THE ROLE OF PARAGLACIAL PROCESSES IN THE EVOLUTION OF HIGH MOUNTAIN LANDSCAPES, THE HIGH HIMALAYA OF INDIA AND NEPAL

Within high mountain environments, erosional and depositional landforms provide evidence for the nature of landscape evolution, and the rates and magnitudes of denudation, sediment transfer and deposition. Using sedimentological and geomorphic techniques, field mapping and cosmogenic radionuclide (CRN) dating, we studied moraines, fans, outwash terraces and strath terraces in the Khumbu (Everest) Himal, Garhwal Himalaya of India, and Langtang Himal of Nepal to examine this evidence and to quantify timing and rates of landscape evolution. Many of the valleys are dominated by major fans that onlap moraines and comprise massive bouldery diamicts, commonly 10-25 m thick, and rarely comprising more than 1 or 2 units. These fans are the result of debris and hyperconcentrated flows, mainly produced by glacial lake outburst floods. CRN dating shows that fan/terrace and moraine ages cluster around 1-2 ka, 4-5 ka, 7-8 ka, 12-14 ka, and 16-17 ka. Each of these clusters can be tied to significant regional or global climatic events, and indicate a strong temporal and hence climatic link between glaciation and fan formation. This catastrophic style of sedimentation that characterizes the fans is consistent across the range of Himalayan areas studied. This suggests that sediment transfer within these environments occurs as paraglacial resedimentation events, while low-energy fluvial reworking dominates during the intervening times. Dating of strath terraces yielded an average denudation rate of ~4-5 mm/a, which is remarkably consistent across the study areas, and provides background denudation rates. This contrasts with fan incision rates of ~17-24 mm/a. Therefore, while climate-driven glacial fluctuations may determine the short-term geomorphic response of the landscape, long-term denudation rates are driven primarily by tectonics.