Glacier dam evolution and knickpoint migration in the Yarlung Tsangpo Gorge, eastern Himalayas, since the last glacial period

Abstract

The repeated cut-and-fill process induced by the late Quaternary glacial damming and catastrophic outburst events in the Yarlung Tsangpo Gorge (YTG) in the eastern Himalaya may reflect the interaction between rock uplift, climate change and river incision in the southeastern Tibetan Plateau. Here we provide the detailed reconstruction of the cut-and-fill history recorded at the entrance of the YTG, based on geomorphological, sedimentological and chronological analyses. The results reveal a catastrophic glacial-dam breach during the last deglaciation (∼18 ka), and the outburst flood eroded a ∼30–45 km long valley floor filled with glacial diamicton and glaciofluvial deposits, which was later further incised to form terrace T2 with a height of ∼350 m. The second dam failure occurred at ∼5 ka, producing a spillway, forming terrace T1 with a height of ∼100 m. The vertical incision and horizontal migration rate of the knickpoint at the glacier dams were estimated as 0.02 m/yr, and 2.5 m/yr since the last deglaciation, based on the age of the terraces and distance of the knickpoints. We consider the glacial dams to be effective in impeding headward river erosion during glacial periods. This, along with rapid river headward erosion during the last interglacial period, results in two knickpoints on the longitudinal profile due to the differences in erosion resistance. The fluvial reaches corresponding to the repeated cut-and-fill processes are confined to the center of the Namche Barwa massif, suggesting that rapid rock uplift counteracted fluvial headward erosion and maintained the stability of the bedrock knickpoint. Therefore, in comparison to glacial dams, tectonic uplift is the most important component in plateau margin stabilization.