Kinematic inversion of the Xianshuihe-Xiaojiang fault system from Oligocene shortening to late Miocene left-lateral slip in the southeastern Tibetan Plateau

Several continental-scale strike-slip faults in the southeastern Tibetan Plateau developed during the Cenozoic eastward growth of the plateau. However, geodynamic mechanisms driving plateau growth and their relationship to intracontinental deformation remain poorly resolved, primarily because the brittle deformation histories of major fault systems—such as the >1000-km-long Xianshuihe-Xiaojiang (XSH-XJ) fault system—are inadequately constrained and difficult to reconstruct. To address this gap, we performed thermochronological analyses of samples from the highest mountains (i.e., Jiaozi Shan and Dahaicao Shan) flanking the XJ segment of the fault system, including apatite (U-Th)/He, apatite fission track, and zircon (U-Th)/He dating, complemented by thermal history modeling. Results reveal differential exhumation and cooling across the XJ fault between ca. 26 Ma and 22 Ma, with greater cooling in the Jiaozi Shan (30−50 °C) than the Dahaicao Shan (<5 °C), implying an exhumation offset of 1−2 km. Since ca. 13 Ma, rapid cooling (50−70 °C) has occurred synchronously on both sides of the XJ fault, corresponding to ∼2 km of accelerated exhumation. Together with structural analyses, it is interpreted that the two phases of exhumation and cooling were kinematically linked to a transition of deformation from late Oligocene east-vergent thrusting to late Miocene left-slip faulting. Integrating structural and geophysical data along the XSH-XJ fault system, we propose that the Oligocene shortening caused crustal thickening and in situ anatexis. These processes produced thermally and mechanically weakened lower crust, facilitating the late Miocene initiation of left-lateral slip. Our work highlights a dynamic linkage between the two phases of deformation, providing new insights into how kinematic inversions of major faults operate in the southeastern Tibetan Plateau.