Crustal anisotropy from Moho converted Ps wave splitting analysis and geodynamic implications beneath the eastern margin of Tibet and surrounding regions |
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| Institution: | 1. State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China;2. Seismological Laboratory, Institute of Crustal Dynamics, China Earthquake Administration, Beijing 100085, China;3. Physics of the Earth, Sciences B, University of Zaragoza, Pedro Cerbuna 12, 50009 Zaragoza, Spain;1. State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China;2. University of Chinese Academy of Sciences, Beijing 100049, China;3. Deutsches GeoForschungsZentrum GFZ, Telegrafenberg, 14473 Potsdam, Germany;4. Physics of the Earth, Sciences B, University of Zaragoza, Pedro Cerbuna 12, 50009 Zaragoza, Spain;1. Department of Geophysics, School of Geodesy and Geomatics, Wuhan University, Wuhan 430079, China;2. Key Laboratory of Geospace Environment and Geodesy of Ministry of Education, Wuhan University, Wuhan 430079, China;3. Collaborative Innovation Center of Geospace Information Science, Wuhan University, Wuhan 430079, China;1. State Key Laboratory for Mineral Deposits Research, School of Earth Sciences and Engineering, Nanjing University, Nanjing 210046, China;2. Institute of Geophysics and Geodynamics, Nanjing University, Nanjing 210046, China;3. Institute of Geophysics, China Earthquake Administration, Beijing 100081, China;1. Institute of Geophysics and Geodynamics, School of Earth Sciences and Engineering, Nanjing University, Nanjing, 210093, China;2. Department of Geology, University of Illinois at Urbana-Champaign, IL 61801, USA |
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| Abstract: | In addition to crustal thickening, distinctly different mechanisms have been suggested to accommodate the huge convergences caused by the continental collision between India and Eurasia. As the transition zone between the two grand tectonic domains of Asia, the Tethys and the Pacific, east Tibet and its surrounding regions are the ideal places to study continental deformation. Pervasive rock deformation may produce anisotropy on the scale of seismic wavelengths; thus, seismic anisotropy provides insight into the deformation of the crust and mantle beneath tectonically active domains. In this study, we calculated receiver function pairs of radial- and transverse-components at 98 stations located in Sichuan and Yunnan provinces, China. We selected 7423 pairs with high signal-to-noise ratio (SNR) and unambiguous Moho converted Ps phases (Pms) to measure the Pms splitting owing to the crustal anisotropy. Both the crustal thickness and the average crustal Vp/Vs ratio were calculated simultaneously by the H–k stacking method. The geodynamic implications were also investigated in relation to surface geological features, GPS velocities, absolute plate motion (APM), SKS/SKKS splitting, and other seismological observations. In addition to the fast polarization directions (FPDs) of the crustal anisotropy, we observed a conspicuous sharper clockwise rotation around the eastern Himalayan syntaxis than was revealed by GPS velocities. The distributed FPDs within and near the main active fault zones also favored the directions parallel to the faults. This implied that the deformation of a continuous medium revealed by GPS motions is a proxy for the deformation of the brittle shallow crust only, while the main active faults and the deep crustal interiors both play important roles in the deep deformation. Our results suggest that the deformation between the crust and upper mantle within the northernmost section of the Indochina block is decoupled due to the large difference in the directions between the observations related to the crust (GPS and crustal anisotropy) and mantle (APM and mantle anisotropy). Focusing on the transition zone between the plateau and the South China and Indochina blocks, we suggest that the motion of the Central Yunnan sub-block is a southeastward extrusion by way of tectonic escape. There is less deformation in the deep crust and the motion is controlled by the active boundary faults of the Ailaoshan–Red River shear zone to the west and the Xianshuihe–Xiaojiang fault to the east; the lower crustal flow within the plateau southeastward reached the Lijiang–Xiaojinhe fault, but further south it was obstructed by the Central Yunnan sub-block. |
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