Eocene thrust vs. Miocene strike-slip: Kinematic transition of the West Qinling Fault driven by localized asthenospheric upwelling

Large-scale strike-slip structures in the northeastern Tibetan Plateau, such as the West Qinling Fault, have accommodated the northeastward extrusion of the plateau crust, thereby recording its outward growth. However, the Cenozoic tectonic evolution of the West Qinling Fault remains incompletely understood. In this study, we integrate new and published apatite (U-Th)/He thermochronology with exiting sedimentary and geomorphological records to refine the exhumation history of the West Qinling in response to the kinematic evolution of the West Qinling Fault. The combined apatite (U-Th)/He dataset and thermal history modeling reveal two significant episodes of accelerated exhumation in the West Qining during the Eocene (~44–36 Ma) and the Miocene (~17–10 Ma). We attribute the Eocene exhumation to enhanced thrusting along the West Qinling Fault, representing a direct far-field response to the India-Asia collision. In contrast, the Miocene exhumation is linked to oblique-slip motion along the West Qinling Fault, marking a kinematic transition of the fault from thrust- to strike-slip-dominated deformation since the middle Miocene. Integration of our findings with geophysical, sedimentary, and petrological evidence suggests that this kinematic change was caused by a rheological weakening of the lower crust due to localized asthenospheric upwelling. The synchronous kinematic shift along the East Kunlun Fault, along with its similar deep lithospheric structure beneath the Hoh Xil region, implies a genetic link between the West Qinling and East Kunlun faults. We propose that localized asthenospheric upwelling thermally weakened the lower crust beneath both fault systems, thereby driving the kinematic shift during the middle Miocene. This analogy underscores the potential major role of small-scale asthenospheric upwelling-induced thermal weakening of the lower crust in affecting surface deformation during the late stage of orogenic plateau formation.