Seismic Radial Anisotropy of the Southern Tibetan Rifts From Ambient Noise Adjoint Tomography: Basal Shear and Localized Crustal Weak Zones Promote Rifting

Abstract North–south‐trending rifts in southern Tibet record E–W extension within the Tibetan Plateau, closely linked to high gravitational potential energy from collisional thickening and northward underthrusting of the Indian lower crust. To reveal how basal shear is transmitted upward and controls rifting, we use ambient noise adjoint tomography with broadband temporary‐array data to construct crustal shear‐wave velocity and seismic radial anisotropy (RA) models beneath three rifts. Positive RA is pervasive in the mid‐to lower crust of the Lhasa Terrane (∼30–55 km), indicating ductile shear deformation related to Indian lower‐crustal underthrusting. In contrast, low velocities (equivalent to ∼3.7%–5.2% partial melt) and strong positive RA in the mid‐to upper crust (∼10–30 km) are mainly confined beneath rift axes. These results suggest that, during upward transmission, deep basal shear preferentially focuses into localized weak zones in the mid‐to upper crust, thereby promoting rifting above.