''Deciphering tectonic and climatic controls on erosion and sediment transfer in the NW Himalaya''

Central to examining orogenic belt formation and evolution are efforts to define the relative roles of tectonics and climate in landscape denudation. Studies in high altitude, high relief mountain settings such as the Himalayan-Tibetan orogen show that denudation can scale with tectonics, precipitation and/or topography. Steep north-south trending gradients in elevation, relief, rock uplift and precipitation make the NW Himalaya an excellent location to examine these controls of landscape denudation. To elucidate the erosional processes involved in sediment mobilization from primary sources at catchment headwaters, rates of lateral rockwall slope erosion are defined for 12 catchments between the Baltistan region of Pakistan and Garhwal, northern India. These rates are derived from 10Be cosmogenic nuclide concentrations in medial moraine sediment, and affirm that between ~0.02 and 7 m of lateral slope erosion through periglacial processes can be achieved across a single millennium in this setting, and >2 km when extrapolated for the whole Quaternary. This erosion is sufficient to influence topographic change at the catchment head, and emphasizes the importance of localized erosion when evaluating broader landscape change. Statistically significant relationships are apparent between slope erosion and apatite fission track cooling ages and inferred rates of exhumation. Rockwall slope erosion in the NW Himalaya is found to be primarily governed by tectonically driven rock uplift, which is the result of the Indo-Eurasian convergence and the geometry and shortening of the Main Himalayan Thrust. Precipitation occupies a secondary control. To examine the nature and timing of sediment transfer from primary sources to transient sinks, 18 new and 29 recalculated 10Be fan surface exposure ages define the timing of surface abandonment for a suite of 10 fans across the Greater Himalaya ranges in northern India. The fans are found to evolve during interglacials and periods of deglaciation, which are associated with a higher intensity monsoon. As glacial conditions or periods of relative cooling and increasing aridity become reestablished, aggradation ceases or becomes more restricted and the fan surfaces stabilize and/or are abandoned. A regional temporal framework of landform abandonment/aggradation cessation and incision events for the monsoon-influenced western Himalaya ranges (MWHR) and the semi-arid western Himalaya ranges (SWHR) are defined for the NW Himalaya. Regional landform abandonment events are consistent with the fan record, occuring during periods of relative cooling and increasing aridity, while regional incision events occur across various climatic conditions. Climate and climate-driven processes largely moderate sediment transfer in the NW Himalaya; however, the distribution of precipitation and temperature are themselves influenced by the structural evolution of the orogen. Although the primary controls of erosion and sediment transfer in this study are shown to be rock uplift and climate respectively, this research overall argues that the various components of Himalayan mountain range evolution cannot each be governed by one mechanism alone. The specific nature and rates of landscape change on the catchment scale are therefore influenced by longstanding catchment-specific feedbacks between topography, geology, surface processes, climate and tectonics, in addition to regional forcings on the mountain range and/or orogen scale.