3D velocity and anisotropy of the southeastern Tibetan plateau extracted by joint inversion of wave gradiometry, ambient noise, and receiver function

Abstract

With the deformation pattern of the southeastern Tibetan Plateau remaining controversial, its anisotropic structure and the three-dimensional (3D) velocity model are powerful tools for understanding this issue. The surface wave dispersion curve (DC) and receiver function (RF) are sensitive to absolute velocity and elasticity contrasts, respectively. In this work, we propose a new method for the joint inversion of 3D shear velocity (Vs) and anisotropy using the datasets of RFs and DCs, which are obtained by combining the wave gradiometry method and ambient noise tomography. Because DCs and RFs are divided into back-azimuth bins, and both datasets in each bin are jointly inverted to obtain velocity in the corresponding direction, the method is called the azimuth-dependent joint inversion method. By applying this new method to the Temporary West Sichuan Array, we obtained a high-resolution 3D Vs model and anisotropic structure of the southeastern Tibetan Plateau. In our model, the vertically consistent fast propagation direction in the Songpan-Ganzi block (SGB) and the north Chuandian block (NCDB) suggest that the deformation in this region is largely coupled vertically. Our model provides anisotropic evidence of the Yangtze crystalline basement extending to the SGB. The low-velocity zone (LVZ) was much thicker and more prominent in the NCDB than in the SGD. Anisotropy in the middle lower crust of the NCDB does not show any sign of an LVZ flowing from the SGB or the Qiangtang block to the NCDB. These observations are contrary to those of the crust flow model, suggesting that the LVZ was generated locally. The mushroom-shaped high-velocity structure beneath the southern Chuandian block and boundary fault zone may be a relic of the Emeishan paleo-mantle plume.