The Structure Of The Crust And Mantle Beneath Southeast Tibetan Plateau From Receiver Function And Surface Wave Dispersion

The Southeast Tibetan is of great scientific value to the problems about the uplift, crustal thickening and escape of the Tibetan Plateau. Through the analysis of observational data recorded by 350 temporary seismic stations from ChinArray and 86 permanent seismic stations from the Chinese National Seismic Network deployed in the study area, crustal thickness, average Vp/Vs ratio and the image of the velocity discontinuity of the upper mantle transition zone have been derived. Then we use cubic splines for model parameterization and use the neighborhood searching algorithm to solve shear velocity of the crust and uppermost mantle with Rayleigh wave phase and group velocity dispersions. Finally, receiver function and surface wave dispersions are jointly inverted to determine crustal thickness, sediment thickness and a high-resolution shear velocity model. Through the analysis of obtained results with geological and geophysical information about the Southeast Tibetan, we get several main conclusions.Regional crustal thickness varies strongly, the crustal thickness becomes thinner gradually from northwest to southeast. The thickest crustal thickness (more than 60 km) is observed at the station in Chuanxibei sub-block while the thinnest crustal thickness (less than 30 km) is located beneath the southern Yangtze block. The results show that the variation of Moho beneath the Southeast Tibetan is consistent with the changes of topography.The distribution of Poisson’s ratio exhibits large variations across the study area. Low Poisson’s ratios are mainly distributed in Yangtze block and Indo-China block, while high Poisson’s ratios appear in the Chuanxibei sub-block, Dianzhong sub-block and Tengchong volcanic area. The highest Poisson’s ratios (-0.31) are observed at the stations in Tengchong volcanic area and boundary zone between Chuandian Block and Sichuan Basin. The high Poisson’s ratios under boundary zone between Chuandian Block and Sichuan Basin may imply the presence of partial melting and the high Poisson’s ratios under Tengchong volcanic implying a melting or partial melting magma. The high Poisson’s ratios under Panzhihua probably are caused by the mafic basalts related to the mantle plume of Emeishan Large Igneous Province.Sediment thickness is consistent with the distribution of sedimentary basins in the research area, such as Sichuan Basin, Chuxiong Basin and Simao Basin. The sediment thickness beneath Sichuan Basin is thicker than the sediment thickness beneath Chuxiong Basin and Simao Basin.There are two main NS-trending low-velocity zones (LVZs) observed in the mid-lower crust. One of the LVZs starts from Chuanxibei sub-block and goes across the Lijiang fault into Dianzhong sub-block. The other one follows the trace of the Xiaojiang fault until about 24°N. These two LVZs are separated by Sichuan Basin and the inner zone of Emeishan Large Igneous Province in the middle crust. However, we do not observe high Vp/Vs ratios beneath south Xiaojiang fault, which may mean the absence of partial melting. The inconsistence between different geophysical results imply the LVZs have complex geometry and evolution mechanism.In the uppermost mantle, the results show that high velocity anomaly exists at the north of 26°N, while low velocity anomaly at the south of 26°N. At the south of 26°N, the velocity structure and other geological and geophysical characteristics differ from upper crust to mantle. It implies that the deformation in the crust and mantle might be decoupled. GPS measurements reveal a clockwise rotation motion around eastern Himalayan syntaxis, while Pn and SKS results show that the fast polarization directions trend near NS in the north and rotate to near EW in the south because of the subduction of Indian block.In the upper mantle transition zone, the low velocity anomaly under Tengchong volcanic area may lead to the depression of the 410. At the west of 104°E, the thickened mantle transition zone with a depressed 660 probably is related to the cold Indian slab. The thicker mantle transition zone and high velocity anomaly under Dianzhong sub-block imply a low temperature environment while the thinner mantle transition zone and low velocity anomaly under Yangtze block imply a high temperature environment.