Complex Lithospheric Deformation in Eastern and Northeastern Tibet from Shear‐wave Splitting Observations and its Geodynamic Implications

The eastern and northeastern Tibetan Plateau is a key region to study the growth and expansion of the plateau and associated extrusion tectonics. We studied the seismic anisotropic structure in this region by shear wave splitting analysis of teleseismic records from a dense linear seismic array, to constrain the lithospheric deformation and processes. We detected small-scale variations in anisotropy, including changes of splitting parameters around major faults and different anisotropy patterns among individual tectonic blocks and units but with consistent interior features. Our results combined with previous observations suggest that, in addition to the dominant effects of lateral extrusion induced by the India-Eurasia collision, major faults and tectonic heterogeneity may have also exerted significant impacts on the deformation and thus anisotropic structure of the lithosphere. In particular, we constructed two-layer anisotropy models for both the Longmenshan subblock in the easternmost Songpan-Ganzi terrane and the western Qinling orogen, indicating crust-mantle decoupling in these areas. The lower anisotropic layer of both areas shows a general NW-SE fast polarization direction. We attribute this feature to the large-scale mantle deformation, due to the lateral extrusion of Tibet associated with the India-Eurasia collision. The upper-layer anisotropy in both areas features an optimal NEE-SWW fast polarization direction. While in the Longmenshan subblock it may stem from crustal deformation under the combined effects of middle-lower crustal flow, faulting, and tectonic heterogeneity, that in the western Qinling orogen is probably resulted from shearing caused by upper crustal displacement along a midcrustal detachment.