Slowing Extrusion Tectonics and Accelerated Uplift of Northern Tibet Since the Mid-Miocene

Mechanisms of northern Tibet crustal thickening are strongly debated. Here we present a high-resolution and continuous record (24-4.8 Ma) of anisotropy of magnetic susceptibility and paleomagnetic declination from the Dahonggou section in the northern Qaidam Basin, northern Tibet. Our results reveal two major clockwise rotations of the regional paleostress field at similar to 15 Ma (similar to 20 degrees) and similar to 8.4 Ma (similar to 15 degrees), coeval with episodes of mountain building and basin deformation in northern Tibet. We suggest that simultaneous stress field rotation and uplift observed in the study area are related to progressively slowing lateral-extrusion tectonics along boundary-parallel strike-slip faults (Altyn Tagh and Haiyuan faults). The 15 Ma event can be explained by the transition from northeastward to eastward extrusion of northern Tibet materials due to the obstruction of the rigid Alxa block. As deformation keeps migrating toward the east, the 8 Ma event is possibly related to a change from eastward to southeastward extrusion of northeastern Tibet materials as a result of the resistance of the rigid Ordos block. We conclude that Tibetan deformation evolved through successive stages of slowing extrusion tectonics since the mid-Miocene. Deformation was initially localized along pre-existing lithospheric structures, and subsequently more distributed under confining boundary conditions, leading to crustal thickening and uniform uplift of northern Tibet during the late stage of plateau development. Plain Language Summary How northern Tibet evolved as a far-field response to the India-Asian collision remains an open question. Here we present high-resolution and continuous record (24-4.8 Ma) of anisotropy of magnetic susceptibility and paleomagnetic declination from a well-exposed sedimentary section in the northern Qaidam Basin, northern Tibet. The results indicate two significant clockwise rotations (similar to 20 degrees and similar to 15 degrees) of the regional paleostress field at 15 and 8.4 Ma. Interestingly, these two ages are characterized by large-scale mountain building, basin deformation and initiation or slowdown of major strike-slip faulting in northern Tibet. Given that the relatively weak northern Tibet crust is surrounded by several rigid blocks on its northern and eastern sides, we suggest that Tibetan deformation patterns since the mid-Miocene was associated with successive stages of a weakening extrusion process (from NE- to E-directed extrusion at 15 Ma and from E- to SE-directed extrusion at similar to 8 Ma) that exploits pre-existing lithospheric weak zones. This study lend support to the dominant role of strain transfer from localized strike-slip faulting to distributed thickening deformation under confining boundary conditions in raising the crust of northern Tibet uniformly during the late stage of plateau development.