All ICDP Publications with Abstracts
From parent-sysfolder "Publications" + 2 folder-levels deep
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.
169.
Episodic thermal perturbations associated with groundwater flow: An example from Kilauea Volcano, Hawaii
Journal of Geophysical Research: Solid Earth,
107
(11)
ECV 13–1 – 13–10
2002
ISSN: 21699313
Publisher: Blackwell Publishing Ltd
Abstract: ▾ 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.
168.
Origin and evolution of the western Snake River Plain: Implications from stratigraphy, faulting, and the geochemistry of basalts near Mountain Home, Idaho
Tectonic and Magmatic Evolution of the Snake River Plain Volcanic Province,
30343--361
2002
167.
Drilling program investigates permafrost gas hydrates
Eos,
83
(18)
193+198
2002
ISSN: 00963941
Publisher: American Geophysical Union
166.
Biot-Gassmann theory for velocities of gas hydrate-bearing sediments
Geophysics,
67
(6)
1711-1719
2002
ISSN: 00168033
Publisher: Society of Exploration Geophysicists
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
Abstract: ▾ 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.
165.
Occurrence of metastable rocks in deeply subducted continental crust from the Dabie Mountains, central China
Episodes,
25
(2)
84 – 89
2002
ISSN: 07053797
Publisher: International Union of Geological Sciences
Keywords:▾
China; Dabie Shan; continental crust; metamorphic facies; metamorphic rock; subduction
Abstract: ▾ 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.
164.
Conjugate thrust faulting associated with the 1999 Chi-Chi, Taiwan, earthquake sequence
Geophysical Research Letters,
29
(8)
2002
ISSN: 00948276
Publisher: Blackwell Publishing Ltd
Keywords:▾
Earthquakes; Geophysics; Underground structures, Chi-Chi , Taiwan; Conjugate faults; Earthquake sequences; Focal mechanism; Geometrical structure; Seismicity pattern; Spatial patterns; Thrust faulting, Faulting
Abstract: ▾ 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.
163.
Chelungpu fault drilling could resolve seismological issues
Eos,
83
(23)
255
2002
ISSN: 00963941
Publisher: American Geophysical Union
162.
High-resolution seismic velocities and shallow structure of the San Andreas fault zone at Middle Mountain, Parkfield, California
Bulletin of the Seismological Society of America,
92
(6)
2493-2503
2002
ISSN: 00371106Keywords:▾
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
Abstract: ▾ 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.
161.
Initial science report of shallow drilling, penetrating into the Chelungpu fault zone, Taiwan
Terrestrial, Atmospheric and Oceanic Sciences,
13
(3)
227-251
2002
ISSN: 10170839
Publisher: Chinese Geoscience Union
Keywords:▾
breccia; Chi-Chi earthquake 1999; dip-slip fault; drilling; fault zone; microstructure; reverse fault, Chelungpu Fault; Taiwan
Abstract: ▾ 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.
160.
Late Pliocene vegetation and climate of the Lake Baikal region, southern East Siberia, reconstructed from palynological data
Palaeogeography, Palaeoclimatology, Palaeoecology,
184
(1-2)
107 – 129
2002
ISSN: 00310182
Publisher: Elsevier B.V.
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
Abstract: ▾ 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.