Rayleigh wave azimuthal anisotropy in the Southeastern Tibetan Plateau from Eikonal tomography

As the frontier zone for lateral extrusion of plateau material, the southeastern margin of the Tibetan Plateau is one of the key areas for studying lithospheric deformation, plateau material lateral escape and deep dynamics. Based on data of 540 broadband digital seismic stations in the study area, we obtain the Rayleigh wave phase velocity and azimuthal anisotropy distribution images at 14 similar to 80 s periods of the southeastern margin of the Tibetan Plateau by Eikonal surface wave tomography. The results show that the azimuthal anisotropy distribution of the surface waves at 14 similar to 20 s period is closely related to the direction of the fault zone and the direction of the maximum principal compressive stress, which may be affected by the fault zones and the regional tectonic stress field. With the increase of the surface wave period near the northern sub-block of the Sichuan-Yunnan diamond block and Lijiang-Xiaojinhe fault zone, the anisotropic fast wave direction gradually changes from NS to NE-SW and is roughly parallel to the fault zone. However, the vicinity of Panzhihua is characterized by high velocity and weak anisotropy. We speculate that there is a lower crust flow in the northern part of the Sichuan-Yunnan active block, and the flow direction is basically the same as the southward extrusion direction of the block. The crustal flow is blocked by the high-speed, high-strength block near Panzhihua in the middle of the Sichuan-Yunnan block and its leading edge flows to the southwest. There is no obvious channel flow in the lower crust in the central region due to the presence of hard blocks. In the west area of the Red River fault, the fast wave direction of surface wave anisotropy at the period of 30 similar to 60 s is roughly parallel to the Red River fault, which is speculated to be closely related to the extrusion of the Indosinian block to SE from the Oligocene to the Early Miocene. In the northeast of the study area, the crustal anisotropy of the southern margin of the Sichuan basin is dominated by NE-SW and NEE-SWW, which are obviously different from the SKS fast wave directions. It is speculated that the anisotropy is mainly related to the early tectonic deformation of the crust in this area, which also indicates that the SKS anisotropy mainly comes from the upper mantle. The anisotropic direction of the mid-long period surface waves in the west of 104 degrees E south of the study area is basically consistent with the nearly EW fast wave direction obtained by SKS splitting research. But the surface wave anisotropy is weaker in the east of 104 degrees E and the fast wave direction is significantly different from SKS splitting. We speculate that the SKS anisotropy source depth in the east is at least below 150 km.