrate occurrences was undertaken to assess the geological setting, sediment associations, pressure and temperature conditions, and the presence of free gas in the Mackenzie Delta-Beaufort Sea region. After an exhaustive review, it was determined that the Mallik L-38 site, drilled by Imperial Oil in 1972, offered the highest probability of encountering a thick gas hydrate occurrence with high gas hydrate concentrations. On the basis of openhole well-log evaluation, it was estimated that about Him of gas-hydrate-bearing strata occur at this location from 810.1 to 1102.3 m, within the zone of predicted methane hydrate stability and below the base of icebearing permafrost, estimated to be at 640 m.
cited By 47

Properties of samples containing natural gas hydrate from the JAPEX/JNOC/GSC Mallik 2L-38 gas hydrate research well, determined using Gas Hydrate And Sediment Test Laboratory Instrument (GHASTLI) - Winters, W.J. (Bulletin of the Geological Survey of Canada, 1999)
As part of an ongoing laboratory study, preliminary acoustic, strength, and hydraulic conductivity results are presented from a suite of tests conducted on four natural-gas-hydrate-containing samples from the Mackenzie Delta JAPEX/JNOC/GSC Mallik 2L-38 gas hydrate research well. The gas hydrate samples were preserved in pressure vessels during transport from the Northwest Territories to Woods Hole, Massachusetts, where multistep tests were performed using GHASTLI (Gas Hydrate And Sediment Test Laboratory Instrument), which recreates pressure and temperature conditions that are stable for gas hydrate. Properties and changes in sediment behaviour were measured before, during, and after controlled gas hydrate dissociation. Significant amounts of gas hydrate occupied the sample pores and substantially increased acoustic velocity and shear strength.
cited By 34

Water contents and hydrogen isotopic ratios of rocks and minerals from the 1991 eruption of Unzen volcano, Japan - Kusakabe, Minoru; Sato, Hiroaki; Nakada, Setsuya; Kitamura, Toshihiro (Journal of Volcanology and Geothermal Research, 1999)
Water contents and hydrogen isotopic ratios were determined for blocks from pyroclastic flow deposits, and bread-crust bombs and blocks from the 1991 Vulcanian eruptions of Unzen volcano, Japan. Groundmass water contents and δD values of samples were calculated by subtracting the contribution of major hydrous minerals (hornblende and biotite) from the bulk rock analyses, and range from 0.1 to 0.5 wt.% and -83 to -49‰, respectively. The samples do not show a systematic H2O-δD relationship, although the block samples tend to have lower δD values than the bomb samples. The non-systematic H2O-δD relationship is likely a result of near surface, kinetically-controlled gas loss. High viscosity of this magma would hinder attainment of hydrogen isotopic equilibrium between exsolved vapor and melt in the final degassing stage. The near surface degassing, however, was accompanied by kinetic fractionation resulting in enrichment of deuterium in the final products as exemplified by bread-crust bombs with high H2O-low δD margins and low H2O-high δD cores. Relatively high δD values of the blocks and bombs as well as high temperature volcanic gas (-30 to -35‰) suggest a closed system degassing of an initial water-rich magma (H2O = 6 wt.%) until its water content was reduced to 0.5 wt.%. The pre-eruptive δD value (-46‰) was estimated from the volcanic gas data and D/H analysis of hornblende phenocrysts coupled with assumed isotopic equilibration in the initial hydrous magma.
Keywords: Japan; degassing; hydrogen isotope; igneous geochemistry; volcanic eruption; water content

Scientific results from JAPEX/JNOC/GSC Mallik 2L-38 gas hydrate research well. - Dallimore S.R., Uchida T., Collett T.S. (Scientific results from JAPEX/JNOC/GSC Mallik 2L-38 gas hydrate research well, Mackenzie Delta, Northwest Territories, Canada. Geological survey of Canada bulletin No.544., 1999)
Natural gas hydrate, a solid form of natural gas and water, is thought to be the single most abundant hydrocarbon source known on Earth. This unique substance occurs in nature in association with deep permafrost and in offshore environments adjacent to continental margins. The scientific results of the JAPEX/JNOC/GSC Mallik 2L-38 gas hydrate research well, which are the successful outcome of a collaborative agreement between the Geological Survey of Canada (GSC) and the Japan National Oil Corporation (JNOC), are presented in this report. For the GSC, the collaboration provided the first detailed characterization of the vast gas hydrate occurrences known to occur beneath the Mackenzie Delta, one of Canada's largest undeveloped hydrocarbon provinces. Although gas hydrate research during the past three decades has been primarily related to geohazard concerns, in recent years the GSC has begun to examine the potential of gas hydrate as an energy resource and the role it may play as a source of greenhouse gases that contribute to climate change.
cited By 0
Keywords: continental margin; energy resource; gas hydrate; greenhouse gas; hydrocarbon exploration; permafrost; Canada

Sedimentology of gas hydrate host strata from the JAPEX/JNOC/GSC Mallik 2L-38 gas hydrate research well - Jenner, K.A. (Bulletin of the Geological Survey of Canada, 1999)
A detailed sedimentological program has been conducted on gas-hydrate-bearing core samples from the JAPEX/JNOC/GSC Mallik 2L-38 gas hydrate research well. Three structurally and texturally distinct sedimentary facies are identified. Facies Csc (952.2-926.5 m) is a weakly bioturbated, clayey silt interbedded with fissile coal and silty sand. Facies Sg (926.5-908.5 m) is comprised of interbedded, finingupward successions of matrix-supported gravel to pebbly sand and fine sand. A dolomite-cemented sandstone (926.5-925 m) forms a distinct basal subfacies (Sst). Facies Ss (908.5-886.2 m) is a fine- to mediumgrained sand interbedded with gravel which fines upward to fine-grained sand with a gradational increase in silt content. The Kugmallit-Mackenzie Bay sequence boundary is interpreted to occur at the base of facies Sg. In situ and self-preserved gas hydrate occurred mainly in the sands and gravels of the Sg and Ss facies. The dolomite-cemented sandstone (subfacies Sst) may be related to complementary geochemical environments resulting from the formation of authigenic pyrite and solute exclusion related to gas hydrate growth within facies Sg.
cited By 14

Seismic- and well-log-inferred gas hydrate accumulations on Richards Island - Collett, T.S. (Bulletin of the Geological Survey of Canada, 1999)
The gas hydrate stability zone is areally extensive beneath most of the Mackenzie Delta-Beaufort Sea region, with the base of the gas hydrate stability zone more than 1000 m deep on Richards Island. In this study, gas hydrate has been inferred to occur in nine Richards Island exploratory wells on the basis of well-log responses calibrated to the response of the logs within the cored gas-hydrate-bearing intervals of the JAPEX/JNOC/GSC Mallik 2L-38 gas hydrate research well. The integration of the available well-log data with more than 240 km of industry-acquired reflection seismic data have allowed us to map the occurrence of four significant gas hydrate and associated free-gas accumulations in the Ivik-Mallik-Taglu area on Richards Island. The occurrence of gas hydrate on Richards Island is mostly restricted to the crest of large anticlinal features that cut across the base of the gas hydrate stability zone. Combined seismic and well-log data analysis indicate that the known and inferred gas hydrate accumulations on Richards Island may contain as much as 187 178106 m3 of gas.
cited By 26

Summary of physicochemical properties of natural gas hydrate and associated gas-hydrate-bearing sediments, JAPEX/JNOC/GSC Mallik 2L-38 gas hydrate research well, by the Japanese research consortium - Uchida, T. (Bulletin of the Geological Survey of Canada, 1999)
The JAPEX/JNOC/GSC Mallik 2L-38 gas hydrate research well was drilled to a depth of 1150 m in the Mackenzie Delta, Northwest Territories, Canada, in February and March, 1998. A highlight of the project was the successful retrieval of natural gas hydrate samples in a variety of sediments. A summary is presented of research conducted by the Japanese research consortium led by the Japan National Oil Corporation with participation by ten Japanese companies and institutes. Fingerprints of the gas hydrate crystal structure and the molar ratio of water to guest-gas molecules occupying lattice sites are described for gas-hydrate-bearing samples as obtained by NMR and Raman spectroscopy. X-Ray CT imagery is used to describe the texture and gas hydrate/sediment characteristics of recovered samples during controlled dissociation testing. In addition, inorganic and organic chemical, thermal geophysical, and physical properties are described for key core horizons. Results are also presented documenting the rate of gas hydrate dissociation in drilling fluids with different chemistry including lecithin.
cited By 15

The 15 September 1991 pyroclastic flows at Unzen Volcano (Japan): A flow model for associated ash-cloud surges - Fujii, Toshitsugu; Nakada, Setsuya (Journal of Volcanology and Geothermal Research, 1999)
Large-scale collapse of a dacite dome in the late afternoon of 15 September 1991 generated a series of pyroclastic-flow events at Unzen Volcano. Pyroclastic flows with a volume of 1 X 106 m3 (as DRE) descended the northeastern slope of the volcano, changing their courses to the southeast due to topographic control. After they exited a narrow gorge, an ash-cloud surge rushed straight ahead, detaching the main body of the flow that turned and followed the topographic lows to the east. The surge swept the Kita-Kamikoba area, which had been devastated by the previous pyroclastic-flow events, and transported a car as far as 120 m. Following detachment, the surge lost its force after it moved several hundred meters, but maintained a high temperature. The deposits consist of a bottom layer of better-sorted ash (unit 1), a thick layer of block and ash (unit 2), and a thin top layer of fall-out ash (unit 3). Unit 2 overlies unit 1 with an erosional contact. The upper part of unit 2 grades into better-sorted ash. At distal block-and-ash flow deposits, the bottom part of unit 2 also consists of better-sorted ash, and the contact with the unit 1 deposits becomes ambiguous. Video footage of cascading pyroclastic flows during the 1991-1995 eruption, traveling over surfaces without any topographic barriers, revealed that lobes of ash cloud protruded intermittently from the moving head and sides, and that these lobes surged ahead on the ground surface. This fact, together with the inspection by helicopter shortly after the events, suggests that the protruded lobes consisted of better-sorted ash, and resulted in the deposits of unit 1. The highest ash-cloud plume at the Oshigadani valley exit, and the thickest deposition of fall-out ash over Kita-Kamikoba and Ohnokoba, indicate that abundant ash was also produced when the flow passed through a narrow gorge. In the model presented here, the ash clouds from the pyroclastic flows were composed of a basal turbulent current of high concentration (main body), an overriding and intermediate fluidization zone, and an overlying dilute cloud. Release of pressurized gas in lava block pores, due to collisions among blocks and the resulting upward current, caused a zone of fluidization just above the main body. The mixture of gas and ash sorted in the fluidization zone moved ahead and to the side of the main body as a gravitational current, where the ash was deposited as surge deposits. The main body, which had high internal friction and shear near its base, then overran the surge deposits, partially eroding them. When the upward current of gas (fluidization) waned, better-sorted ash suspended in the fluidization zone was deposited on block-and-ash deposits. In the distal places of block-and-ash deposits, unit 2 probably was deposited in non-turbulent fashion without any erosion of the underlying layer (unit 1).
Keywords: Japan; ash flow; pyroclastic flow; volcanic ash; volcanic eruption

The loss from the upper layer of bottom sediments in the cores from boreholes BDP-96-1 and BDP-96-2 (drilled on lake Baikal) - Karabanov, E.B.; Kuz'min, M.I.; Geletii, V.F.; Kalraychkov, G.V.; Gvozdkov, A.N. (Geologiya i Geofizika, 1999)
In 1996, boreholes BDP-96-1 and BDP-96 penetrated the bottom sediments of Lake Baikal. From the lithological and geochemical correlation of the sediments, the loss from the upper layer of sediments in the cores is estimated: 630 cm (BDP-96-1) and 19 cm (BDP-96-2). Correlation of these cores with the 120 cm long core from borehole GC-1, taken with a gravity corer during the drilling through the lake ice, permits compilation of a general continuous record of climatic changes in Central Asia for the last 5 Ma and prevention of loss of Holocene sediments.

The Sr, Nd and O isotopic studies of the 1991-1995 eruption at Unzen, Japan - Chen, Chang-Hwa; Nakada, Setsuya; Shieh, Yuch-Ning; DePaolo, Donald J. (Journal of Volcanology and Geothermal Research, 1999)
The magma generation at Unzen volcano may be considered as the product of crustal material mixed with mantle magma accompanied by fractional crystallization (AFC). The magma in the Unzen volcano is estimated to consist of about 50-80% of residual magma (F) and about 30-70% assimilated crustal material (A) relative to the original magma. Concerning the 1991-1995 eruption, it is estimated that the magma formed as the result of mixing of about 50-60% crustal material and about 55-65% of residual magma. An alternative magma eruption model for the 1991-1995 eruption is proposed here. In the early stage, the isotopic characteristics of 1991 eruption are defined by AFC process in the deeper magma chamber. Later, the magma ascended through the conduit and quiescently stayed for a long time in a shallow reservoir before eruption. The minerals continuously crystallized as phenocrysts especially at the chilled top and outer margin in the shallow chamber. The crystallized phenocryst mush was reworked into the central part of the magma chamber by means of magma convection and rapid magma ascent. Therefore, the reaction between phenocrysts and melt occurs only in internal chemical disequilibrium in the magma chamber. In contrast, the isotopic compositions of the original magma shall be little influenced by the above processes throughout its eruptive history. The 1991-1995 eruptive rocks of the Unzen volcano show their characteristics in Sr and Nd isotopic values independent of their two previous eruptions. However, the isotopic values of early eruptive product could represent the original magma value. This result also supports the previous work of Chen et al. (1993) [Chen, C.H., DePaolo, D.J., Nakada, S., Shieh, Y.N., 1993. Relationship between eruption volume and neodymium isotopic composition at Unzen volcano. Nature 362, 831-834], that suggested the εNd of early or precursory eruptive products could be a qualitative indicator of the maximum size of a continuing or impending eruption.
Keywords: Japan; isotopic analysis; magma chemistry; phenocryst; volcanic eruption

Thermochronologic constraints on deformation and cooling history of high- and ultrahigh-pressure rocks in the Qinling-Dabie orogen, eastern China - Webb, Laura E.; Hacker, Bradley R.; Ratschbacher, Lothar; McWilliams, Michael O.; Dong, Shuwen (Tectonics, 1999)
The Hong'an block is the best place to study the exhumation of high-pressure (HP) and ultrahigh-pressure (UHP) metamorphic rocks in the Qinling-Dabie orogen of eastern China because it lacks the extensive Cretaceous tectonic and thermal overprint observed in the Dabie Shan. We measured timing of deformation and rate of cooling of the HP-UHP rocks by 40Ar/39Ar analyses of synkinematic minerals from tectonites of key structural zones in the Hong'an and Tongbai Shan. Normal-sense shear along the north dipping Huwan detachment at the northern edge of the Hong'an block occurred between 237 and 231 Ma; this detachment facilitated the bulk of the exhumation of the HP-UHP rocks. Our new 40Ar/39Ar ages, combined with U/Pb zircon and Sm/Nd ages of 245-240 Ma, suggest that exhumation of UHP rocks from mantle depths occurred at rates of 5-25 mm/yr from 245 to 230 Ma. The mountain range is a warped extensional footwall, within which white mica cooled from 225 to 205 Ma. Locally, younger extension is recorded by white mica recrystallization at 198-194 Ma, after which the entire block had cooled to below 300 °C. Early Cretaceous 40Ar/39Ar ages from the Tongbai shear zone indicate that dextral shear along the southwest boundary of the orogen was contemporaneous with normal to sinistral-oblique slip along the Xiaotian-Mozitan fault along the northern boundary. Coeval dextral and sinistral shear zones along the northern and southwestern margins of the Hong'an and Dabie Shan would have caused eastward lateral extrusion of these two blocks, perhaps driven by collision of the Lhasa block with Eurasia.
Keywords: China; Qinling-Dabie Orogen; Deformation; Geochronology; Mica; Rocks; Silicate minerals; Early Cretaceous; Lateral extrusion; Mountain ranges; Qinling-Dabie orogen; Rate of cooling; Sinistral shear; Ultra-high-pressure metamorphic rocks; Ultrahigh pressure rocks; cooling; deformation; petrology; ultrahigh pressure metamorphism; Metamorphic rocks

Velocity analysis of vertical seismic profile (VSP) survey at JAPEX/JNOC/GSC Mallik 2L-38 gas hydrate research well, and related problems for estimating gas hydrate concentration - Sakai, A. (Bulletin of the Geological Survey of Canada, 1999)
A VSP survey was conducted at the JAPEX/JNOC/GSC Mallik 2L-38 gas hydrate research well to determine elastic-wave velocities that were estimated by traveltime inversion of zero-offset VSP and wavefield inversion of offset VSP data. Shear-wave velocity is estimated to be slower from VSP data than from wireline DSI measurements in the depth interval from 677 m to 889 m. The compressional-wave velocity difference between the VSP- and DSI-derived velocities are comparatively small. Synthetic seismograms from the drift-corrected DSI velocity log correlate well with VSP sections, especially for compressional waves. Azimuthal anisotropy is suggested in VSP shear-source data and the mode of anisotropy appears to change around the base of permafrost. By comparing computed elastic velocities with drift-corrected DSI velocity logs, two opposing gas hydrate saturation models are examined. Shear wave velocity proved to be the key data to select the correct model. The observed elastic velocity fits the computed elastic velocity for the model of gas hydrate disseminated in pore-space with little cementation at the grain boundaries.
cited By 42

Vertical seismic profile (VSP) in the JAPEX/JNOC/GSC Mallik 2L-38 gas hydrate research well - Walia, R. (Bulletin of the Geological Survey of Canada, 1999)
As part of the JAPEX/JNOC/GSC Mallik 2L-38 field program, a vertical seismic profiling(VSP) survey was carried out at zero and offset-source positions with multicomponent receiver tools and multipolarized vibrators. The results will be integrated with downhole logs and regional seismic data to evaluate the effect of gas hydrate on seismic velocity and to estimate gas hydrate concentrations. The excellent data quality allows accurate compressional-and shear-velocity depth profiles. There are down-going and up-going waves from numerous reflectors, and corridor stacks provide comparison with surface multi-channel data. Velocities in the permafrost zone above 600 m are enhanced, to more than 2500 m/s. In the largely unfrozen section from 600 m to 850 m, the velocities are lower, about 2000 m/s. The gas hydrate zone is well defined below about 900 m, with velocities of 2500-2700 m/s. Poisson's Ratio is ∼0.39 in both the permafrost and gas hydrate sections, compared to -0.44 in the unfrozen sections.
cited By 6

Vertical seismic profile in the mallik 2L-38 gas hydrate research well in the Canadian arctic - Mi, Y.; Walia, R.; Hyndman, R.D.; Sakai, A. (1999 SEG Annual Meeting, 1999)
Natural gas hydrate is receiving recent attention and exploration interest because it may represent a large clean energy resource. Gas hydrate is an ice-like nonstoichiometric inclusion compound with water molecules forming a three-dimensional network in which small molecules (guests) can be trapped. For most occurrences, the gas is biogenic methane. A large volume of methane in natural gas hydrate may exist beneath some deep sea continental slopes and arctic land areas. This paper presents preliminary VSP results from a gas hydrate research well drilled in the Canadian Arctic. © 1999 Society of Exploration Geophysicists. All rights reserved.
cited By 0
Keywords: Energy resources; Gases; Hydration; Methane; Molecules; Natural gas; Natural gas deposits; Petroleum prospecting; Seismic prospecting; Seismology; Well drilling; Canadian Arctic; Continental slope; Inclusion compounds; Non-stoichiometric; Small molecules; Three-dimensional networks; Vertical seismic profiles; Water molecule; Gas hydrates

Volcanic deformation at Unzen, Japan, visualized by a time-differential stereoscopy - Yamashina, Ken'Ichiro; Matsushima, Takeshi; Ohmi, Shiro (Journal of Volcanology and Geothermal Research, 1999)
Using a pair of old and new photographs taken at the same location, a method named as a time-differential stereoscopy is successfully applied to detect visually the deformation at Unzen volcano, Japan. On September 6, 1992, gradual movement of a lava lobe was visible in a stereo image as relief surrounded by immobile areas. In this case, accelerated movement of a lava block was detected immediately before the time of its collapse, which resulted in the occurrence of a pyroclastic flow. In November 1994, growth of a lava spine was also measured with time-differential stereoscopy. A possible fluctuation of the growth rate might have correlated with an abnormal oscillation of tilt, with a period of about 40 h. This convenient photographic method is recommended for various occasions to detect volcanic and any other deformations as a supplement to precise instrumental measurements.
Keywords: Japan; crustal deformation; lava flow; stereo image; volcanic eruption

Well-log interpretation of gas-hydrate-bearing formations in the JAPEX/JNOC/GSC Mallik 2L-38 gas hydrate research well - Miyairi, M. (Bulletin of the Geological Survey of Canada, 1999)
Techniques for evaluating subsurface natural gas hydrate were part of the JNOC/GSC/J APEX joint research project. The physical properties of pure methane hydrate, related to well-log responses, were directly measured and/or calculated based on its physico-chemical properties. A petrophysical model of the pore-filling gas hydrate was built considering the existence of thermally dissociated free gas in the pores of the formation. Tool sensitivity to gas hydrate content was analyzed, and formation resistivity and acoustic transit time were found to show distinct sensitivity. Three practical methods for evaluating gas hydrate content were proposed and were tested to confirm their applicability: 1) the resistivity method, 2) the acoustic-velocity method, and 3) the statistical-inversion-analysis method. The porosity and gas hydrate saturation results calculated from these methods agreed quite well. Thus, reasonable interpretations can be achieved using these methods if the drilling and log measurements are carefully designed, and the zoning and parameter settings are made properly in pore-filling-type gas hydrate occurrences similar to those found in the JAPEX/JNOC/GSC Mallik 2L-38 gas hydrate research well.
cited By 14

Occurrences and X-ray computerized tomography (CT) observations of natural gas hydrate, JAPEX/JNOC/GSC Mallik 2L-38 gas hydrate research well - Uchida, T. (Bulletin of the Geological Survey of Canada, 1999)
The JAPEX/JNOC/GSC Mallik 2L-38 gas hydrate research well was drilled through a thick interbedded sequence of gas-hydrate-bearing sediments between 896 and 1106 m. In total, 37.3 m of core were collected between 886 and 952.6 m, using a variety of coring systems. Visual observations at the drill site identified a predominance of pore-space gas hydrate in varying concentrations within framework-supported sands and pebbly sands. Gas hydrate was mainly fine grained (<2 mm), filling the intergranular pores and/or coating mineral grains. Although rare, thin veins (1-2 mm) and clasts or nodules of gas hydrate (up to 0.5 mm) were also observed. The largest gas hydrate occurrence (2 cm in diameter) formed the matrix of a granular sand at 913 m. X-ray CT imagery, carried out in Japan, has identified eight well constrained signatures of the constituent components of the gas-hydrate-bearing sands and granular sands. These images have been used to establish textural characteristics and the relationship between sediment grains and gas hydrate.
cited By 9

Micropaleontology of the JAPEX/JNOC/GSC Mallik 2L-38 gas hydrate research well - McNeil, D.H. (Bulletin of the Geological Survey of Canada, 1999)
Core and cuttings samples from the JAPEX/JNOC/GSC Mallik 2L-38 gas hydrate research well in the Mackenzie Delta have yielded sparse terrestrial microfossils and abundant reworked Cretaceous foraminifers (silicified) and plant microfossils. No definitely in situ marine microfossils were recovered in the borehole (total depth 1150m; gas hydrate at 896-1110m). Cores from 110-118 m and 173-175 m contained terrestrial microfossils including fungi, seeds, insect fragments, and abundant macerated plant fragments typical of the Pliocene-Pleistocene Iperk Sequence in the Mackenzie-Beaufort Basin. Core from 886-951 m and cuttings samples from 670-870 and 960-1140 m yielded reworked algal cysts, seed casings, and megaspores. In addition, cuttings contained reworked Cretaceous agglutinated foraminifers. Core and cuttings samples were also characterized by quartz, chert, brownish-black lignite, coaly fragments, and rare amber. The lithology of the section below 670 m is characteristic of the Oligocene Kugmallit Sequence in the Mackenzie-Beaufort Basin.
cited By 2
Keywords: algae; Foraminifera; Fungi; Hexapoda

Mount Dutton volcano, Alaska: Aleutian arc analog to Unzen volcano, Japan - Miller, T.P.; Chertkoff, D.G.; Eichelberger, J.C.; Coombs, M.L. (Journal of Volcanology and Geothermal Research, 1999)
Holocene eruptions from Mount Dutton, a small Late Quaternary volcano near the tip of the Alaska Peninsula, bear strong physical and petrologic similarities to the 1990-1995 Unzen Fugendake eruption in Japan. The volcano had a protracted phase of effusive calcalkaline andesitic (54-59 wt.% SiO2) cone-building in the late Pleistocene followed by an abrupt switch to more silicic ( 65 wt.% SiO2) lavas, emplaced as a central summit cluster of steep-sided domes beginning in the early Holocene. The flanks of the volcano are mantled by pyroclastic flows, debris flows, and talus formed as a result of gravitational dome collapse. Disequilibrium mineral assemblages, including coexisting quartz and olivine in eruptive episodes ranging from the initial cone-building basaltic andesite lavas to the latest Holocene dacite domes, suggest extensive magma mixing. In addition, up to meter-sized, pillow-like cognate mafic enclaves of hornblende + plagioclase + glass are common in the latest of the summit dacite domes. Mineralogical evidence and bulk chemical data indicate the enclaves represent a high-alumina basalt parent with variable and subordinate reservoir contaminant, and the host lava is reservoir magma with variable and subordinate basaltic contaminant. Mount Dutton's history and petrology can be interpreted as reflecting the monotonous repetitive intrusion of mantle-derived mafic magma into a silicic crystal-rich crustal reservoir. During the Holocene, these injections resulted in the extrusion of partially crystallized, viscous, 'sticky' central domes which typically failed by collapse resulting in small volume Merapi-type flowage deposits. We speculate that slow introduction of mafic magma into the silicic chamber leads both to enclave formation and to the effusive eruption style. Mount Dutton volcano experienced severe shallow earthquake swarms in 1984, 1988, and to a lesser extent in 1991; although none of these swarms resulted in an eruption, their epicenter distribution and volcanic-tectonic character indicate they recorded the movement of magma at shallow depth beneath the volcano. The lessons of Unzen and Montserrat suggest that, despite limited tephra production, these small volcanic centers with central dome complexes present a serious hazard to life and property.
Keywords: Japan; United States; Holocene; lava dome; magma chemistry; volcanic eruption

Generation of block and ash flows during the 1990-1995 eruption of Unzen Volcano, Japan - Ui, Tadahide; Matsuwo, Norimichi; Sumita, Mari; Fujinawa, Akihiko (Journal of Volcanology and Geothermal Research, 1999)
Processes generating block and ash flows by gravitational dome collapse (Merapi-type pyroclastic flow) were observed in detail during the 1990-1995 eruption of Unzen volcano, Japan. Two different types were identified by analysis of video records and observations during helicopter flights. Most of the block and ash flows erupted during the 1991-1993 exogenous dome growth stage initially involved crack propagation due to cooling and flowage of the dome lava lobes. The mass around the crack became unstable, locally decreasing in tensile strength. Finally, a slab separated from the lobe front, fragmented progressively from the base to the top within a few seconds, and became a block and ash flow. Rock falls immediately followed, in response to local instability of the lobe front. Clasts in these rock falls fragmented and merged with the preceding flow. In contrast, block and ash flows during the endogenous dome growth stage in 1994 were generated due to local bulge of the dome. Unstable lava blocks collapsed and subsequently fragmented to produce block and ash flows.
Keywords: Japan; ash flow; lava dome; volcanic ash; volcanic eruption

A new regime of volcanic eruption due to the relative motion between liquid and gas - Yoshida, Shigeo; Koyaguchi, Takehiro (Journal of Volcanology and Geothermal Research, 1999)
In explosive magma eruptions, magma ascends through a conduit as a Poiseuille flow at depth, and gas exsolves gradually and expands as the pressure decreases (bubbly flow regime). When the volume fraction of gas becomes sufficiently large, liquid or solid parts of magma fragment into droplets or ashes, and the flow dynamics becomes governed by the gas phase (gas-ash flow regime). We propose a new flow regime, which we call fractured-turbulent flow regime, between the bubbly flow regime and the gas-ash flow regime. In the new regime, both liquid magma and gas are continuous phases. The high connectivity of the two phases allows the relative velocity between them to increase significantly. We present one sample calculation, which displays basically explosive characteristics, but has three features distinct from previous models. The explosive characteristics are manifested as the fragmentation of the magma and the high speed jet that issues from the vent. The first distinct feature is a nearly lithostatic pressure distribution, which results from the increase of the height of the fragmentation surface. The second one is the atmospheric pressure at the vent; the flow is not choked. The third one is that the relative velocity between the gas and the ash is large at the vent despite the large interaction force between the two phases. The large relative velocity is established in the fractured-turbulent regime, and is maintained in the subsequent gas-ash flow regime.
Keywords: explosive volcanism; magmatism; two phase flow; volcanic eruption

Microbial communities from core intervals, JAPEX/JNOC/GSC Mallik 2L-38 gas hydrate research well - Colwell, F.S. (Bulletin of the Geological Survey of Canada, 1999)
Microbial cell distribution in sediment core samples from the JAPEX/JNOC/GSC Mallik 2L-38 gas hydrate research well on the Mackenzie Delta (Canada) was studied using acridine orange direct counts of stained cells for total cell estimation, and by most probable number statistical enumeration for culturable methanogens. The purpose was to characterize the microbial communities in gas-hydrate-bearing sediments. Results indicated that the total cell count values were in the range of 1.1 × 105 cells/g to 2.8× 106 cells/g with culturable methanogens present at 1×10-4% to 1.0% of those values. These results also indicated that culturable methanogens may be more numerous in the porous sandy strata of the Mackenzie Bay Sequence than in clay and silt units of the Kugmallit Sequence. These data expand the known distribution of methanogens in deep sediments and establish the presence of microbial communities in subpermafrost environments.
cited By 7

Chemical and physical analysis of natural gas hydrate from the JAPEX/JNOC/GSC Mallik 2L-38 gas hydrate research well - Tulk, C.A. (Bulletin of the Geological Survey of Canada, 1999)
Gas hydrate samples from the JAPEX/JNOC/GSC Mallik 2L-38 gas hydrate research well were analyzed on both macroscopic and molecular scales using several complementary experimental techniques. These included gas volume analysis, thermogravimetric analysis, precision gas analysis, powder X-ray diffraction, differential scanning calorimetry, Fourier transform infrared spectroscopy, and Raman Spectroscopy. Powder X-ray diffraction indicated that the samples were Structure I gas hydrate. Enclathrated gas species were identified to be mostly methane (98-100%); however, some samples contained significant amounts of heterogeneously dispersed propane and carbon dioxide (at least 1.5-2.0%). These samples were found to be significantly more stable than samples containing methane only. In addition, Raman spectra indicate subtle variations in the cage occupancies of the mixed gas hydrate as compared to those in pure methane hydrate.
cited By 18

Clay minerals in the sediments of Lake Baikal: A useful climate proxy - Yuretich, Richard; Melles, Martin; Sarata, Brenn; Grobe, Hannes (Journal of Sedimentary Research, 1999)
Lake Baikal, in south-central Siberia, has been the focus of an international effort (the Baikal Drilling Project; BDP) to obtain continuous long cores (upwards of 100 m) from this unique rift-valley lake and to interpret the paleoclimatic history from various proxy data. As part of this effort, the clay minerals were examined by two research teams. A consistent clay-mineral assemblage, containing illite, inter-stratified illite-smectite, chlorite, and kaolinite as the major minerals, characterizes much of the modern sediments. The relative abundance of these minerals changes with depth in both short piston cores from various parts of the lake and in 100-m-long cores taken from the distal toe of the Selenga Delta (BDP-93). Independent analyses of the abundance changes and correlation with other data from the cores show that the clays are responding to two influences: (1) climatic fluctuations, particularly in the upper 40 m of the sedimentary record, which show the relative amount of illite-smectite (and sometimes kaolinite) increasing during warmer climate episodes, and (2) source-area changes, which are most evident below 40 m in the BDP-93 cores and mark a shift from an eastern (Buguldeika River) to a western (Selenga River) source. The clay-mineral-based climate fluctuations are correlative with the marine oxygen-isotope record through stage 7, and provide a relatively simple and cost-effective tool for gaining insight into the paleoclimate of this interior continental site.
Keywords: Russian Federation; clay mineral; proxy climate record; stratigraphic correlation