Lithospheric Deformation of the Western Qinling Revealed by Shear‐Wave Splitting: Insights Into Northeastward Expansion of the Tibetan Plateau

Abstract As a critical boundary zone of the northeastern Tibetan Plateau (NETP), the Western Qinling is key to understanding the deformation mechanism responsible for the plateau's northeastward expansion. In this work, we perform shear‐wave splitting (SWS) measurements using teleseismic waveforms recorded by a dense linear seismic array traversing this region. Our results provide new information on small‐scale variations in seismic anisotropy and lithospheric deformation patterns, which can be used to constrain the deformation mechanisms. The dominant WNW‐ESE fast polarization direction aligns with surface fault strikes and crustal deformation, supporting vertically coherent crust‐mantle deformation. Crucially, we identify two distinct two‐layer anisotropy structures: one beneath the southern Western Qinling and the other near the Western Qinling Fault (WQLF) and the Shangdan Fault Zone (SDF). The southern region is characterized by NW‐oriented lithospheric deformation in the upper layer and E‐W‐oriented asthenospheric flow in the lower layer. Beneath the WQLF and SDF, the upper layer exhibits ENE‐oriented anisotropy, attributed to the combined effects of fossil anisotropy preserved within the Mesozoic ductile shear zone and Cenozoic modification associated with the northeastward expansion of the Tibetan Plateau, whereas the lower layer shows WNW‐oriented deformation involving both the lithospheric mantle and the asthenosphere. Combined with previous observations, our results indicate that vertically coherent deformation dominated the mid‐late Cenozoic tectonic evolution of the NETP. In light of geochronological constraints and the lateral variations in the deformation, we infer that the lateral extrusion likely dominated the early stage of plateau expansion.