Crustal structure beneath the northeastern margin of the Tibetan plateau and its surrounding regions revealed by direct surface wave radial anisotropy tomography

We assemble the continuous waveform data recorded by 571 broadband temporary seismic stations from 2014 to 2015 in the northeastern (NE) margin of the Tibetan plateau and its surrounding regions. Rayleigh and Love wave phase velocity dispersion curves (T = 5-50 s) are obtained by the image transformation technique. Then the phase velocity dispersion data are used to jointly invert the 3-D shear wave velocity (V-s) and radial anisotropy (xi) models in the study region with the direct surface wave radial anisotropy tomography (DRadiSurfTomo) method. The xi is negative and positive in the upper crust and mid-to-lower crust, respectively, indicating that the crustal deformation pattern is depth-dependent. The middle crust beneath the NE margin of the Tibetan plateau generally exhibits positive xi. Two low-velocity zones (LVZs) are detected beneath the Songpan-Garze block and the western Qinling orogen as well as the northern Qilian orogen. High-velocity zone (HVZ) and positive xi are characterized in the mid-to-lower crust beneath both the Alxa block and the Ordos basin. Combining with the previous results, we propose these two blocks, as a whole belonging to a part of the North China Craton (NCC), effectively restrict the northeastward growth of the Tibetan plateau. The uneven crustal deformation along the boundary between the NE margin of the Tibetan plateau and its surrounding regions is likely to be affected by the far-field stress originating from the collision between the Indian and Eurasian plates.