CLIMATIC AND TECTONIC INFLUENCES ON CATCHMENT-MEAN DENUDATION RATES IN THE ARUN VALLEY, EASTERN NEPAL

A link between climate, tectonics, and denudation has been widely debated in the Himalaya and other orogens, with fractured consensus. We investigate possible connections between driving factors using 10-Berrylium cosmogenic radionuclides (CRN) and Digital Elevation Model (DEM) analysis in the Arun Valley of eastern Nepal. One of the largest trans-Himalayan rivers, the Arun forms a natural cross-section through all major Himalayan geological units and structures from Tibet to the Ganges Plain. The valley receives large amounts of Indian Summer Monsoon (ISM) rainfall at the mountain front (>2 m yr^-1); however, due to the steep orographic barriers of the Lesser Himalaya (LH) and Higher Himalaya (HH), local rainfall varies significantly along a steep, two-tiered north-south gradient. Topography also varies from south to north, with elevation ranging from ~0.1 km to 8.4 km asl. Hillslope and channel steepness show a step-wise increase coincident with the topographic rise of the LH and HH. Abundant ISM precipitation and warm annual temperatures in the deeply incised Arun Gorge result in dense vegetation cover, ranging from tropical to alpine.

In order to quantify denudation variability in the Arun with respect to climate, vegetation, and tectonics, we collected 48 river sand samples for catchment-mean CRN analysis. Samples were collected from the main stem of the Arun and its tributary watersheds. Denudation rates across the Arun valley range from 0.33 ± 0.03 mm yr^-1 (n=10) in the Lesser Himalaya to 0.77 ± 0.07 mm yr^-1 (n=14) in the Higher Himalaya. Catchment-mean denudation rates do not strongly correlate with standard uplift-related topographic metrics (e.g., slope, channel steepness, relief), and negligible relation to lithology or mean annual rainfall. Instead, denudation is best described by TRMM-weighted specific stream power (SSP), while vegetation density appears to buffer erosion in landslide-dominated catchments. We argue that, in contrast to recent studies in the central and western Himalaya, centennial to millennial denudation in the Arun Valley is not only dominated by tectonics. While tectonic uplift sets the large-scale spatial pattern of denudation, we conclude that the interplay of multiple factors and not a single tectonic or climatic signal drive denudation.