Tectonophysics [T]

T41A MCC:Level 2 Thursday

Lithospheric Structure of East Asia I Posters

Presiding: P Tiwari, ITT Bombay; M McRivette, University of California, Los Angeles

T41A-1267

A Thinned Lithospheric Image beneath the Tanlu Fault Zone, Northeastern China: Constructed from Wave Equation Based Receiver Function Migration

* Chen, L (lchen@mail.igcas.ac.cn) , Institute of Geology and Geophysics, Chinese Academy of Sciences, Deshengmenwai, Qijiahuozi, P.O.BOX 9825 Chaoyang District, Beijing, 100029 China
Zheng, T (tyzheng@mail.igcas.ac.cn) , Institute of Geology and Geophysics, Chinese Academy of Sciences, Deshengmenwai, Qijiahuozi, P.O.BOX 9825 Chaoyang District, Beijing, 100029 China
Xu, W (wwxu@mail.iggcas.ac.cn) , Institute of Geology and Geophysics, Chinese Academy of Sciences, Deshengmenwai, Qijiahuozi, P.O.BOX 9825 Chaoyang District, Beijing, 100029 China

Lithospheric reactivation and thinning have long been proposed as a dominant process occurred in the cratonic Northeastern China continent during late Mesozoic and Cenozoic, yet direct seismic observations of fine-scale lithospheric structure have been scarce. We apply the newly proposed wave equation-based receiver function poststack migration method to the NCISP broadband data to image the lithospheric structure of the Tanlu Fault Zone area in Northeastern China. Our migration result reveals a 60-to-80-km-thick present-day lithosphere beneath the study region, significant thinned from the Paleozoic lithosphere of > 180 km. The lithosphere-asthenosphere boundary (LAB) is coherently imaged along an about 300-km east-west profile, with its apex roughly coincident with the transverse location of the Tanlu Fault Zone at the surface. An obvious uplift from around 36 km to around 32 km of the Moho is also clearly detected right below this fault zone. The coincidence of the imaged Moho uplift and the LAB apex with the surface location of the Tanlu Fault Zone provides seismological evidence for the steep geometry and deep penetration of the fault system, and indicates that the Tanlu Fault Zone might have facilitated anthenosphere upwelling during the Mesozoic-Cenozoic continental extension and lithospheric thinning. Frequency analysis and synthetic modeling suggest that both the Moho and the LAB are sharp and strong. The latter, in particular, is constrained to be within a less-than-10 km depth interval with a shear-wave velocity reduction of at least 4%. The revealed structural feature of the LAB is unlikely to be fully explained by the upper mantle thermal structure of the study region. Our lithospheric image suggests that the uppermost portion of the Archean lithospheric mantle beneath the Tanlu Fault Zone area might have survived the lithospheric reactivation and thinning occurred during the late Mesozoic and Cenozoic time. The compositional contrast between the preserved cold cratonic lithospheric mantle veneer and the uplifted hot asthenospheric materials may partially account for the sharp seismic velocity gradient at the base of the lithosphere.

T41A-1268

Structure of the Dayangshu Basin in Northeast China Inferred from Magnetotelluric Data

Cao, J (jxcao@mit.edu) , MIT, Earth Resources Laboratory, Cambridge, MA 02142 United States
Cao, J (jxcao@mit.edu) , Chengdu University of Technology, 1 East 3rd Rd Erxianqiao, Chengdu, 610059 China
* Sun, Y (youshun@mit.edu) , MIT, Earth Resources Laboratory, Cambridge, MA 02142 United States
Li, X , Chengdu University of Technology, 1 East 3rd Rd Erxianqiao, Chengdu, 610059 China
Chen, S , Exploration & Production Research Institute, Daqing Oilfield Company Limited, Daqing, 163712 China

The Dayangshu basin is a NNE trending fault-bounded basin, located at longitude 123°21' - 125°38' east and latitude 47°55' - 50°59' north within the southern part of the Xingan fold system. It is one of the largest sedimentary basins in Northeast China with an area of 15,460 km$^2$. The sedimentary rock in the basin is approximately 3,000 m thick, and is composed mainly of clastic rocks and volcanic rocks. The base of the basin consists mainly of Paleozoic low-grade metamorphic rocks. In the early 1990s, the Daqing Oilfield Company started petroleum exploration programs in the area. By the end of 1990s, gravity, magnetic, magnetotelluric (MT), and 2D seismic reflection measurements had been carried out in the basin and several prospects had been drilled. It has been determined that the majority, about 75%, of the basin is composed of volcanic rocks. However, the location and extent of the lava and main hydrocarbon source rock are still unclear. The distribution of these materials is very important for understanding the geological history and evaluating the petroleum resource capacity of the basin. The volcanic rocks within the basin are highly magnetic. The average susceptibility of the lava in the basin is about 0.18 SI units. It has been demonstrated that highly magnetic rocks significantly affect the MT response. By simultaneously inverting MT data for electric resistivity and magnetic susceptibility estimate, we develop a new approach for mapping magnetic strata. We apply the approach to the interpretation of MT data collected at 3,243 sites within the basin, and obtain a volcanic rock distribution model characterized by high susceptibility. The model shows that volcanic rocks are predominantly located at the boundary and middle of the basin. The volcanic rocks located at the middle of the basin cut the basin into southern, middle and northern sub-depressions. The hydrocarbon source rocks should be buried in the sub-depressions, and the lava close to the source rocks is potentially an oil or gas reservoir. The volcanic rock distribution mapped by the MT approach coincides with surface geological and magnetic surveys and drilling records.

T41A-1269

Geometry and Kinematics of Active Faults in the Mongolian-Chinese Altai Mountains: Results From Analysis of ASTER and DISP Satellite Images

* Ganev, P N (pganev@whittier.edu) , Whittier College, 13406 Philadelphia Str., Whittier, CA 90608 United States
Yin, A (yin@ess.ucla.edu) , Department of Earth and Space Sciences and Institute of Geophysics and Planetary Physics, University of California, Los Angeles, 595 Charles Young Drive East, Los Angeles, CA 90095 United States

It is well established that the Cenozoic India-Asia collision has created the Himalaya and the Tibetan plateau. However, little research has been conducted on how the collision may have controlled the geometry and kinematic evolution of Cenozoic intracontinental deformation in central Asia that lies some 2000 km north of the Indo-Asian convergent front in the Himalaya. Previous geologic investigations in the Altai region have revealed the existence of a 1200 km long, NNW-striking right-slip fault system parallel to the general strike of the Altai Range extending from Goni Altai of Russia in the north to the Mongolian-Chinese Altai in the south in central Asia. However, an important question remains on how and why the fault system was created so far into the continent from the collision front. There are two end-member possibilities: (1) the faults were created as new structures that break through old structural grains via Coulomb fracture mechanism, and (2) the faults were reactivated from existing weakness following old structure grains. In the first case, relatively high magnitude of shear stress is required in the lithosphere transmitted from the Himalaya, while in the second case relatively smaller magnitude of shear stress is needed. In order to test the competing hypotheses, we determine the geometrical and kinematic relationships between secondary contractional structures and the primary strike-slip fault system by mapping active fault traces, their offset risers, deflected drainages across the active faults, and alluvial fans using CORONA, ASTER, and LANDSAT satellite images and SRTM topographic data. Our observations along the Mongolia segment of the Altai right-slip fault system reveal the presence of several prominent echelon thrusts that are aligned at an oblique angle between 25- 40° from the main trace of the right-slip fault system. This finding supports the view that the Altai right-slip fault was created as a new structure rather than reactivating along pre-existing weakness in the Paleozoic Altai accretionary orogen. The latter would require weak faults and thus subparallel contractional structures near the main strand of the Altai strike-slip system. Our results imply that the stress in the Asian lithosphere was transmitted efficiently from the Himalayan front to central Asia and its magnitude was sufficiently large to create new fractures. T41A-1270 A Seismological and Geodynamic Study of the Seismic Zones Surrounding the Ordos Plateau, China Wang, L (lswang@nju.edu.cn) , Department of Earth Sciences, Nanjing University, Nanjing, 210093 China * Sandvol, E (sandvole@missouri.edu) , Department of Geological Sciences, 101 Geology Building University of Missouri, Columbia, MO 65211 United States Liu, M (lium@missouri.edu) , Department of Geological Sciences, 101 Geology Building University of Missouri, Columbia, MO 65211 United States Chen, J (johnyc@pku.edu.cn) , Department of Geophysics, Peking University, Beijing, 100871 China Mi, N (lswang@nju.edu.cn) , Department of Earth Sciences, Nanjing University, Nanjing, 210093 China Li, H (lswang@nju.edu.cn) , Department of Earth Sciences, Nanjing University, Nanjing, 210093 China Yu, D (yudy@nju.edu.cn) , Department of Earth Sciences, Nanjing University, Nanjing, 210093 China Weiwei, T (lswang@nju.edu.cn) , Department of Earth Sciences, Nanjing University, Nanjing, 210093 China Zhou, S (zsy@pku.edu.cn) , Department of Geophysics, Peking University, Beijing, 100871 China The fault zones surrounding the Ordos Plateau in China are marked by intense seismicity; the lithospheric structure of these active fault zones are also critical for understanding diffuse plate boundary deformation. On the west side of the plateau is the Liupan Shan thrust belt, which is structurally connected to the Altyn Tagh-Qilian-Haiyuan fault system bounding the northern margin of the Tibetan Plateau and marks the change from shortening and uplift in the Tibetan Plateau to widespread extension in the North China block. The Weihe graben on the south side of the Ordos Plateau is connected to the Qinling-Dabie strike-slip fault that plays an important role in accommodating the eastward escape of the South China block driven by the Indo-Asian collision. During summer 2004, a team from Nanjing University and the University of Missouri deployed 15 stations across the Liupan Shan fold and thrust belt. We have calculated receiver functions that are consistent with a rapid increase in the crustal thickness 20 km to the east of the Lupen Shan mountains. This suggests that the Ordos crust is substantially thicker than the surrounding crust, implying that the topography in the plateau may be overcompensated unless the lower crust consists primarily of dense mafic material. These results have important implications for the interaction of the stable Ordos block and the surrounding lithosphere. We have also observed preliminary evidence of changes in the seismic anisotropy across the Lupen Shan mountains. With additional instruments from Beijing University, we are currently in the process of installing another 30 seismometers across the Weihe graben that should further constrain the seismic structure in this important region. T41A-1271 Lithosperic Image Across the Eastern Tian Shan Mountains from Migrated Receiver Functions * Li, H (li_hai_ou@126.com) , China University of Geosciences (Beijing), 29 Xueyuan Road, Haidian District, Beijing, 100083 China Jiao, W (wjiao@multimax.com) , Multimax Inc., 1441 McCormick Drive, Largo, MD 20774 United States Chan, W (winston@multimax.com) , Multimax Inc., 1441 McCormick Drive, Largo, MD 20774 United States To the north of the Tibetan plateau, a series of mountain ranges were built under the compression from the Indian plate where Tian Shan Mountains are situated between the Kunlun/Altyn Mountains to the south, and the Altai Mountains to the north. Also situated between these mountain ranges are large cratonic basins. The topography changes drastically across the eastern Tian Shan Mountains, with Turfan basin in the southern front being the lowest point in China. Several large, deep fault zones were suggested to have ruptured into the mantle. In order to get a better picture of the Moho and these fault zones, we processed a seismic profile 700km in length across the eastern Tian Shan Mountains consisting of 39 portable and 8 permanent seismic stations. 172 receiver functions were migrated to produce an image of the lithosphere in the profile. The Moho depth is mapped across the profile. Under the Dzungarian basin to the north of the Tian Shan Mountains, the Moho is gently tilted southward, from about 40km to 60km. Under the eastern Tian Shan Mountains, the Moho is downward convex, with the lowest depth around 70km. In this part of the profile it is also clearly seen that the Moho is ruptured by several thrust fault zones. Under the Northern Tian Shan fault, the image shows a clear overlap of the Moho due to overthrusting, where the most active seismic zone is situated. Our results are different in some details from the previous profile studies to the west of our profile. T41A-1272 Crustal Structure Beneath China Inferred from Receiver Function and Apparent SS wave Splitting * Niu, F (niu@rice.edu) , Department of Earth Science, Rice University, 6100 Main Street, MS 126, Houston, TX 77005 United States Mooney, W D (mooney@usgs.gov) , U.S. Geological Survey, 345 Middlefield Road, MS 977, Menlo Park, CA 94025 United States Global crustal models have traditionally been compiled mainly based on active-source refraction and reflection profiles. However, such profiles are usually not evenly distributed. In addition, S-wave velocities are poorly constrained due to the lack of shear wave energy in active sources. Ideally, information on both the P- and S-wave velocities (or P-wave velocity and the Poissons ratio) is preferred in order to infer the crustal composition. In this study, we demonstrate how earthquake data can improve the global coverage of crustal models. Two types of data, receiver function and apparent SS waveform splitting data have been compiled to obtain the desired information. As a feasibility test, we have chosen China as our starting point. We have analyzed receiver function data at more than 270 stations, deployed largely by previous PASSCAL experiments. Very large variations in crustal structure are observed in the study region. The crustal thickness varies from ~25 km to about 90 km and shows a good correlation with geologic terranes, as observed in many previous studies. The crust beneath the extended crust of eastern China and the North China Craton is about 35 km thick. Large P to S conversions are observed at stations in these regions, which suggests a sharp Moho-discontinuity with a large velocity and density contrast. This is suggestive of the lack of a high-velocity mafic layer in the lowermost crust. In contrast, the crust beneath the deformed regions tends to be thick with a diffuse Moho boundary. At large epicentral distance the transverse component of the SS wave always appears to be leading the radial component by a few to tens of seconds. This apparent splitting is produced by several arrivals that include the precursory reflection/conversion at the Moho, followed by several later Moho reverberations. From synthetic data we confirmed that the reflection/conversion series, when filtered to the low-frequency band, produce a large apparent SS splitting. We also found that the apparent splitting time is proportional to the crustal thickness. Our preliminary analysis of several locations in China with the SS data yield very consistent crustal thickness with the receiver function results. The teleseismic SS dataset, which has a very good global coverage, thus could be very useful in developing global crustal models. Crustal Structure beneath China Inferred from Receiver Function and Apparent SS wave Splitting. T41A-1273 Amplitude Tomography From ML-Magnitude Data Beneath China * Wang, S (suyunw@163bj.com) , Geophysical union of China, Minzuxueyuan nanlu, Beijing, BJ 100081 China Hearn, T (thearn@nmsu.edu) , American Geophysical union, 2000 Florida Avenue, Washington, DC 20009 United States Pei, S (peisp@itpcas.ac.cn) , Geophysical union of China, Minzuxueyuan nanlu, Beijing, BJ 100081 China Xu, Z (xuzh@cea-igp.ac.cn) , Geophysical union of China, Minzuxueyuan nanlu, Beijing, BJ 100081 China Ni, J (jni@nmsu.edu) , American Geophysical union, 2000 Florida Avenue, Washington, DC 20009 United States Yu, Y (yyx@cea-igp.ac.cn) , Geophysical union of China, Minzuxueyuan nanlu, Beijing, BJ 100081 China Using the data of S wave amplitude and period reported in the Annual Bulletin of Chinese Earthquakes(1984-2004) for the calculation of ML estimates, we studied the S wave attenuation in the crust and the regional variation of material quality factor Q. Over 54,000 S wave amplitudes were used to map regional attenuation beneath China. These amplitudes were measured from the horizontal components of short-period (about 0.1-2.0 seconds) instruments and generally correspond to regional S waves which propagate within the upper and middle crust. Data are corrected for event size and geometric spreading, but not for radiation pattern. Classical, two-dimensional tomography methods are adapted to find regional attenuation variations, site gain effects, and source size corrections. Features larger than 2 degrees are well resolved. The attenuation Q0 value for these data averages around 360 beneath China. The regional Q0 fluctuation varies from -200 to +200. Regions with the highest Q0 value (low attenuation) are beneath the Tarim Basin, Sichuan Basin, Ordos platform and the southeast China fold belt. Regions with the lowest Q0 value (high attenuation) are beneath Bohai Bay, the North China plain in eastern China and the Panxi rift along the eastern margin of the Tibet Plateau. In general, Q0 values in tectonically stable regions such as the cratonic platform tend to be high, while those in tectonically active regions tend to be low. T41A-1274 Short-period surface-wave amplitude tomography of China * Hearn, T M (thearn@nmsu.edu) , New Mexico State University, Physics Department, Las Cruces, NM 88003 United States Wang, S (suyunw@163bj.com) , Institute of Geophysics China Earthquake Administration, China Earthquake Administration, Beijing, 100081 China Pei, S (peisp@itcas.ac.cn) , Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Beijing, 100085 China Ni, J F (jni@nmsu.edu) , New Mexico State University, Physics Department, Las Cruces, NM 88003 United States Zhonghai, X (xuzh@cea-iqp.ac.cn) , Institute of Geophysics China Earthquake Administration, China Earthquake Administration, Beijing, 100081 China Yu, Y (yanxiang@cdsn.org.cn) , Institute of Geophysics China Earthquake Administration, China Earthquake Administration, Beijing, 100081 China Amplitude data from the Annual Bulletin of Chinese Earthquakes are routinely collected to estimate the surface-wave magnitudes for regional events. We have used these data to construct amplitude tomography map of China with regionally varying attenuation. Qs values average around 200. There are clear correlations between the attenuation and regional structures. Sedimentary basins are clearly identifiable by their high attenuation - particularly Bohai Basin. Continental rifts surrounding the Ordos Plateau and the Panxi rift also have high attenuation. Amplitude data are measured along with a period measurement that comes from the pulse width at maximum amplitude. Periods range from 0.2 to 24 seconds corresponding to surface waves that propagate primarily within the upper crust. Both source size and propagation path affects these periods. Larger sources have larger periods and these are used to investigate source models. Attenuation maps made for different periods ranges have similar structures; however, attenuation at higher frequencies is stronger than that of lower frequencies. T41A-1275 Crustal Structure of the NE Tibetan Plateau, China, From Deep Crustal Seismic Profiling Data * Mooney, W (mooney@usgs.gov) , US Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025 United States Wang, Y (mooney@usgs.gov) , China University of Geoscience, 29 Xueyuan Lu, Beijing, 100081 China Yuan, X (mooney@usgs.gov) , China Geological Survey, 31 Xueyuan Lu, Beijing, 100081 China Okaya, N (mooney@usgs.gov) , US Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025 United States We discuss crustal structure across the northeastern Tibetan plateau based on active-source seismic data recorded along a 1600-km-long profile crossing, from NW to SE, the southern Tarim basin, the NE Tibetan plateau, and the Sichuan basin. Within the Tibetan plateau, from NW to SE, are the western flank of the South-Qilian Shan, the NE margin of the Qaidam basin, the East-Kunlun Shan (mountain range containing the Kunlun fault), and the Songpan-Ganzi terrane. The crustal P- and S-wave velocity structure and Poisson's ratio outline the characteristics of the crustal structure and provide constraints on the crustal composition. The derived crustal cross section shows several significant features. (1) The crustal thickness varies considerably. North of the Kunlun fault variations in crustal thickness and topography correlate well. The crust thickens from 48 km below the Tarim basin to 70 km beneath the northeastern margin of the Qaidam basin, and then thins to about 56 km depth beneath the eastern flank of the Qaidam basin. The crust subsequently thickens again to 70 km beneath the East-Kunlun Shan. Across the Songpan-Ganzi terrane, the crust steadily thins from 70 km just south of the Kunlun fault to 48 km beneath the Sichuan basin, despite relatively constant topography across the Songpan-Ganzi terrane followed abruptly by a 10% drop in elevation in the Sichuan basin. (2) North of the Kunlun fault differences in crustal thickness are mainly caused by variations in lower crustal thickness, whereas south of the Kunlun fault they are caused by variations in upper-, middle- and lower-crustal thickness. (3) North of the Kunlun fault we detect a mid-crustal low-velocity zone, which is not apparent south of the fault. Across the plateau the Poisson's ratio is nearly constant with a value of 0.24-0.25 in the upper and middle crust, indicating a felsic bulk composition. In the lower crust the Kunlun fault seems to act as a boundary, with a Poisson's ratio of 0.29 north of the fault (Kunlun-Qaidam) and 0.26 south of the fault (Songpan-Ganzi). The Poisson's ratio and P-wave velocity values suggest that the lower crust throughout the Tibetan plateau (South-Qilian Shan, margins of the Qaidam Basin, East-Kunlun Shan, Songpan-Ganzi terrane) is of intermediate composition. Thus, along our profile, the NE Tibetan plateau is missing a mafic crustal layer at the base of the crust. In contrast, the Tarim basin, which borders the Tibetan plateau to the north, shows a typical platform-like crustal structure with a felsic upper and middle, and a mafic lower crust. The Sichuan basin, which borders the Tibetan plateau to the east, also has a felsic upper and middle crust, and an intermediate or mafic lower crust. T41A-1276 Qimen Tagh Uplift Constrained From Tectonostratigraphy of Southwest Qaidam Basin, Northern Tibet Plateau, China. * Hertz, M (mhertz@indiana.edu) , Indiana University Dept. of Geological Sciences, 1001 East 10th St., Bloomington, IN 47405-1405 United States Ritts, B (britts@indiana.edu) , Indiana University Dept. of Geological Sciences, 1001 East 10th St., Bloomington, IN 47405-1405 United States Bovet, P (pbovet@indiana.edu) , Indiana University Dept. of Geological Sciences, 1001 East 10th St., Bloomington, IN 47405-1405 United States Kent-Corson, M (malkc@stanford.edu) , Stanford University Geological and Environmental Sciences Dept., Braun Hall (Geo Corner) #118 450 Serra Mall, Building 320 Stanford, CA, Stanford, CA 94305 United States The initiation of Qimantagh uplift is an important tectonic event in the evolution of the northern Tibet plateau. The Qimantagh bounds the SW edge of Qaidam basin, and is the northern extent of the east Kunlun Mountains. Cenozoic sedimentary rocks located in SW Qaidam basin have recorded tectonic activity from this adjacent uplift. The tectonostratigraphy of Qaidam basin suggests that Qimantagh uplift began in early Miocene. A section of Oligocene - Miocene sedimentary rocks located on the SW edge of Qaidam basin near Lao Mangnai was studied. Its total thickness is 2166 m and is subdivided into two units. The Oligocene-Miocene? unit is 1246 m thick. It consists of laterally-continuous red and green intervals interbedded on a 5 - 10 m scale. The green intervals are composed of green to grey marl, siltstone and fine sandstone, and contain oscillation ripples, small-scale hummocky cross-stratification and ostracods. The red intervals are composed of red massive mudstone, siltstone and plane-bedded or ripple cross-stratified fine to medium mud-chip bearing sandstone. Paleocurrent indicators show a wide spread of generally north-directed flow, with strong north-directed flow near the top. This unit coarsens upward over the upper 247 m, with systematically more abundant and coarser red intervals toward the top. These red intervals contain coarse sand and m-scale packages of boulder-bearing pebble to cobble conglomerate. Clast composition consists of granite, marble, limestone, meta-volcanic, meta-sediment, and mylonite. The overlying Miocene unit is 920 m thick. It is composed of massive boulder-bearing pebble to cobble conglomerate. Clast composition is similar to that of the underlying transitional package. Paleocurrent in the Miocene conglomerate was north-directed. The fine-grained Oligocene-Miocene? represents a marginal lacustrine environment, with no evidence for nearby coarse sediment sources or steep depositional gradients, in spite of proximity to the present Qimantagh front. An increase in depositional energy is indicated by coarsening upward into the massive Miocene conglomerate. Clast composition and paleocurrent indicators suggest a Qimantagh provenance. Thus, the increase in depositional energy indicates that proximal thrusting had propagated in SW Qaidam basin - northern Qimantagh by Miocene, replacing earlier, finer-grained distal or tectonically quiescent deposits. T41A-1277 Structural framework of the central and northern Tibetan plateau as constrained by geologic investigation from Tangula Pass to the southern Qilian Shan thrust belt * McRivette, M W (mmcrivet@ucla.edu) , m.W. McRivette, Department of Earth and Space Sciences, UCLA, Los Angeles, CA 90095 United States Yin, A (yin@ess.ucla.edu) , m.W. McRivette, Department of Earth and Space Sciences, UCLA, Los Angeles, CA 90095 United States Chen, X (xhchen@cags.net.cn) , Institute of Geomechanics, Chinese Academy of Geological Sciences, Xueyuan Nan Lu #11, Haidian District, Beijing, 100081 China Burgess, W P (wburgess@ucla.edu) , m.W. McRivette, Department of Earth and Space Sciences, UCLA, Los Angeles, CA 90095 United States A preliminary cross section is constructed across central and northern Tibet based on field mapping, reinterpretation of existing geologic maps, analysis of satellite images, and interpretation of seismic-reflection profiles. Our cross section can be divided into 6 segments from south to north. (1) Between the Tanggula Pass and Wenquan over a distance of 80 km are dominated by south-directed thrusts entirely within Jurassic strata. The thrusts are cut by the younger and active east-dipping Wenquan normal fault system that bounds a 70-km long rift directly south of the Tanggula Pass. (2) The 90-km long Yanshiping section to the north is dominated by three major south-verging folds involving both Jurassic and Triassic strata. The folds are tight at their cores, suggesting that they may have detached along a decollement. (3) Between Tuotuo He in the south and Wudaoliang in the north over a distance ~130 km, contraction is expressed by south-directed thrusts that place Carboniferous-Permian strata over Triassic strata or Cretaceous strata over Tertiary strata. The leading-edge south-directed thrust near Tuotuo He truncates the south-verging Yanshiping fold system and places Carboniferous strata atop Triassic strata. Along this segment we also document for the first time the presence of a well-exposed south-dipping normal fault that places Cretaceous red beds over Triassic flysch sequences. (4) The 80-km long section between Wudaoling and the left-slip Kunlun fault in Xidatan is dominated by WNW-striking, south-directed thrusts and folds. They are aligned in an en echelon pattern and merge with the east-striking Kunlun fault at their western ends. (5) The Eastern Kunlun segment consists of the left-slip Kunlun fault system and the south-directed East Kunlun-Qimen Tagh thrust belt. The active Kunlun fault system consists of two main strands: (a) the well-known Xidatan strata as the main Kunlun fault, and (b) the Yieniugou branch to the north. The Yieniugou strand, documented for the first time by this study, has a prominent morphological expression (i.e., prominent fault scarps, left-lateral offset of gullies, en echelon tension fractures against the fault trace). The Yieniugou fault is linked to the east with the Kunlun fault by a NW-striking thrust, whereas to the west it links with the NW-striking and SW-directed Qimen Tagh thrust belt. (6) The structures over the Qaidam basin is dominated by a major south-directed contraction zone consisting of imbricate and duplex systems in the north and a gentle northward tilting of the basin floor in the hanging wall of the south-directed Eastern Kunlun thrust belt. Our geologic investigations lead to the following conclusions: (A) The Eastern Kunlun-Qimen Tagh thrust belt is part of the broad left-slip Kunlun shear zone that aligns in an en echelon fashion against Kunlun fault system. This thrust belt is dominantly south-directed, with major thrusts moving towards the Kunlun fault. (B) The Qaidam basin has been developed as a piggyback basin in the hanging wall of the Kunlun-Qimen Tagh thrust belt and a foreland basin inn the footwall of the southern Qilian Shan thrust belt. (C) The lack of extensional structures in the Paleocene-Eocene Fenghuo Shan Group adjacent to our observed normal fault suggests that Cretaceous extension may have occurred in the northern Qiangtang terrane. T41A-1278 Crustal Structure of the Northeastern Margin of the Tibetan Plateau From the Songpan-Ganzi Terrane to the Ordos Block * Liu, M (shane@usgs.gov) , Geophysical Exploration Center, China Earthquake Administration, 104 Wenhua Road, Zhengzhou, 450002 China Mooney, W (shane@usgs.gov) , U.S. Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025 United States Li, S (shane@usgs.gov) , Geophysical Exploration Center, China Earthquake Administration, 104 Wenhua Road, Zhengzhou, 450002 China Okaya, N (shane@usgs.gov) , Geophysical Exploration Center, China Earthquake Administration, 104 Wenhua Road, Zhengzhou, 450002 China Detweiler, S (shane@usgs.gov) , U.S. Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025 United States The 1000-km-long Maqin-Lanzhou-Jingbian seismic refraction profile is located in the NE margin of the Tibetan Plateau, and crosses the northern Songpan-Ganzi terrane, the Qinling-Qilian fold zone, the Haiyuan arcuate tectonic region, and the stable Ordos block. The profile and calculated Poisson's ratio reveal many significant characteristics in the P-wave and S-wave velocity structure. The crustal thickness increases from northeast to southwest, with the average observed crustal thickness increasing from 42 km in the Ordos block to 63 km in the Songpan-Ganzi terrane. The crust becomes obviously thicker south of the Haiyuan fault and south of Zikog. The crustal velocities vary significantly along the profile. The average crustal P-wave velocities are 6.30 km/s in the Ordos block, 6.22 km/s in the Haiyuan arcuate tectonic region, 6.25 km/s in the Qilian block, 6.20 km/s in the Qinling block, and 6.10 km/s in the Songpan-Ganzi terrane. The average crustal velocity is 6.22 km/s along the profile, which is 0.23 km/s lower than the global average (6.45 km/s). Combined P-wave velocity and Poisson's ratio suggest that the crust is dominantly felsic in composition with an intermediate base. A mafic lower crust is absent in the NE margin of the Tibetan Plateau from the Songpan-Ganzi terrane to the Ordos block. Felsic lithologies are inferred to be greenschist-to-amphibolite facies. There are low velocity zones in the Qinling block and the Haiyuan arcuate tectonic region. The low velocity zones have low S-wave velocities and high Poisson's ratios, so it is possible that partial melting has caused the formation of the low velocity zones. The crust is divided into two layers, the upper and the lower crust, with crustal thickening mainly in the lower crust in the NE Tibetan Plateau. The profile shows that the thickness of the lower crust increasing from 22 km to 38 km as the crustal thickness increases from 42 km in the Ordos block to 63 km in the Songpan-Ganzi terrane south of the Kunlun fault. Both the Conrad discontinuity and Moho in the Qinling block and in the Haiyuan arcuate tectonic region are laminated interfaces, implying intense tectonic activity. The arcuate faults and large earthquakes in the Haiyuan arcuate tectonic region are the result of interaction between the Tibetan Plateau and the Ordos and Alxa block. T41A-1279 Extreme Exhumation Within the Sichuan Basin - Late Cenozoic Erosion, Sedimentation and Tectonics at the Eastern Margin of the Tibetan Plateau Based on Apatite Fission Track Thermochronology. * RICHARDSON, N (nick@erdw.ethz.ch) , ETH Zurich (Swiss Federal Institute of Technology), Geological Institute, Department of Earth Sciences, Sonneggstrasse 5, ETH Zentrum, Zurich, CH-8092 Switzerland Densmore, A L (alexander.densmore@erdw.ethz.ch) , ETH Zurich (Swiss Federal Institute of Technology), Geological Institute, Department of Earth Sciences, Sonneggstrasse 5, ETH Zentrum, Zurich, CH-8092 Switzerland Seward, D (diane.seward@erdw.ethz.ch) , ETH Zurich (Swiss Federal Institute of Technology), Geological Institute, Department of Earth Sciences, Sonneggstrasse 5, ETH Zentrum, Zurich, CH-8092 Switzerland Yong, L (liy@cdut.edu.cn) , Chengdu University of Technology, National Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Sichuan, Chengdu, 610059 China The Tibetan Plateau represents the most impressive geomorphic expression of the effects of continental collision on the Earth's surface. Despite being the focus of much research over the last decades, there are still numerous outstanding questions regarding the mechanisms and timing of plateau formation, and regional lithospheric structure. The eastern margin of the plateau is a particularly key location for the assessment of these issues. This margin, commonly referred to as the Longmenshan, presents a formidable topographic front, yet appears not to have experienced significant shortening since the Triassic, and is not accompanied by major Cenozoic thrusting and associated foreland basin deposits within the adjacent Sichuan Basin. The means by which such extreme relief is produced and maintained are therefore somewhat enigmatic. Previous studies have suggested that focused late Cenozoic exhumation took place along the eastern margin of the Tibetan Plateau, but in this study, we demonstrate that significant exhumation also took place within the adjacent Sichuan Basin. Using new apatite fission track thermochronological data and compilations of vitrinite reflectance data, we place constraints on the magnitude and timing of Cenozoic exhumation, and of post-exhumation deformation, adjacent to the eastern Tibetan Plateau. We calculate that between 2 and 4 km of sedimentary basin fill was removed during the late Tertiary across the entire Sichuan Basin. This event is marked by a widespread regional unconformity between Cretaceous to Eocene fine-grained clastic rocks and late Neogene conglomerate within the western Sichuan Basin. Determination of the location of the base of the apatite partial annealing zone allows us to establish exhumation patterns across the Tibetan Plateau margin and into the Sichuan Basin. Field observations, heavy minerals and clast compositional analyses of late Neogene sediments within the basin also permit us to constrain the timing of the latest deformational phase along the margin, and allow us to reconstruct major drainage pattern reorganization that has taken place between the late Neogene and Recent. Clearly, the rapid removal of such large quantities of material from the Sichuan Basin has important implications for the surrounding lithosphere, and for our understanding of the tectonic and sedimentary history of this margin of the Tibetan Plateau. We close by speculating on the erosional mechanisms involved in such large-scale basin evacuation. T41A-1280 Clockwise Rotation of Upper-Mantle Strain and Crust-Mantle Coupling Beneath the Eastern Syntaxis Tibet * sol, S (sol@lehigh.edu) , Lehigh University, 31 Williams drive, Bethlehem, PA 18055 United States Meltzer, A (ameltzer@lehigh.edu) , Lehigh University, 31 Williams drive, Bethlehem, PA 18055 United States Zurek, B (zurek@lehigh.edu) , Lehigh University, 31 Williams drive, Bethlehem, PA 18055 United States Zeitler, P (peter.zeitler@lehigh.edu) , Lehigh University, 31 Williams drive, Bethlehem, PA 18055 United States Zhang, X (cdxuanyang@cgs.gov.cn) , Institute of Geology and Mineral resources, Chengdu Institute of Geology, Chengdu, 610065 China Zhang, J , Institute of Geology and Mineral resources, Chengdu Institute of Geology, Chengdu, 610065 China We performed shear-wave splitting of core phases using a subset of 33 selected events within an array composed of 48 broadband stations that spans a 650x350 km area crossing the Lhasa and the southern part of the Qiangtang terranes. Our results reveal the presence of azimuthal anisotropy with small-to-moderate delay times (0.5-1.4s). The consistency of the splitting parameters with respect to back-azimuth, as well as the absence of splitting along propagation paths parallel to the symmetry axis, suggest that a single layer of horizontal anisotropy in the lithospheric mantle adequately explains the data at most of our stations. At regional scale, the most striking feature is the remarkable southeastward clockwise rotation of fast polarization axis around the eastern Himalayan syntaxis, a characteristic compatible with geomorphology, structural geology, paleo-magnetism and GPS. At a more local scale, our study region can be divided into various zones based on similarity in the splitting pattern. The most remarkable zone is the one running parallel to the strike-slip fault associated with the Bangong suture, where we observe a suture-parallel fast direction rotating from E-W to SSE at the eastern edge of the syntaxis. This indicates a possible extent of faulting into the lithospheric mantle highlighting the role of strike-slip faults to accommodate the rotation of material around the syntaxis. Another significant feature is observed in the western part of the Lhasa terrane, where a clear change in fast direction from collision parallel to collision perpendicular occurs within a narrow N-S transition zone. The small-scale variations in the fast polarization orientation and the tendency of the fast polarization direction to align close to the direction of the surficial structures argue against anisotropy induced by absolute plate motion and support a general consensus in which anisotropy in the plateau has been developed by finite strain in the lithospheric mantle with a potential contribution from the crust. This argues for the presence of an effective crust-mantle coupling beneath the eastern syntaxis, in contrast with the presence of a low strength (weak) decoupling lower crust relative to the upper mantle that has been suggested by data from the central plateau and some geodynamic modelling of the whole orogen. Our results indicate that although the lithosphere in the syntaxis appears to deform internally, fault block rotation via strike-slip tectonics plays an important role in the southeastward extrusion of the plateau. T41A-1281 New geochronological constraints on the movement of Jiali and Gaoligong shear zones in SE Tibet, and its tectonic implication * Lin, T (f90224101@ntu.edu.tw) , Departments of Geosciences, National Taiwan University, No. 245, Choushan Rd., Taipei, 106 Taiwan Lo, C (loch@ntu.edu.tw) , Departments of Geosciences, National Taiwan University, No. 245, Choushan Rd., Taipei, 106 Taiwan Chung, S (sunlin@ntu.edu.tw) , Departments of Geosciences, National Taiwan University, No. 245, Choushan Rd., Taipei, 106 Taiwan Lee, T (t44001@cc.ntnu.edu.tw) , Department of Earth Sciences, National Taiwan Normal University, No. 88, Sec. 4, Tingjou Rd., Taipei, 116 Taiwan Lee, H (f88224102@ntu.edu.tw) , Departments of Geosciences, National Taiwan University, No. 245, Choushan Rd., Taipei, 106 Taiwan Hsu, F (r87224208@ntu.edu.tw) , Departments of Geosciences, National Taiwan University, No. 245, Choushan Rd., Taipei, 106 Taiwan As a consequence of the northward moving and indentation of India into Asia, crustal fragments around the eastern Himalaya syntaxis were rotated clockwise, and extruded southeastward along the NW-trending Karakoram-Jiali, N-trending Gaoligong and Sagaing faults. Much of the information about the crustal deformation in the India-Asia collision has therefore been registered and recorded in the fault zones. As the easternmost segment of the right-lateral Karakoram-Jiali fault system, the Jiali fault, and its south extension, the Gaoligong fault, provides a great opportunity in revealing the crustal deformation history in Southeast Asia during the collision of India-Asia collision. New laser 40Ar/39Ar dating results for migmatites and sheared granitoids in the Bomi and Shama areas along the Jiali fault zone show ages in the range of 11-22 Ma . Whereas hornblende, muscovite, and biotite separates from the Gaoligong fault zone display shorter but concordant ages ranging form 13-18 Ma. 40Ar/39Ar ages from the Jiali fault appear to be younger than those reported for the Sagaing fault in Burma (16- 27 Ma, Bertrand et al., 2001). Comparison of these 40Ar/39Ar ages suggests that the deformation may start from the south along the Sagaing fault in Indochina, and jump to the areas in the front of the Himalaya, resulting in the shearing of the the Jiali fault, and then propagate toward the south along the Gaoligong fault. Such a deformation history may reflect two stages of extrusion tectonic events during the northward indentation of the India plate into the Asia plate in the Tertiary. T41A-1282 Crustal Structure Beneath the Southeastern Tibetan Plateau and Yunnan Province Using Teleseismic Data * Bhaskar, A (Bhaskar@Brown.edu) , Brown University, Box 2760, Providence, RI 02912-2760 Savage, B (savage13@dtm.ciw.edu) , Department of Terrestrial Magnetism, Carnegie Institution of Washington, 5241 Broad Branch Road, NW, Washington, DC, 20015-1305 Silver, P (silver@dtm.ciw.edu) , Department of Terrestrial Magnetism, Carnegie Institution of Washington, 5241 Broad Branch Road, NW, Washington, DC, 20015-1305 To explain the development of the India-Eurasian continent-continent collision, it is necessary to constrain the characteristics of the lithosphere of the Tibetan Plateau and surrounding regions. The crustal structure beneath the SE Tibetan Plateau and adjacent off-Plateau Yunnan Province was illuminated using the receiver function method. The Moho depth in Tibet was found to be ~70 km, decreasing off-Plateau to ~60 km in northern Yunnan, and further decreasing to ~40 km in southern Yunnan. The gradual change in crustal thickness matches the gradual topography change over this region. The lower crustal flow model (Clark and Royden, 2000) predicts anisotropy and a low velocity (weak) zone in both Tibet and Yunnan Province. The Flesch et al. (2005) model predicts anisotropy and shear between a decoupled crust and mantle only in Yunnan. Crustal anisotropy manifests as backazimuth variability on the radial and transverse components (Sherrington et al., 2004). A positive velocity discontinuity at ~40 km depth in northern Yunnan is observed on the radial component with backazimuth variability (strongest at backazimuths of 100 degrees to 150 degrees). However, no backazimuth variability was observed on the transverse component in either Tibet or Yunnan. We see a positive velocity discontinuity in Yunnan at ~20km depth and in SE Tibet at ~43km, but we observe no negative velocity discontinuities or low velocity zones. We see no evidence of anisotropy or crustal flow in Tibet, and possible evidence of anisotropy in Yunnan. These results agree with the Flesch et al., 2005 model in Tibet, disagree with the lower crustal flow model in Tibet, and are inconclusive in Yunnan Province. T41A-1283 Measurements of crustal thickness and Poisson's ratio in southeastern Tibet from receiver functions * Zurek, B D (zurek@lehigh.edu) , Lehigh University Dept Earth & Environmental Sciences, 31 Williams Dr, Bethlehem, PA 18018 Meltzer, A (ameltzer@lehigh.edu) , Lehigh University Dept Earth & Environmental Sciences, 31 Williams Dr, Bethlehem, PA 18018 Sol, S (Sol@lehigh.edu) , Lehigh University Dept Earth & Environmental Sciences, 31 Williams Dr, Bethlehem, PA 18018 Zhang, X (cdzxuanyang@cgs.gov.cn) , Institue of Geology and Mineral Resources, #82 the 3rd Northern Segment of the First Ring Road, Chengdu, 610082 China Zhang, J (cdzxuanyang@cgs.gov.cn) , Institue of Geology and Mineral Resources, #82 the 3rd Northern Segment of the First Ring Road, Chengdu, 610082 China In this study we present new measurements of crustal thickness and Poisson's ratios for southeastern Tibet extending east from Lhasa across the eastern syntaxis. The syntaxis is characterized by the end of Himalayan orogen and the start of 'escape' tectonics, as material is extruded from the central plateau and rotates around the corner of the Indian plate indentor. Depth to Moho and determination of Poisson's ratio were made by measuring the travel times of direct and multiple P to S converted phases from the Moho and solving jointly for depth and Poisson's ratio assuming a crustal average Vp of 6.4 km/s. The depth to Moho ranged in value from 55 to 75 km below sea level. These depth measurements can be divided into two regions, a moderately uniform, deep Moho, extending from the central plateau toward the eastern margin and a higher relief, shallower Moho within the core of the syntaxis. Poisson's ratio ranged from 0.21 to 0.29. These measurements can be divided into three regions, fairly homogenous low to intermediate values within the central plateau, predominantly low to intermediate values but localized high values (0.29) within the eastern margin, and a wide range of values (0.21-0.29) over short scale lengths within the central syntaxis. No correlation is seen between Poisson's ratio and depth to Moho. The predominantly low to intermediate Poisson's ratios are indicative of a crust that is relatively felsic in composition compared to "average" continental crust. The lack of consistent high Poisson's ratio means there is no evidence for widespread partial melt within the crust, but does not rule out the presence of at least localized pockets of melt. To see if the depth of the Moho correlates with the surface topography correlation analysis was preformed with the surface topography smoothed over a range of 20-400 km. The best correlation occurs (r=0.75) when the surface topography is smoothed to 180km. This correlation can not however be explained by simple Airy Isostasy alone, it requires at least in part another mechanism controlling the thickness of the crust in this region. Such mechanisms could be variations in density or rheology within the lithosphere or for the region to be out of isostatic equilibrium. T41A-1284 Onset of Middle Crustal Flow, Southern Tibet: Evidence From U/Pb Zircon Ages * Lee, J (jeff@geology.cwu.edu) , Dept. of Geological Sciences, Central Washington Univ., Ellensburg, WA 98926 United States Whitehouse, M (Martin.Whitehouse@nrm.se) , Laboratory for Isotope Geology, Swedish Museum of Natural History, Stockholm, SE-104 05 Sweden New U/Pb zircon ion-microprobe ages from core rocks of Mabja Dome constrain the timing and duration of migmatite formation, peak metamorphism, and onset of middle crustal flow in southern Tibet. Mabja Dome is one of several North Himalayan gneiss domes which are characterized by doubly plunging antiforms cored by middle crustal rocks including high grade metapelites, orthogneisses, and migmatites. These rocks record two primary deformational events: D1 characterized by EW-trending folds of S0 with an associated N-dipping axial planar foliation S1, and D2 characterized by a domed mylonitic foliation S2, and associated NS-trending stretching lineation. Peak metamorphism, ranging from 500°C/300 MPa in chtd-zone rocks to 705°C/870 MPa in sill-zone rocks, was syntectonic with D2 and defines a set of isograds which are subparallel to structure. D2 horizontal extension and vertical thinning was simultaneous with development of migmatites and emplacement of a pegmatite and aplite dike swarm; two-mica granites were emplaced after D2 ductile deformation had ceased. Migmatites from the deepest structural levels at Mabja yielded zircons with oscillatory zoned cores and recrystallized or newly grown high U rims and concordant ion-microprobe ages of 532 ± 16 Ma and 35.0 ± 0.8 Ma, respectively. Zircons from a syn-tectonic pegmatite dike yielded an U/Pb age of 23.1 ± 0.8 Ma (Lee et al., in press). The deepest post-tectonic granite yielded zircons with oscillatory zoned cores and overgrowths of high U oscillatory zoned rims; these zircons yielded ion-microprobe ages of ~450 Ma and 16.1 ± 0.4 Ma, respectively. These data suggest that D2 vertical thinning and horizontal stretching, simultaneous with peak metamorphism and migmatization, in the middle crust of southern Tibet began at 35 Ma, was ongoing at 23 Ma and had ceased by 16 Ma. The structural, metamorphic, intrusive, and geochronologic histories in the North Himalayan gneiss domes are similar to those recorded in the Greater Himalayan sequence, suggesting that during Eocene to Miocene time high grade middle crustal rocks were once continuous from beneath the high Himalaya northward beneath southern Tibet. Exposures of the Greater Himalayan sequence have been interpreted as the leading edge of an eroding and southward extruding tabular or wedge-shaped body of ductile middle-crustal rocks bounded above by the STDS and below by the MCT. We suggest that the once hot and weak middle crustal rocks now exposed in the core of the North Himalayan gneiss domes represent the interior of such a middle-crustal channel and that onset of flow within this channel began in the late Eocene. To maintain strain compatibility, slip along the STDS and MCT must have begun in the late Eocene to accommodate the onset of middle crustal flow. T41A-1285 Kinematics of a continental triple junction in the western Tibetan plateau explains variation in structural style along the Karakoram fault * Raterman, N S (raterman@geology.ucdavis.edu) , Department of Geology, University of California Davis, One Shileds Ave., Davis, CA 95616 United States Cowgill, E (cowgill@geology.ucdavis.edu) , Department of Geology, University of California Davis, One Shileds Ave., Davis, CA 95616 United States Lin, D (dinglin@mail.igcas.ac.cn) , Institute of Geology and Geophysics, Chinese Academy of Sciences, Deshengmenwai Street, Beijing, 100029 China Although microplate theories of continental deformation are commonly employed to develop kinematic models of entire continental collision zones, they are rarely used to investigate individual fault intersections within them. The India-Asia collision zone contains several large strike-slip faults that resemble plate boundaries providing an excellent opportunity to apply the principles of plate triple junctions to the intersection of major intracontinental fault systems. Using Landsat TM remotely sensed imagery, SRTM90 topographic data, and a new three dimensional visualization software package developed to interpret these datasets in unison, we created a neotectonic map of the intersection of the Altyn Tagh, Ghoza-Longmu Co, and Karakoram faults in the western portion of the India-Asia collision zone. Our mapping suggests that structural style along the Karakoram fault varies from essentially pure strike-slip in the north (37° to 34°N) to transtension in the south (34° to 32°N). This transition is coincident with a slight bend in the Karakoram fault that coincides with its intersection with the Ghoza-Longmu Co extension of the Altyn Tagh fault. Rigid block kinematic analyses using fault geometries derived from the neotectonic observations, reconnaissance field observations, and published geodetic and Quaternary slip-rates indicate that the variation in structural style along the Karakoram fault can be explained by differential motion between India, the Tianshuihai terrane, and the Tibetan plateau. Results from geodetically determined slip-rates imply a 6-8 Ma onset of left slip along the Ghoza - Longmu Co fault, bending and transtension along the central and southern Karakoram respectively, and extrusion of the Tibetan plateau at a rate of 3.7 mm/yr towards 107°. The correspondence between the neotectonic interpretations, kinematic analyses, and previous studies of the bedrock geology along the Karakoram fault demonstrate the applicability of microplate theories of continental deformation in assessing short-term (0-10 Myr) deformation patterns along intracontinental strike-slip fault systems. T41A-1286 Very Small 660-km Discontinuity beneath Tibet: Evidence for Detached Lithosphere? * Tseng, T (tseng1@uiuc.edu) , Department of Geology, University of Illinois, 1301 W Green St., Urbana, IL 61801 Chen, W (wpchen@uiuc.edu) , Department of Geology, University of Illinois, 1301 W Green St., Urbana, IL 61801 The fate of mantle lithosphere during continent-continent collision is a key question in continental dynamics. Unfortunately there were few tight constraints on mantle processes beneath the Himalayan-Tibetan orogen - the most prominent, active collision zone on the Earth. We address this issue with high-resolution seismic results concerning the nature of the 660-km discontinuity beneath central Tibet. Furthermore, we obtain similar constraints beneath the Indian shield which provide a crucial baseline for comparison with the mantle transition zone (TZ) under Tibet. Using data recorded by several permanent and temporary broadband arrays, we construct a sequence of large-aperture (over 1,000 km) seismic profiles that sample the TZ beneath Tibet and India. By modeling both the timing and amplitude of triplicate waveforms, our results are particularly sensitive to seismic anomalies near the TZ. It turns out that the contrast in P-wave speeds across the 660-km discontinuity (Î” VP660) is remarkably small beneath central Tibet: only 3.1± 0.5% at a depth of 670 ± 10 km. The contrast is only about half of the global average (cf. 5.6% in the iasp91 model) which also seem to characterize regions to the north of Tibet. More important, the corresponding value beneath stable India is also about 1/3 larger (about 4.2% in Î” VP660) than that under central Tibet. In other words, even though the Tibetan plateau is the result of collision between India and Eurasia, Î” VP660 beneath Tibet resembles neither end-member. It follows that a small Î” VP660, most likely due to a thermal anomaly of high VP in the bottom of the TZ, must have been allochthonous in origin, due to impounding of either subducted oceanic lithosphere or convective removal of thickened continental lithosphere. Since major episodes of subduction ceased about 50 Ma ago, recent sinking of once thickened Asian lithosphere, now resting on the 660-km discontinuity, seems to the most plausible cause of a small Î” VP660 under central Tibet. T41A-1287 Seismicity, Structure, and Rheology of the Lithosphere in the Lake Baikal Region * Emmerson, B (emmerson@esc.cam.ac.uk) , Department of Earth Sciences University of Cambridge, Bullard Labs Madingley Road, Cambridge, CB3 OEZ United Kingdom Jackson, J (jackson@esc.cam.ac.uk) , Department of Earth Sciences University of Cambridge, Bullard Labs Madingley Road, Cambridge, CB3 OEZ United Kingdom McKenzie, D (mckenzie@esc.cam.ac.uk) , Department of Earth Sciences University of Cambridge, Bullard Labs Madingley Road, Cambridge, CB3 OEZ United Kingdom Priestley, K (keith@esc.cam.ac.uk) , Department of Earth Sciences University of Cambridge, Bullard Labs Madingley Road, Cambridge, CB3 OEZ United Kingdom Our investigation of rheological properties of the lithosphere in the lake Baikal region combines observations of seismicity, gravity, topography, and thermal and velocity structures. We use teleseismic waveform modelling and published relocations of microearthquakes to examine the seismogenic thickness (T_s$), and conclude that there is no convincing evidence for a seismogenic mantle in the Baikal region. Using the admittance between free air gravity and topography, we estimate the effective elastic thickness ($T_e$) in the region to be between 5--20~km. Nowhere do the data require that$T_{e}>T_{s}$, allowing the simple interpretation that the long term strength of the lithosphere resides in its seismogenic layer, and that the mantle is relatively weak. A weak mantle in the Baikal region can be explained by its high temperature, which we estimate by combining local geotherm estimates with the regional upper mantle velocity structure, obtained from fundamental and higher-mode surface waves. Geotherms are fitted to pressure and temperature estimates from mantle nodules at four sites, both within and outside the Siberian shield. In order to constrain the temperatures at the Moho, we estimated crustal thicknesses using teleseismic receiver functions. Moho temperatures exceed ~550$\rm ^o\$C beneath the Siberian shield and are higher in the more recently deformed mountain belts to the south. Based on a recent reassessment of oceanic geotherms and seismicity, it seems likely, therefore, that the mantle in the Baikal region is too hot to be a source of long term strength. This is consistent with the suggestion that the distribution of mantle seismicity in both the oceans \textit{and} the continents is dependent on temperature alone. Finally, we note that results from S-wave tomography studies, combined with the observed locations of rift-related earthquakes, lead us to suspect that the frequently published position of the edge to the Siberian shield at the surface provides a poor description of that same boundary at depth.

T41A-1288

Korean Peninsula Average Velocity and Q Models From Six Earthquake Events in the Yellow Sea and South Korea

* Nguyen, B V (bao@aftac.gov) , HQ AFTAC/TTR, 1030 S. Highway A1A, Patrick AFB, FL 32925-3002 United States

The source parameters of these six events were presented by the author at previous AGU Meetings. Table 1: Source Parameters [Event Number][Date (dd/mm/yy)][USGS mb][Depth (km)][Slip (deg)][Dip (deg)][Strike (deg)][Seismic Moment (E+23 dyne-cm)] 1 03/11/92 4.8 07.0 015 65 110 0.758 2 22/01/92 5.1 02.0 070 40 345 4.720 3 23/07/02 4.7 09.0 140 65 010 1.900 4 22/03/03 4.8 13.0 165 75 025 1.750 5 30/03/03 4.7 06.0 050 70 035 0.550 6 29/05/04 5.3 11.0 065 55 160 3.790 The paths from these earthquake sources in the Yellow Sea and South Korea to the CDSN station MDJ cross the Korean Peninsula region. In this study, average Love-wave and Rayleigh-wave group-velocity dispersion data of these paths were used to invert for velocity model, and average anelastic attenuation coefficient data obtained using the technique of Tsai and Aki (1969) were used to invert for Q-beta-inverse (Qb-1). Table 2: Velocity and Qb-1 Models [Layer Number][Layer Thickness (km)][P (km/s)][S (km/s)][Density (g/cm3)][Model Std Dev (E-02)][Qb-1 (E-02)][Qb-1 Std Dev (E-03)][Qa/Qb] 1 2.00 4.37 2.52 2.37 7.08 0.8520 1.739 264/117 2 6.00 6.22 3.59 2.77 1.92 0.8199 0.337 274/122 3 10.0 6.11 3.53 2.73 0.75 0.6075 0.358 370/165 4 10.0 7.59 4.38 3.17 2.02 0.9566 0.802 235/105 5 10.0 6.63 3.83 2.88 2.71 1.2720 0.818 177/079 6 12.0 8.35 4.82 3.44 3.20 1.0270 1.926 219/097 7 0.00 8.28 4.65 3.41 2.57 0.4994 0.581 451/200

T41A-1289

Preliminary Results of the Second Seismic Refraction Experiment in Korea

Baag, C (baagce@snu.ac.kr) , School of Earth and Environmental Sciences, Seoul National University, Seoul, 151-742 Korea, Republic of
* Sheen, D (sheendh@snu.ac.kr) , School of Earth and Environmental Sciences, Seoul National University, Seoul, 151-742 Korea, Republic of
Lee, J (jung@knu.ac.kr) , Department of Geology, Kyungpook National University, Daegu, 702-701 Korea, Republic of
Moon, W M (wmoon@eos1.snu.ac.kr) , School of Earth and Environmental Sciences, Seoul National University, Seoul, 151-742 Korea, Republic of
Jung, H (hijung@kunsan.ac.kr) , Department of Ocean System Engineering, Kunsan National University, Kunsan, 573-701 Korea, Republic of
Kim, K (kykim@kangwon.ac.kr) , Department of Geophysics, Kangwon National University, Chuncheon, 200-701 Korea, Republic of

The second crustal refraction experiment in Korea was carried out along a 335 km long survey profile in October 2004. The principal purpose of the study is to understand the crustal velocity structure and the tectonic setting of the Korea peninsula. The survey line runs along NNW-SSE direction in the central part of the southern Korea. A total of 196 portable seismometers including 26 three-component ones were deployed at approximately 1.7 km interval. Along the profile line, four shots were exploded in 95-150 m deep drill wells with charge sizes between 500 and 1000 kg of explosives. Seismic signals were recorded at a rate of 120 samples per second. The traveltimes of P wave phases, Pg, PiP, PmP, and Pn were identified and shear wave phases, Sg were also observable on a reduced traveltime plot. An iterative combination of ray tracing and least-squares inversion scheme (Zelt and Smith, 1992) resulted in a layered crustal P-velocity model in which the thickness varies from 30 km to 33 km. The crust appears thickest around the Okcheon belt in the central part, which coincides with the result from the first experiment (Cho et al., 2004). This also agrees with previous researches on crustal structure of the Korean Peninsula (Park et al., 2003; Chang et al., 2004; Kim et al., 2005). Several moderate mid-crustal reflections were recognized, one of which was considered to be representative of the Conrad.

T41A-1290

Evidence for Triassic sinistral shear along the Altyn Tagh fault, northern Tibet (China)

* Li, H (lihaibing@ccsd.org.cn) , Haibing Li Jingsui Yang Cailai Wu Zhiqin Xu, Key Laboratory for Contiental Dynamics of MLR, Institute of Geology, Chinese Academy of Geological Sciences, 26 Baiwanzhuang Road,, Beijing, 100037 China
Yang, J (yangjsui@ccsd.cn) , Haibing Li Jingsui Yang Cailai Wu Zhiqin Xu, Key Laboratory for Contiental Dynamics of MLR, Institute of Geology, Chinese Academy of Geological Sciences, 26 Baiwanzhuang Road,, Beijing, 100037 China
Wu, C (wucailai@ccsd.cn) , Haibing Li Jingsui Yang Cailai Wu Zhiqin Xu, Key Laboratory for Contiental Dynamics of MLR, Institute of Geology, Chinese Academy of Geological Sciences, 26 Baiwanzhuang Road,, Beijing, 100037 China
Xu, Z (xuzhiqin@ccsd.cn) , Haibing Li Jingsui Yang Cailai Wu Zhiqin Xu, Key Laboratory for Contiental Dynamics of MLR, Institute of Geology, Chinese Academy of Geological Sciences, 26 Baiwanzhuang Road,, Beijing, 100037 China
Tapponnier, P (tappon@ipgp.jussieu.fr) , Paul Tapponnier, Laboratoire de Tectonique, Institut de Physique du Globe de Paris, 4 Place Jussieu, 75252 Paris Cedex 05, France., Paris, 75252 France
Arnaud, N (Nicolas.Arnaud@dstu.univ-montp2.fr) , Nicolas Arnaud, Universite Montpellier II and UMR 5573, Centre National de la Recherche Scientifique (CNRS), Montpellier France., Montpellier, 34095 France

The strike-slip faults of north Tibet accommodate part of the Cenozoic convergence between India and Asia. Along the Xorkol basin west-North of Qaidam, the active traces of the Altyn Tagh fault follow narrow belts of granitic and amphibolitic mylonites. The deformation recorded in those mylonites is sinistral strike-slip. Three types of zircon may be sorted out from the mylonites: anatectic (magmatic), long columnar zircons, magmatic columnar zircons, and residual, metamorphic, sub-rounded zircon. Three groups of U-Pb ages measured by ion microprobe (SHRIMP) on single zircon were obtained: 530-550Ma for the columnar, magmatic zircon, 460-510Ma for the sub-rounded, residual metamorphic zircon, and 235-245Ma for the long-columnar anatectic (magmatic) zircon. The latter type of zircon is well oriented with the crystal long axis parallel to the stretching lineation. Mineral inclusions in the oriented zircons are also parallel to the stretching lineation, which coincides with the direction of maximum tectonic stress in the process of the strike-slip. Raman spectrum study indicates that the inclusion minerals show the melting phase feature, and cathodoluminescence images show that this type of zircon has a relatively homogeneous internal structure. Therefore, the long columnar zircons resulted from rapid oriented growth in a partial melting regime in the ductile shear process. It not only denotes the direction of shear strain in the strike-slip shear, but the growth age (crystallization age) of this type of zircon denotes the age of strike-slip shear. 40Ar/39Ar ages of directionaly grown hornblendes and biotite in the same samples are 220-230Ma and 190-200Ma, respectively. This suggest syntectonic anatexis and cooling occurred during strike-slip shear along the Altyn Tagh fault in Triassic time (to Early Jurassic). The Triassic shear may be related to oblique collision between the Bayan Har and the Kunlun- Qaidam blocks. 120Ma (Arnaud, et al., 2003) and 90Ma (Liu, et al., 2001) of 40Ar/39Ar ages of deformation from syntectonic fabrics formed in the Karakax segment and central-eastern segment of the Altyn Tagh fault, respectively. The volcanic eruption occurred in early Cretaceous in the eastern end of the Altyn Tagh fault, simultaneity. The Altyn Tagh (Xorkol) shear zone may have formed a unique, continuous boundary in the Triassic, which was later reactivated and sinistral sheared during Cretaceous, and finally reused by the Tertiary strike-slip fault, leading to potentially calculable offsets along the Altyn Tagh fault.