All ICDP Publications with Abstracts
From parent-sysfolder "Publications" + 2 folder-levels deep
159.
Late-Quaternary lowstands of Lake Titicaca: Evidence from high-resolution seismic data
Palaeogeography, Palaeoclimatology, Palaeoecology,
179
(1-2)
97 – 111
2002
Keywords:▾
Bolivia; Peru; lake level; paleoclimate; Quaternary; seismic data; seismic reflection; seismic stratigraphy
Abstract: ▾ 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.
158.
Mapping the northern portion of the Chelungpu fault, Taiwan by shallow reflection seismics
Geophysical Research Letters,
29
(16)
37-1-37-3
2002
ISSN: 00948276
Publisher: American Geophysical Union
Keywords:▾
Cost effectiveness; Seismology; Structure (composition); Surveys, Fault slippages, Earthquakes, borehole; earthquake rupture; fault slip; seismic reflection; seismic survey, Chelungpu Fault; Taiwan
Abstract: ▾ 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.
157.
Late Pliocene sedimentation in Lake Baikal: Implications for climatic and tectonic change in SE Siberia
Palaeogeography, Palaeoclimatology, Palaeoecology,
174
(4)
305 – 326
2001
ISSN: 00310182Keywords:▾
Russian Federation; Bacillariophyta; geochemistry; lacustrine deposit; paleoclimate; Pliocene; sedimentology; tectonic evolution; weathering
Abstract: ▾ 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.
156.
Thermal properties of bottom sediments of Lake Baikal (according to data on temperature field recovery in underwater boreholes)
Geologiya i Geofizika,
42
(1-2)
298 – 307
2001
ISSN: 00167886Abstract: ▾ 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.
155.
The sponge fauna of lake baikal in the late pliocene (according to studies of core samples from deep borehole BDP-96-1)
Geologiya i Geofizika,
42
(1-2)
130 – 137
2001
ISSN: 00167886Abstract: ▾ 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.
154.
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
Geologiya i Geofizika,
42
(1-2)
373 – 383
2001
ISSN: 00167886Abstract: ▾ 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.
153.
The new BDP-98 600-m drill core from Lake Baikal: A key late Cenozoic sedimentary section in continental Asia
Quaternary International,
80-8119 – 36
2001
ISSN: 10406182Keywords:▾
Asia; Russian Federation; Cenozoic; lacustrine deposit; sediment core
Abstract: ▾ 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.
152.
The link between tectonic and paleoclimatic events at 2.8-2.5 Ma BP in the Lake Baikal region
Quaternary International,
80-8137 – 46
2001
ISSN: 10406182Keywords:▾
Russian Federation; lacustrine deposit; neotectonics; paleoclimate; paleomagnetism; sediment core
Abstract: ▾ 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.
151.
Paleoclimate record in bottom sediments of lake Baikal, from magnetic susceptibility data
Geologiya i Geofizika,
42
(1-2)
87 – 97
2001
ISSN: 00167886Abstract: ▾ 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.
150.
Multiwave XRF-SR determination of U and Th in bottom sediments of Lake Baikal: Brunhes paleoclimatic chronology
Geologiya i Geofizika,
42
(1-2)
186 – 193
2001
ISSN: 00167886Abstract: ▾ 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).
149.
Mineral inclusions in zircons of para- and orthogneiss from pre-pilot drillhole CCSD-PP1, Chinese Continental Scientific Drilling Project
Lithos,
59
(4)
199 – 215
2001
ISSN: 00244937Keywords:▾
China; drilling; mineralogy; orthogneiss; paragenesis; zircon
Abstract: ▾ 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.
148.
Late quaternary climate and hydrology of tropical South America inferred from an isotopic and chemical model of Lake Titicaca, Bolivia and Peru
Quaternary Research,
56
(1)
1 – 9
2001
Keywords:▾
Bolivia; Lake Titicaca; Peru; geochemistry; lake level; paleoclimate; paleohydrology; paleolimnology; Quaternary
Abstract: ▾ 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.
147.
Biostratigraphic significance of new fossil species of the diatom genera Stephanodiscuts and Cyclotella from Upper Cenozoic deposits of Lake Baikal, Siberia
Micropaleontology,
47
(1)
47 – 71
2001
ISSN: 00262803DOI:10.2113/47.1.47
Keywords:▾
Russian Federation; biostratigraphy; Cenozoic; diatom; lacustrine deposit; new species; paleoclimate
Abstract: ▾ 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 (Kützing) Brébisson (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.
146.
Lake Titicaca: An archive of South American paleoclimate
Geotimes,
46
(12)
20 – 21
2001
145.
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
Geologiya i Geofizika,
42
(1-2)
98 – 107
2001
ISSN: 00167886Abstract: ▾ 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.
144.
Geochemical constraints of the eclogite and granulite facies metamorphism as recognized in the Raobazhai complex from North Dabie Shan, China
Journal of Metamorphic Geology,
19
(1)
3 – 19
2001
ISSN: 02634929Keywords:▾
China; eclogite; geochemistry; granulite; metamorphism
Abstract: ▾ 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.
143.
Elastic properties of gas hydrate-bearing sediments
Geophysics,
66
(3)
763-771
2001
ISSN: 00168033
Publisher: Society of Exploration Geophysicists
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
Abstract: ▾ 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.
142.
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
Contributions to Mineralogy and Petrology,
141
(3)
322 – 346
2001
ISSN: 00107999Keywords:▾
China; deformation; eclogite; exhumation; fluid flow; metamorphism; P-T conditions; P-T-t path
Abstract: ▾ 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.
141.
Differences in glacial and interglacial clay mineral associations of Baikal sediments from BDP-93-2 and BDP-96 cores
Geologiya i Geofizika,
42
(1-2)
146 – 156
2001
ISSN: 00167886Abstract: ▾ 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.
140.
Detailed diatom biostratigraphy of Baikal sediments during the brunhes chron and climatic factors of species formation
Geologiya i Geofizika,
42
(1-2)
108 – 129
2001
ISSN: 00167886Abstract: ▾ 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.
139.
Density of basalt core from Hilo drill hole, Hawaii
Journal of Volcanology and Geothermal Research,
112
(1-4)
221 – 230
2001
ISSN: 03770273Keywords:▾
United States; Landslides; Stratigraphy; Thermal effects; Volcanic rocks; basalt; density; hyaloclastite; pillow lava; stratigraphy; Hyaloclastite; Volcanoes
Abstract: ▾ 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.
138.
Dating of deep-water sediments of Lake Baikal from the ratios of thorium and uranium isotopes in various fractions of their authigenic part
Geologiya i Geofizika,
42
(1-2)
194 – 205
2001
ISSN: 00167886Abstract: ▾ 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.
137.
Crustal structure of the eastern Dabie Shan interpreted from deep reflection and shallow tomographic data
Tectonophysics,
333
(3-4)
347 – 359
2001
ISSN: 00401951Keywords:▾
China; crustal structure; deep seismic sounding; seismic tomography; ultrahigh pressure metamorphism
Abstract: ▾ 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.
136.
Kinematics of exhumation of high- and ultrahigh-pressure rocks in the Hong'an and Tongbai Shan of the Qinling-Dabie collisional orogen, eastern China
Memoir of the Geological Society of America,
194231 – 245
2001
ISSN: 00721069
Publisher: Geological Society of America
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
Abstract: ▾ 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.
135.
22 - Comparison of persistent organochlorine pollutant behavior in the food webs of lakes baikal and superior
In Koji Minoura, Editor
,
Lake Baikal
Page 247-261
Publisher
Elsevier Science B.V.
,
Amsterdam
2000
247-261