Multi-stage river incision processes since 15 Ma and the formation of the Yarlung Tsangpo River in the southeast Tibet

The Himalayan-Tibetan Plateau presents an exemplary setting to explore the intricate interactions among tectonics, erosion, and climate. Since achieving its elevated stature in the Miocene, the plateau's landscape has undergone significant transformation, largely influenced by several major rivers. The Yarlung Tsangpo River, the largest river on the plateau, has been instrumental in this geomorphic evolution. Throughout the Neogene and Quaternary periods, this river has facilitated the extensive removal and transportation of massive rock volumes from the plateau into the southern Himalayas. Consequently, it has profoundly affected the patterns and intensities of erosion and uplift within the orogenic system, contributed to the reorganization of river networks, and influenced sedimentary processes in the adjacent foreland basin. Nevertheless, the specifics of river erosion evolution process in southeast Tibet and its driving factors remain a subject of considerable debate. In this study, we present an in-depth analysis of both long- and short-term denudation processes in southeast Tibet, particularly along the Yarlung Tsangpo River. The long-term denudation history is elucidated through exhumation rate simulations derived from published low-temperature thermochronological data. Near the hanging wall of the Woka normal fault (upstream), the data indicates an average exhumation rate of 0.23 km/Ma, predominantly from samples older than 10 Ma. In contrast, the footwall experienced an initial rapid exhumation phase around 10.25 0.81 Ma, with rates approximating 0.53 km/ myr. This rate was comparatively steady at 0.31 0.01 km/ myr further from the fault. Subsequently, at 7.12 0.36 myr, the exhumation rate increased to 0.42 0.02 km/myr. Post 5 Ma, rapid exhumation, reaching rates of 0.57 0.05 km/ myr, was confined to the Jiacha Gorge, continuing up to ~1 Ma as indicated by AHe dating. Short-term erosion processes were assessed through millennium-scale catchment erosion rates, determined by cosmogenic nuclide analyses of river sediments. A sample from the hanging wall of the Woka normal fault indicated a catchment-wide erosion rate of 19.9 m/myr. Conversely, samples from outside the Jiacha Gorge, including two from main river tributaries and two from secondary tributaries, demonstrated significantly higher erosion rates, ranging from 47.5 to 67.3 m/myr. Subsequently, we employed 3D thermo-kinematic modeling to reconstruct the region's topography as it appeared approximately 15 million years ago, integrating both long-term exhumation and short-term erosion rates. The model suggests the formation of a peneplain in southern Tibet around 15 Ma, after notable uplift in the early Miocene and substantial exhumation between 20 and 15 Ma. The drainage patterns during this period in southern Tibet likely differed markedly from the present, as the eastward-flowing Yarlung Tsangpo River had not yet formed. It is hypothesized that the river flowed directly towards the Himalayan foreland until around 6 Ma. At this time, the river channel was altered through capture by the Jiacha Gorge, redirecting its flow eastward.