TIMING AND NATURE OF ALLUVIAL/ DEBRIS FLOW FAN FORMATION IN THE NW HIMALAYA OF NORTHERN INDIA

Alluvial/debris flow fans (here simplified as fans) offer well-preserved records of local and/or regional paleoenvironmental change and are important for understanding sediment flux and landscape evolution of high mountain settings. Fan chronologies are defined and examined across several contrasting settings in the NW Himalaya of northern India using new and published ¹⁰Be cosmogenic nuclide surface exposure ages. Fans in Lahul and Garhwal of the monsoon-influenced Greater Himalaya include small debris fans that are largely formed during the Holocene. Large alluvial fans formed throughout the last glacial, and small debris fans formed after ~21 ka are identified in semi-arid Ladakh, south of the Transhimalaya. The polygenetic nature of the investigated fans and the inherent complexities of these depositional systems, means that no single causal mechanism or trigger for fan formation can be identified. Fan formation can however be tentatively linked to broad periods of climatic, environmental and/or geomorphic transition. Although Himalayan landscape change is conditioned by glaciation, the timing and nature of fan development is governed by individual catchment dynamics, which vary across space and time. Traditional paraglacial and process-response models cannot therefore be applied to fan development in the NW Himalaya. Instead we propose that fans are formed by a continuum of sediment transfer cycles that are governed by internal and external forcing factors which influence the sediment flux for individual catchments over 10^3-4 year timescales.