3-D shear wave velocity and radial anisotropy models of northeast India and eastern Tibetan Plateau

The south-eastern margin of the Tibetan Plateau is crucial to understanding the dynamics of the Indian-Eurasian continental collision and potential causes of the plateau upliftment. In the present study, we investigate the crustal and upper-mantle shear-velocity structure below the north-eastern India and Tibetan plateau with 1°X1° resolution using fundamental-mode surface-wave group-velocity tomography performed by adopting the Fast-Marching Method. The group-velocity dataset within periods of 5-100 s is compiled from waveforms of 568 regional earthquakes recorded at 326 seismic stations across the study area. Dispersion curves extracted from each node point of the Rayleigh and Love wave group velocity maps have been inverted using the non-linear damped least square method to obtain a well constrained quasi-3D isotropic shear wave velocity tomographic image down to 100 km depth. Spatial discrepancy across Vsv and Vsh observations provided further insight into the radial anisotropy distribution across the study area. Our 3-D shear wave velocity and radial anisotropy models indicate significant lateral variations within the crust. Consequently, the observed ubiquitous low-velocities and radial anisotropy variations along with crustal thickness, supports the concept that the region serves as a pathway for material migration from the Tibetan Plateau eastward and southeast-wards. Low velocity zones within the crust are observed beneath the Songpan-Ganzi Terrane and north-western Qilian Orogen, possibly reflecting eastward crustal extrusion resisted by the Sichuan Basin. The channel flows, delineated at crustal depths, are redirected southeast-wards at the edge of the western margin of the Sichuan Basin, which rotate around the Eastern Himalayan Syntaxis. Observations suggest a possibility of accumulation of flow close to 25-26° leading to a thicker northwestern Yunnan crust in comparison to southern Yunnan represented by an abrupt increase in shear velocity in the latter.