Landslide erosion coupled to tectonics and river incision

The steep topography of mountain landscapes arises from interactions among tectonic rock uplift, valley incision and landslide erosion on hillslopes. Hillslopes in rapidly uplifting landscapes are thought to respond to river incision into bedrock by steepening to a maximum stable or 'threshold' angle(1-3). Landslide erosion rates are predicted to increase nonlinearly as hillslope angles approach the threshold angle(1-7). However, the key tenet of this emerging threshold hillslope model of landscape evolution-the coupled response of landslide erosion to tectonic and fluvial forcing-remains untested. Here we quantify landslide erosion rates in the eastern Himalaya, based on mapping more than 15,000 landslides on satellite images. We show that landslide erosion rates are significantly correlated with exhumation rates and stream power and that small increases in mean hillslope angles beyond 30 degrees translate into large and significant increases in landslide erosion. Extensive landsliding in response to a large outburst flood indicates that lateral river erosion is a key driver of landslide erosion on threshold hillslopes. Our results confirm the existence of threshold hillslopes and demonstrate that an increase in landslide erosion rates, rather than steepened hillslope angles, is the primary mechanism by which steep uplands respond to and balance rapid rates of rock uplift and bedrock river incision in tectonically active mountain belts.