Eroding the Himalaya; topographic and climatic control of erosion rates and implications for tectonics

New geomorphic tools for determining regional erosion rate patterns provide opportunities to better understand the tectonics of active orogens so long as short-term erosion rates faithfully reflect longer-term exhumation rates. Here we test the applicability of this approach to the Himalayan orogenic system and explore implications for patterns of active rock uplift. The transition between the rugged High Himalaya to the north and the more subdued foothills of the Low Himalaya is marked by a sharp physiographic boundary we term the High Himalayan Topographic Front (HHTF). We show that the HHTF is continuous for >2100 km along strike of the Himalayan arc, from 73 degrees E to 92 degrees E. As the HHTF is associated with both a pronounced increase in topographic relief and high orographic rainfall to the north, it is most sharply defined from a geomorphic perspective by discharge-based channel steepness, ksnQ - a proxy for flood unit stream power. This is significant because we find a single, robust correlation between ksnQ and published catchment-mean millennial-scale erosion rates in the Himalaya. Capitalizing on this relationship, we show that erosion rates estimated from ksnQ exhibit a strong inverse correlation with low-temperature thermochronologic cooling ages. This implies that topography, modern rainfall, and millennial-scale erosion rates are useful for inferring long-term tectonic deformation patterns in the Himalaya. Although the tectonic implications of this pattern remain a subject of much debate for the central approximately 1270 km of the Himalayan arc (76-89 degrees E), the HHTF corresponds to active frontal or out-of-sequence thrust faults for the 40% of the Himalayan arc west of 76 degrees E and east of 89 degrees E, suggesting that this may be true for the debated central HHTF as well. Here we show that the HHTF defines a continuous, significant northward increase in millennial-scale erosion rate and inferred Plio-Pleistocene exhumation rates along the entire the Himalayan arc, suggesting that it has been a persistent tectonic and physiographic feature for the past several million years.