Sedimentary records of a palaeo-lake in the middle Yarlung Tsangpo: Implications for terrace genesis and outburst flooding

The middle reaches region of the Yarlung Tsangpo in the southern Tibetan Plateau consist of broad valley reaches, alternating with gorges, which were easily dammed by glaciers and/or glacial moraines during the last glacial period. We investigated three palaeo-lake sedimentary sequences on terraces located in the broad Xigazê valley reach and the Dazhuka-Yueju gorge in the middle reaches of the Yarlung Tsangpo. We found that the altitude of the palaeo-lake was 3811 m above sea level, while its minimum area and volume were ∼679 km2 and ∼22 km3, respectively. The palaeo-lake was formed during the period from about 30.2 to 32.3 cal. kyr BP, and it failed at about 13.2 cal. kyr BP. The dam was possibly located at the eastern end of the Dazhuka-Yueju gorge and may have been a glacier and/or a moraine during the late part of the last glacial. The failure of the dam and related flooding from the Dazhuka-Yueju gorge probably triggered a chain reaction of dam failures downstream, forming a megaflood. Terrace genesis and valley evolution in the investigated region can be divided into four phases: Phase Ⅰ (>32.3 cal. kyr BP), river occupation; Phase Ⅱ (<32.3-13.2 cal. kyr BP), palaeo-lake occupation; Phase Ⅲ (after 13.2 cal. kyr BP), erosion and aeolian deposition; Phase Ⅳ, modern downward incision.