Mantle Upwelling Beneath the Hoh‐Xil Basin in the Northern Tibetan Plateau: Evidence From Seismic Anisotropy

Abstract The non‐uniform growth of the Tibetan Plateau has generally been attributed to continental subduction and delamination, yet how the downgoing lithosphere affects mantle flow and shapes the plateau deformation remains unclear. Here, we present new shear wave splitting measurements from a N‐S‐trending profile across the Hoh‐Xil Basin at approximately 89°E, closing the previous gap in seismic anisotropy observations in the northern Tibetan Plateau. Our results reveal significant E‐W‐oriented splittings in the northern Qiangtang and Songpan‐Ganzi Terranes, closely aligned with the absolute plate motion of the Tibetan Plateau, with a peak delay time of ∼1.8 s. The delay time of splittings gradually decreases to ∼0.5 s in the northern margin of the plateau, where a large delay in the SKS arrivals is observed, indicating pronounced low velocities in the underlying mantle. Combined with the northward migration of post‐collisional magmatism, this pattern suggests significant mantle upwelling induced by lithospheric delamination beneath the Hoh‐Xil Basin. In the southern portion of the profile, the splitting direction changes to ENE‐WSW, parallel to the Indian Plate's motion, which can be attributed to a poloidal flow associated with the Indian subduction. This subduction‐driven flow is separated from the delamination‐driven mantle upwelling to the north by a deep‐seated relic terrane, as suggested by a sharp transition in splitting parameters over a ∼100‐km distance beneath the central Qiangtang Terrane. These findings provide compelling evidence for differential mantle flow beneath the Tibetan Plateau and offer new insights into how the downgoing continental lithosphere drives deformation in collisional orogens.