All ICDP Publications with Abstracts
From parent-sysfolder "Publications" + 2 folder-levels deep
194.
Characteristic magnetic behavior of subaerial and submarine lava units from the Hawaiian Scientific Drilling Project (HSDP-2)
Geochemistry, Geophysics, Geosystems,
4
(2)
2003
ISSN: 15252027Abstract: ▾ 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.
193.
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)
Geochemistry, Geophysics, Geosystems,
4
(5)
2003
ISSN: 15252027192.
Finding fault
Engineer,
292
(7638)
21-24
2003
ISSN: 00137758Abstract: ▾ 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.
191.
Earthquake locations and three-dimentional fault zone structure along the creeping section of the San Andreas fault near Parkfield, CA: Preparing for SAFOD
Geophysical Research Letters,
30
(3)
12-1
2003
ISSN: 00948276Abstract: ▾ 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.
190.
Geology and geochemistry of shallow drill cores from the Bosumtwi impact structure, Ghana
Meteoritics and Planetary Science,
38
(8)
1137 – 1159
2003
ISSN: 10869379
Publisher: University of Arkansas
Keywords:▾
Bosumtwi Impact Structure; Ghana; breccia; ejecta; geochemistry; geological structure; impact structure; tektite
Abstract: ▾ 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.
189.
Late Quaternary clay mineral record in Central Lake Baikal (Academician Ridge, Siberia)
Palaeogeography, Palaeoclimatology, Palaeoecology,
193
(1)
159 – 179
2003
ISSN: 00310182Keywords:▾
Russian Federation; Bacillariophyta; clay mineral; illite; lacustrine deposit; paleoclimate; proxy climate record; Quaternary; smectite
Abstract: ▾ 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.
188.
Mid-holocene climate change in Lake Bosumtwi, Ghana
Quaternary Research,
60
(2)
133 – 141
2003
ISSN: 00335894
Publisher: Academic Press Inc.
Keywords:▾
Ghana; Lake Bosumtwi; aridity; Holocene; lacustrine deposit; nitrogen isotope; paleoclimate; paleolimnology
Abstract: ▾ 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.
187.
Hawaiian hot spot dynamics as inferred from the Hf and Pb isotope evolution of Mauna Kea volcano
Geochemistry, Geophysics, Geosystems,
4
(2)
2003
ISSN: 15252027Abstract: ▾ 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.
186.
Lithium isotope geochemistry of the Hawaiian plume: Results from the Hawaii scientific drilling project and koolau volcano
Geochemistry, Geophysics, Geosystems,
4
(3)
2003
ISSN: 15252027Abstract: ▾ [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.
185.
Late Quaternary lake-level changes constrained by radiocarbon and stable isotope studies on sediment cores from Lake Titicaca, South America
Global and Planetary Change,
38
(3-4)
273 – 290
2003
Keywords:▾
Lake Titicaca; South America; geochronology; lake level; paleolimnology; radiocarbon dating; sediment core; stable isotope
Abstract: ▾ 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.
184.
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
Tectonophysics,
370
(1-4)
49 – 76
2003
ISSN: 00401951
Publisher: Elsevier B.V.
Keywords:▾
China; Sulu Belt; eclogite; mantle; microstructure; petrofabric; rheology; seismic reflection; subduction
Abstract: ▾ 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.
183.
Investigating a 65-Ma-Old smoking gun: Deep drilling of the chicxulub impact structure
Eos,
84
(14)
125+130
2003
182.
Inferences on the hydrothermal system beneath the resurgent dome in Long Valley Caldera, east-central California, USA, from recent pumping tests and geochemical sampling
Journal of Volcanology and Geothermal Research,
127
(3-4)
305-328
2003
ISSN: 03770273
Publisher: Elsevier
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
Abstract: ▾ 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.
181.
High-resolution MIS 11 record from the continental sedimentary archive of Lake Baikal, Siberia
Geophysical Monograph Series,
137223 – 230
2003
DOI:10.1029/137GM16
180.
Hawaiian lava flows in the third dimension: Identification and interpretation of pahoehoe and ̀àa distribution in the KP-1 and SOH-4 cores
Geochemistry, Geophysics, Geosystems,
4
(2)
2003
ISSN: 15252027Abstract: ▾ 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.
179.
Preservation of primary volcanic textures in the ultrahigh-pressure terrain of Dabie Shan
Geology,
30
(8)
699 – 702
2002
ISSN: 00917613Keywords:▾
China; High pressure effects; Levees; Zeolites; Pillow lava; pillow lava; preservation; ultrahigh pressure metamorphism; volcanic feature; volcaniclastic deposit; Volcanoes
Abstract: ▾ 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.
178.
[English]
Temperature Memory Survey Results and Future Plan for the Recently Erupted Unzen Volcano
Page 633-637
2002
177.
The Corinth Rift Laboratory: Monitoring of active faults
First Break,
20
(2)
91 – 97
2002
ISSN: 02635046Keywords:▾
active fault; hydraulic property; pressure solution; rheology; stress change
Abstract: ▾ 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.
176.
Studies of sedimentary facies, stratigraphy, and deformation structures of the Chelungpu fault zone on cores from drilled wells in Fengyuan and Nantou, Central Taiwan
Terrestrial, Atmospheric and Oceanic Sciences,
13
(3)
253-278
2002
ISSN: 10170839
Publisher: Chinese Geoscience Union
Keywords:▾
Chi-Chi earthquake 1999; deformation; fault zone; sequence stratigraphy; Tertiary; thrust fault, (Central); Taiwan
Abstract: ▾ 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.
175.
Structural mapping of the 1999 Chi-Chi earthquake fault, Taiwan by seismic reflection methods
Terrestrial, Atmospheric and Oceanic Sciences,
13
(3)
211-226
2002
ISSN: 10170839
Publisher: Chinese Geoscience Union
Keywords:▾
Chi-Chi earthquake 1999; fault; geological mapping; seismic reflection, (Central); Taiwan
Abstract: ▾ 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.
174.
Sonic waveform attenuation in gas hydrate-bearing sediments from the Mallik 2L-38 research well, Mackenzie Delta, Canada
Journal of Geophysical Research: Solid Earth,
107
(5)
EPM 1-1 - EPM 1-12
2002
ISSN: 21699313
Publisher: Blackwell Publishing Ltd
Abstract: ▾ 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.
173.
Shallow reflection seismics aiding geological drilling into the Chelungpu fault after the 1999 Chi-Chi earthquake, Taiwan
Terrestrial, Atmospheric and Oceanic Sciences,
13
(2)
153-170
2002
ISSN: 10170839
Publisher: Chinese Geoscience Union
Keywords:▾
drilling; earthquake; fault; seismic reflection, Chichi; Taiwan
Abstract: ▾ 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.
172.
Seismic investigation of the Lake Bosumtwi impact crater: Preliminary results
Planetary and Space Science,
50
(7-8)
735 – 743
2002
ISSN: 00320633Keywords:▾
Drilling; Earth (planet); Geophysics; Hydrophones; Rocks; Topography; Seismic prospecting
Abstract: ▾ 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.
171.
Researchers propose earthquake observatory inside San Andreas fault
Civil Engineering,
72
(7)
34-35
2002
ISSN: 08857024Abstract: ▾ 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.
170.
Relationship of gas hydrate concentration to porosity and reflection amplitude in a research well, Mackenzie Delta, Canada
Marine and Petroleum Geology,
19
(4)
407-415
2002
ISSN: 02648172Keywords:▾
Pore spaces, Gas hydrates; Porosity; Reflection; Sand; Sediments; Seismic prospecting; Well logging, Geology, amplitude; gas hydrate; porosity; seismic data; seismic reflection; well logging, Canada
Abstract: ▾ 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.