CLIMATE VERSUS TECTONIC CONTROLS ON LANDSLIDE EROSION IN THE EASTERN HIMALAYA

Landslide erosion is a key driver of landscape evolution in high relief terrain. However, little is known about the relative roles of tectonics and climate in driving landslide erosion at broad spatial scales. Within the eastern Himalaya there are strong spatial gradients in tectonic rock uplift and mean annual precipitation, making the region ideal for assessing controls on landslide erosion. We mapped more than 15,000 landslides, estimated their volumes, and quantified multi-decadal landslide erosion rates in the landscape surrounding the Tsangpo-Brahmaputra River as it flows through the Tsangpo Gorge and Siang Valley in the eastern Himalaya. We also measured in situ-produced ¹⁰Be in sediment to quantify watershed-scale erosion rates for 15 tributaries in the Siang Valley. We use the spatial pattern of exhumation rates estimated from published mineral cooling ages and satellite-derived precipitation data to assess the relative roles of tectonics and climate on landslide erosion. High rates of landslide erosion in the Tsangpo Gorge are spatially focused within a zone with high exhumation rates and low (< 1 m yr^-1) mean annual precipitation. Moreover, landslide erosion rates increase with increasing rates of exhumation and stream power and increase non-linearly as hillslope angles increase, which is diagnostic of tectonic-driven landslide erosion on threshold hillslopes. Mean annual precipitation increases by more than a factor of four to the south in the Siang Valley relative to the Tsangpo Gorge. Preliminary landslide mapping in the Siang Valley indicates that landslide erosion rates are much lower, as are the ¹⁰Be-based watershed-scale erosion rates, which is consistent with exhumation rates that are an order of magnitude lower than those in the Tsangpo Gorge. Together these results indicate that tectonics, rather than climate, is the main driver of decadal-scale landslide erosion and long-term exhumation in the eastern Himalaya.