P-wave velocity and radial anisotropy beneath Sichuan-Yunnan area  revealed by teleseismic traveltime tomography

Abstract

The Sichuan-Yunnan area is located on the southeastern margin of the Tibet Plateau, and it interacts with the Tibet Plateau because of the the southeastward motion of the plateau material since the Cenozoic. Therefore, the Sichuan-Yunnan area acts an important window to understand the uplift and deformation processes of the largest and highest plateau in the world. In this study, we invert the teleseismic relative traveltime differentials extracted from waveform cross-correction to invert the P-wave and radial anisotropy (RAN) beneath the Sichuan-Yunnan area. In the tomographic inversion, we utilize an eikonal equation-based tomography method, and obtain the subsurface P-wave velocity and radial anisotropic structure down to a depth of 700 km. Our P-wave tomography results show that high-velocity anomalies are beneath Sichuan basin at depths from ~ 30 km to ~ 200 km, and remarkable low-velocity anomalies south of Sichuan basin are below the Xiaojiang fault and Red River fault at the same depths. Notably, low-velocity anomalies west of Sichuan basin are under the Songpan-Ganzi block at depths less than 200 km. In addition, our tomography results do not reveal high-velocity anomalies in the mantle transition zone (MTZ) beneath Xiaojiang fault. Evidently, the lithosphere of Sichuan basin exhibits negative radial anisotropy, and the Emeishan large igneous province shows visible negative radial anisotropy. Remarkably, positive radial anisotropy is found underneath Red River fault and it extends a depth of at least 170 km.