GSA QUATERNARY GEOLOGY AND GEOMORPHOLOGY DIVISION DISTINGUISHED CAREER AWARD 2021: THE MORAINE DAM OF THE LATE-QUATERNARY GIANT OUTBURST FLOODS FROM THE YARLUNG TSANGPO RIVER IN TIBET

The Yarlung Tsangpo (YT) drains most of the northern Himalaya and much of the Tibetan Plateau and has generated huge outburst floods (eg peak discharge 5x10⁶ m³ s^-1). On the plateau it flows gently east (~1.9‰) before turning south and plunging through the Himalaya between the 7+ km asl peaks Namcha Barwa and Gyala Peri. Above its knick point during the late Quaternary the YT, impounded by the Zelunglung moraines and outwash from Namcha Barwa, hosted giant lakes (eg 835 km³) that sourced the outburst floods. Smaller floods have resulted from surging glaciers and from the collapse of mountain walls during earthquakes. Recent landslide-generated floods down the YT have inundated towns in India. Studying the floods gives insight into the evolution of the incision by the YT, uplift of the Tibetan Plateau and hazards. However, establishing the chronology has been problematical.

Analysis of the lake sediments has led to different interpretations of the chronology, with two OSL studies suggesting a single long-lived lake (eg >51–13 ka (OSL): Xu et al. 2020) and others a more complex history (eg Huang et al. 2014). The Holocene chronology is clearer: from ¹⁴C-dated lake terraces at ~3500 and ~3165 m asl Montgomery et al. (2004) identified two floods. ¹⁰Be dating of nearby moraines indicates a standard staccato sequence of advances that is hard to reconcile with the lake interpretations: Zhou et al. (2007) recognized MIS 6, MIS 2, and post-glacial advances and we have added an MIS 3 advance.

Some Zelunglung moraines appear to be much bigger than the moraines that they largely bury and also indicate a lower ELA than the equivalent neighbors. These observations can be explained if the 2-km high west arête of Namcha Barwa collapsed onto the Zelunglung Glacier, insulating it and permitting transport the debris farther than otherwise. This observation suggests that climate is not the only driver of glacial advance here, and the regional glacial record cannot be relied upon when interpreting local glacial sequences or when inferring climatic detail here. The difficulty in establishing a complete chronology and the multiple causes of flooding imply that predictive models for disasters downstream will be hard to make here and the flood history cannot be unravelled without considering the sedimentary record in the flood plains south of the Himalaya.