LANDSCAPE EVOLUTION AND MORPHOLOGY OF THE TANGTSE RIVER VALLEY CONTROLLED BY CATASTROPHIC PARTIAL DRAINAGE OF PANGONG TSO

Roche moutonnées, strath terraces, and flood deposits in the Tangtse River valley, Ladakh–northern India, were dated using ¹⁰Be terrestrial cosmogenic nuclide surface exposure methods to define the timing of partial drainage of Pangong Tso, one of the largest lakes in Tibet. Spillways in fractured marble rockbars that dammed Pangong Tso are ~17 m deep, and potentially allowed 15.7 km³ of water to flow through them as the lake drained. The main resultant flood deposit, composed of imbrecated granitic boulders up to 4.5 m in diameter, dates to 11.1±1.0. Reconstructed discharge based on the five largest boulders is 8260 m³ s^-1, while peak discharge estimated from the Pangong Tso spillway is 39,000 m³ s^-1. One set of strath terraces, up valley of the flood deposit and along the drainage path, shows that the rate of fluvial incision increased from 0.3±0.1 mm/yr to 1.5±0.5 mm/yr at 10.5 ka while the second set shows an increase from 0.6±0.3 mm/yr to 1.1±0.3 mm/yr at 23.0 ka. The age of the main flood deposit (11.1±1.0 ka) overlaps with the initiation of increased strath incision (10.5-23.0 ka) as result of the reactivation of the Pangong spillway (11.0-6.8 ka) possibly due to an intensified monsoon phase (10.7–9.6 ka). Roche moutonnées, up valley from strath terraces, date to 35.8±3.0 ka, but there are no moraines or till deposits in the Tangtse River valley. The broad range of ages for the main flood deposit suggest that the missing glacial debris was reworked during the flooding event, enabled increased strath incision, and were preserved as the main flood deposit. This study illustrates how extreme events in the Himalayan-Tibetan orogen have a profound influence on landscape evolution.