Catastrophic partial drainage of Pangong Tso, northern India and Tibet

Catastrophic partial drainage of Pangong Tso, one of the largest lakes in Tibet, is supported by the geomorphology of the Tangtse Valley, Ladakh, northern India and cosmogenic ¹⁰Be nuclide ages of roche moutonnées, strath terraces, and a flood deposit downstream from the former spillway. The former spillway for Pangong Tso is ~20-m-high and likely allowed ~18km³ of water to drain catastrophically down the Tangtse Valley over a period of about 2days sometime during the latest Pleistocene to early Holocene. The largest flood deposit, composed of imbricated granitic boulders up to 4.5m in length, is present ~33km downvalley of the spillway. These boulders have a cosmogenic ¹⁰Be exposure age of 11.1±1.0ka, the age of the outburst flood. The minimum calculated discharge was ~110,000m³s^-1. One set of strath terraces, upvalley of the flood deposit along the flood's drainage path, shows that the rate of fluvial incision 0.3±0.1mmy^-1 during 122-10.5ka increased to 1.5±0.5mmy^-1 during 10.5ka to the present. The temporal overlap of this increase in the rate of fluvial incision with the main flood deposit suggests that the flood was important in defining the incision along the Tangste valley. A second set of strath terraces shows little change in incision, from ~0.6-0.9 to ~0.9-1.4mmy^-1, sometime between 18 and 27ka. Roche moutonnées, upvalley from strath terraces, yield a cosmogenic ¹⁰Be age of 35.8±3.0ka, defining the time when glaciers last occupied the Tangtse valley. However, the lack of glacial sediment along the Tangste valley suggests that the flood eroded glacial depositional landforms and sediments resulting in high sediment loads in the floodwater, which in turn increased fluvial incision to form strath terraces. Much of the eroded glacial sediment was subsequently redeposited as the main flood deposit. The catastrophic drainage of Pangong Tso may be the result of breaching of the Pangong-Tangste spillway during very high lake levels in a period of intensified monsoon (10.7-9.6ka) and/or possibly the consequence of seismic activity along the Karakoram Fault that is associated with the initial formation of Pangong Tso.