Program     Evolution of Deep Sea Processes in the South China Sea       Poster Crustal and mantle structure of the Indochina Peninsula and its implications for the tectonic evolution of the South China Sea P-SPS6-13 Youqiang Yu* , State Key Laboratory of Marine Geology, Tongji University Ting Yang, School of Oceanography, South University of Science and Technology of China Mei Xue, State Key Laboratory of Marine Geology, Tongji University Stephen Gao, Geology and Geophysics Program, Missouri University of Science and Technology Kelly Liu, Tran Danh Hung, State Key Laboratory of Marine Geology, Tongji University Khanh Phon Le, Faculty of Oil and Gas, Hanoi University of Mining and Geology Presenter Email: yuyouqiang@tongji.edu.cn While the northward indentation of the Indian into Eurasian plates has been intensively investigated, its oblique subduction beneath the Indochina Peninsula (ICP) and the role it played on crust and mantle structure and dynamics remain enigmatic. Here, we have conducted receiver function investigation of the crust and mantle transition zone (MTZ) discontinuities and employed the shear wave splitting (SWS) technique to systematically illuminate upper mantle anisotropy beneath the Indochina Peninsula with an unprecedented data coverage. Crust beneath the Khorat Plateau, which occupies the core of the Indochina Block, has relatively large thickness with a mean of 36.9 km and small Vp/Vs measurements with an average of 1.74, which indicates an overall felsic bulk composition. The laterally heterogeneous distribution of crustal properties and its correspondence with indentation-related tectonic features suggest that the Indochina lithosphere is extruded as rigid blocks rather than as a viscous flow. Systematic spatial variations of MTZ thickness with departures between -21 and +24 km from the globally averaged value are revealed, providing independent evidence for the presence of slab segments in the MTZ beneath the central and a slab window beneath the western ICP. The results also support the existence of broad mantle upwelling adjacent to the eastern edge of the slab segments, which might be responsible for the widespread Cenozoic volcanisms and pervasively observed upper mantle low velocities in the area. The resulting 409 SWS measurements show that upper mantle anisotropy beneath the vast majorty of the study area is characterized by dominantly E-W fast orientations which are nearly orthogonal to the strike of most of the major tectonic features in the study area, ruling out significant lithospheric contributions to the observed anisotropy. This observation, when combined with results from seismic tomography, numerical modeling, surface movement, and focal mechanism investigations, suggests that the observed azimuthal anisotropy is mostly the consequence of absolute plate motion or the westward rollback of the oceanic Indian slab. The flow system induced by the rollback or absolute plate motion may experience regional alteration from mantle upwelling along the eastern edge of the slab and through the detected slab window, leading to local variations in the observed splitting parameters.