Temporal deceleration of fluvial incision since the Middle Pleistocene along the Salween-Nu River in southeast Tibet

Fluvial terraces record intermittent river aggradation and down cutting regulated by the competition between stream power and sediment supply. Therefore, reconstructing the magnitude and rate of fluvial incision can provide insights into the interaction of tectonics, surface processes and climate change in shaping the landscape. The southeast Tibetan plateau is characterized by gently-dipping topography and deeply incised valleys with inset levels of fluvial terraces along the Salween, Mekong, and Yangtze Rivers. In this study, we focused on fluvial terraces well preserved at the two steeper segments of the Salween-Nu River (Bingzhongluo and Exi) to reconstruct the aggradation and incision history by integrating field investigation, unscrewed aerial vehicle (UAV) photogrammetric survey, and K-feldspar post-infrared infrared stimulated luminescence (pIRIR) dating. For the downstream Bingzhongluo reach with higher steepness, three levels of strath terraces at 20-160 m above the trunk river are overlain by three episodes of fluvial deposits at 370 ka, 275 ka, and 130 ka, and this yield the incision rates decreasing from 0.4 to 0.2 mm/yr with time. For the upstream Exi reach with lower steepness, five levels of strath terraces occurs at 10-350 m high, and the middle three terrace deposits are dated at 430 ka, 380 ka, and 300 ka. The corresponding incision decelerated from 0.6 mm/yr to 0.1 mm/yr. To summarize, fluvial terraces along two steeper reaches of the Salween-Nu River reveal temporal deceleration of river incision since the Middle Pleistocene varying at 0.6-0.1 mm/yr. Field investigation for terrace deposits revealed that there occur thick alluvial deposits overlying fluvial deposits by the Salween-Nu River, either tributary-derived boulder conglomerates or hillslope-derived angular conglomerates. We tend to believe that the large amounts of sediment supply may protect the underlying bedrock strath from erosion for a certain interval of time, and therefore slow the pace of river incision and regional landscape evolution in southeast Tibet.