OSL dating of past lake levels for a large dammed lake in southern Tibet and determination of possible controls on lake evolution

Fluvio-lacustrine terraces along Phung Chu (river) on the central southern Tibetan Plateau indicate that a large palaeo-dammed-lake formerly existed in this area. Based on landscape survey, optically stimulated luminescence (OSL) dating and sedimentary analyses, this research shows that the Phung Chu was blocked and a dammed-lake over 2500 km in size formed before 30 ka ago. OSL dating analysis suggests the fluvio-lacustrine sediments were well bleached and yield accurate age estimates for two lake drainage events. The first drainage event took place after 30 ka, resulted in river incision and formed a high terrace at 50 m height from the present river level. The second drainage happened after 3.7 ka, resulted in further river incision and formed the second terrace at 25 m height from the present river level. According to the distribution of the fluvio-lacustrine sediments, active normal faults (particularly the Kharta Fault) in this region and the high gradient slopes after Phung Chu enters the Yo Ri gorge, seismically-induced landsliding is regarded as highly likely to have been the cause of river blockage and associated formation of a dammed-lake, although glacial damming is also a possible cause. The volume of drainages from this dammed-lake may have led to catastrophic flooding and analogous modern lakes represent significant geo-hazard risks to down-river human settlements. As dammed-lakes are special phases in fluvial evolution, often involving river blockage, breakthrough and drastic catchment change, these processes can reveal how tectonic or climatic events modify landforms. However, such tectonic-derived landform changes can also impact palaeo-climate of the region. Thus this study has added new evidence regarding the evolutionary history of a dammed lake including its formation, duration, extent and final drainage, which is crucial for understanding its general landscape process mechanisms and for better assessing geo-hazard risks.

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