Late Cenozoic eastward growth of the Tibetan Plateau: evidence from geomorphic indices and river-profile inversion around the Hanzhong Basin

During the late Cenozoic, the outward growth of the Tibetan Plateau significantly influenced the tectonic, climatic, and geomorphic evolution of surrounding regions. The Qinling Mountains, at the eastern front of the Tibetan Plateau, have been involved in plateau expansion since the late Cenozoic, and the Hanzhong Basin, its unique late Cenozoic intermontane basin, preserves rich information on plateau growth. In this study, geomorphic indices, apatite fission track dating, and river-profile inversion were conducted on catchments around the Hanzhong Basin. Results reveal that drainages north of the Hanzhong Basin generally exhibit high steepness indices, especially those in the west, but southern drainages show greater variation. River-profile inversion documents two phases of accelerated relative rock-uplift at 15-10 Ma and 5-2 Ma on northern drainages. We interpret that high steepness indices and uplift rates in the west reflect tectonic forcing, expressed as pronounced relative rock uplift and enhanced subsidence of the western basin, whereas the heterogeneous steepness in the south indicates the differential uplift. Integrating tectonic and sedimentary evidence, we propose a new surface deformation model in which the outward expansion of the Tibetan Plateau since ~15 Ma has forced the rigid Bikou Terrane to wedge eastward, thus reactivating the Mianlue Fault and inducing extensional faulting within the Qinling Mountains and subsidence of the Hanzhong Basin. Synchronously, a series of transpressive faults formed in the Micang Shan and governed the landscape.