The Crustal Anisotropy And S Wave Velocity Structures Beneath The Southeastern Margin Of Tibetan Plateau

The southeastern margin of Tibetan Plateau (SE Tibetan Plateau) has always suffered from intense deformation under the collision of Indian and Eurasian plates. It has complex tectonics and frequent seismic activities, which is an important area to study the crustal deformation pattern and evolution mechanisms of Tibetan Plateau. According to the crust-mantle anisotropy and velocity structure beneath the SE Tibetan Plateau, we could discuss the relationship between the crustal deformation and regional stress, ineractions of different blocks, dynamic processes, deformation of lithosphere and the causes of strong earthquakes under the effects of eastward extrusion of Tibetan Plateau.For obtaining the detailed deep velocity structure of the SE Tibetan Plateau, studing its dynamic processes, understanding the causes and trends of the earthquakes, there have been deployed a large number of temporary seismic stations by Institute of Geophysics, China Earthquake Administation and other institutes in recent years. Including the permanent stations in this area, the total number of the stations is more than 900. In this work, we took full use of the observations from the densely temporary and permanent stations in the SE Tibetan Plateau, extracted the crustal anisotropic parameters with Pms phase in receiver functions, and obtained the crust-mantle S wave velocity structures with the joint inversion of receiver functions and surface waves.We obtained robust anisotropy estimations at 217 stations in the south of～26°N The measured splitting time of the Pms phase varies from 0.02 to 0.88 s, with an average value of approximately 0.28 s, which is much larger than the splitting time in the upper crust (top 15 km), implying that the crustal anisotropy is mainly generated within the middle and lower crust (below 15 km). The fast polarization directions of Pms in the sub-blocks are in good agreement with the directions of principle compression stress. Near the Xiaojiang fault, the Red River fault, and the Lancangjiang fault, the directions are parallel with the strikes of the faults. The splitting time shows a conspicuous contrast with a small value in the east of the Xiaojiang fault zone and a large value in the west. We assume that the differential movement in the ductile middle-to-lower crust caused the alignment of materials along the movement directions and developed the crustal anisotropy in the sub-blocks. Near these major faults, the low-velocity zone, high Poisson’s ratio and high heat flow in the crust provide evidence for the existence of partial melt. The crustal deformation near these faults is mainly dominated by the strike-slip motion, resulting in the alignment of minerals and partial melts along the strikes of these faults. Analyzing the anisotropy pattern from SKS and Pms splitting, we are inclined to conclude that the deformation in the crust and mantle is mechanically decoupled and the mantle deformation is mainly caused by the asthenospheric flow in this area.We investigated the S wave velocity structure beneath the SE Tibetan Plateau by applying joint inversion of receiver functions and surface wave phase velocity and group velocity dispersion curves. The results show that: a low velocity anomaly widely distributes in the middle-to-lower crust and upper mantle beneath the western Sichuan sub-block; near Xiaojiang fault zone, the lower crust exists low veloctity zone, which is narrow in the north and south of the fault and wide in the middle; in Panzhihua region and the southwest of Sichuan basin, the crust presents relatively high velocity; the velocity in the western Yunnan is on the whole low, but not uniform and continuous; the crustal thickness of the Sichuan-Yunnan block deceases from about 60 km in the north to 40 km in the south, and the thickness of the Sichuan-Yunnan block is thicker than that of the blocks in its two sides. We speculate that the high velocity zone beneath the southeast of Sichuan basin and Panzhihua region has relation to the Emeishan basalt distribution, which plays a certain impediment to the movement of low velocity materials of the western Sichuan sub-block, inducing the crustal thickening and uplift of the topography. The low velocity zone becoming narrow in the north of Xiaojiang fault zone may be caused by the intrusion of the Emeishan basalt. The SE Tibetan Plateau shows generally weak zone with low velocity, but the low velocity zone is not uniform as is expected. If the materials move from the Tibetan Plateau with crustal flow pattern, the crustal flow may be complicated, which is affected by the high velocity zone.