DOES LANDSLIDE EROSION KEEP PACE WITH EXHUMATION OF THE TSANGPO GORGE?

Erosion and crustal deformation in the eastern Himalayan syntaxis are thought to be so closely coupled that the landscape has reached a steady state, where denudation equals rock uplift. The steady state inference is based upon close spatial coupling of extreme topographic relief, high river incision potential, and young mineral cooling ages within the syntaxial antiform. Thermochronology data from the eastern syntaxis indicate exhumation rates of 3-10 mm yr^-1 have been sustained over million-year timescales and that rapid exhumation rates of 7-21 mm yr^-1 occur in the deeply incised Tsangpo Gorge. Though topographic metrics suggest the region is eroding rapidly, there are no erosion data to compare against the exhumation rates to explicitly test whether the landscape is in a steady state. We present preliminary landslide erosion data for a 1.3x10³ km² area comprising the sub-alpine portion of the Tsangpo Gorge. Fresh landslide scars were mapped on LANDSAT and ASTER images. Scar area was converted to landslide volume using empirical scaling relationships based on depth-area measurements from over 400 Himalayan landslides, which yielded a total landslide volume of 2.4x10⁸ m³. The time over which our landslide volume estimate is averaged is approximated by the time required for vegetation to obscure landslide scars, which is constrained to 10-40 years by field observations. These measurements indicate decadal-scale landslide erosion rates are on the order of 5-19 mm yr^-1. The close agreement between the short-term landslide erosion rates and long-term exhumation rates provides strong evidence that the Tsangpo Gorge is a steady-state landscape. The tight spatial coupling between the extremely rapid erosion and exhumation rates is consistent with the “tectonic aneurism” model and suggests the Tsangpo Gorge is a site of localized feedbacks among erosion, rock uplift, thermal weakening of the crust, and deformation.