Lithospheric S-wave Velocity Structure In Central Yunnan And Its Geodynamic Implications

The central Yunnan area is located in the south of the "Sichuan Yunnan diamond block" in the southeast corner of the Qinghai Tibet Plateau.The tectonic environment of the area is complex and the crustal deformation is also very complex,which is an ideal area for testing different geological evolution models.The inner belt of Emeishan igneous rock province is located in the middle of Yunnan.Previous studies show that there is an obvious high-speed anomaly in the central Yunnan area.The high wave velocity,low attenuation,high wave velocity ratio and high density anomaly in the area indicate that the crust strength in Central Yunnan is very high,which is probably related to magmatic activity in Emeishan igneous province of Permian.In this paper,the three components of enz are extracted by using the remote events recorded by 107 seismic stations in the region,and the coordinate rotation is carried out after decapitation,averaging and filtering.The receiver function is calculated by iterative deconvolution method in time domain,and then the receiving function is selected.Finally,2620 P-wave receiving functions with low frequency(Gaussian parameter is 1.0)and 2620 high frequency(Gaussian parameter 2.5)are selected The velocity structure and dynamic significance of S-wave in the lithosphere in Central Yunnan are studied by the method of combining the receiving function with the surface wave inversion method,combining the dispersion curve of Rayleigh surface wave in0-60 s period and lef surface wave 0-30 s period.In this paper,three joint inversion methods are used,which are the combination of receiving function and Rayleigh surface wave,the combination of receiving function and lef surface wave,and the combination of receiving function and Rayleigh and Leff surface wave,so as to give full play to the effectiveness of different data sets to speed structure constraints.The results obtained by the combination of low frequency receiver function and surface wave are used as the initial model of joint inversion of high frequency receiver function,which reduces the non uniqueness of the understanding and effectively reduces the dependence of inversion results on the initial model.The results are counted and expanded by bootstrap resampling technology,and the best results under the station are returned to obtain the fine and reliable crustal structure characteristics.The results show that:(1)the depth of the middle and upper crust(10-26km),the high-speed anomaly in the S-wave velocity distribution profile below the inner belt of Emeishan igneous province is speculated to be due to the residual part of basic ultrabasic material in the crust during the eruption process of Emeishan igneous province.As the boundary of the inner belt of Emeishan igneous province,Xiaojiang Fault and Lijiang xiaojinhe fault are distributed with obvious S-wave low velocity anomaly area under them,which is presumed to be related to deep and large fault activities.(2)At 30 km depth,the inner zone of Emeishan igneous province in Central Yunnan Province began to show some low-speed anomaly.(3)The S wave velocity in the north and South shows the difference of velocity at 100 km depth.It is considered that the S-wave velocity in the north is mainly high-speed anomaly and the south is characterized by low-speed anomaly.(4)The low speed anomaly is limited to the south of Honghe fault at a depth of 70-120 km.