Seismic Azimuthal Anisotropy of Northeastern Tibetan Plateau From Ambient Noise Double Beamforming Tomography: Implications for Crustal Deformation

In this study, we obtain a three-dimensional (3-D) high resolution seismic anisotropy model of NE Tibet using ambient noise double beamforming (DBF) tomography with ChinArray-Himalaya II data. DBF tomography technique allows for the simultaneous derivation of local phase velocity and azimuthal anisotropy from ambient noise cross-correlations without performing tomographic inversion. Based on the DBF tomography results, we have constructed a high resolution 3-D azimuthal anisotropic shear wave velocity model of NE Tibet. Our model indicates the presence of prominent low velocities and strong azimuthal anisotropy in the mid-to lower crust beneath the Songpan-Ganzi belt, while the mid-to lower crust of the Qilian belt exhibits relatively weak low velocity and azimuthal anisotropy. We propose that crustal channel flow may be the primary mechanism driving crustal deformation in the Songpan-Ganzi belt, which is in the mature part of the plateau. As the Qilian belt is at an early stage of plateau formation, it is characterized by crustal shortening with the development of a sub-vertical foliation. Additionally, the misalignment between predicted and observed shear wave splitting reveals decoupled crust and upper mantle deformation west of the Ordos block where eastward mantle flow is deflected by the lithospheric keel of Ordos.