Anisotropic Tomography Beneath Northeast Tibet: Evidence for Regional Crustal Flow

We present high-resolution tomographic images of isotropic P wave velocity and azimuthal anisotropy in the crust and uppermost mantle beneath NE Tibet by jointly inverting 62,339 arrival times of the first P and later PmP waves from 6,602 local earthquakes and 9 seismic explosions. Widespread low-velocity zones in the middle crust contribute most of seismic anisotropy in the crust beneath NE Tibet. The predominant fast-velocity directions of azimuthal anisotropy are closely correlated with the stress field revealed by GPS observations and focal mechanism solutions in the transition zones among the Alxa block, the Ordos basin, and the Tibetan Plateau. We attribute this feature to regional crustal flow that has intruded northeastward into NE Tibet and possibly affected vertical ground motions, whereas the flow has been resisted by the surrounding rigid blocks and so failed to further extrude eastward between the Ordos basin and the Sichuan basin. The crustal flow is responsible for the intracrust and crust-mantle decoupling beneath the transition zones of NE Tibet. High-velocity zones with depth-consistent anisotropy are found to border the southwestern Ordos basin between 105° and 106°E. The rigid blocks, major active faults (e.g., the Haiyuan, Qinling, and Kunlun faults), and their interactions cause the regional tectonic features and seismic activities. Accommodation of the different deformation patterns and the tectonic interactions may explain the complicated geodynamic evolution of the crust beneath NE Tibet.