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 ∼15 Ma (∼20°) and ∼8.4 Ma (∼15°), 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.