GEOMORPHIC-GEODYNAMIC COUPLING AT THE OROGEN SCALE: A HIMALAYAN TRANSECT IN CENTRAL NEPAL

This project was designed to assess the intriguing proposition that climatically modulated erosion strongly influences spatial variations in deformation within a collisional orogen. We focused on the central Nepalese Himalaya where precipitation in the High Himalaya during the summer monsoon exceeds that in almost any other Himalayan catchment. We installed a dense, high-altitude meteorological network across the Annapurna Range that revealed a 10-fold gradient in monsoonal rainfall. The highest rainfall (>4 m/yr) is localized where relief abruptly increases ~15 km south of the range crest. The band of high rainfall roughly parallels the trace of the Main Central Thrust (MCT) and lies near the toe of the Greater Himalaya. A stream-gauging network defined a north-to-south increase in the modern sediment flux, ranging from ~1 to >3 mm/yr. Cosmogenic nuclide concentrations in river sediment on the range's south flank also indicate average erosion of >3 mm/yr over the past 200 years. Fission-track and Ar-Ar dating of valley-bottom and vertical-relief transects indicate that, across the trace of the MCT, cooling ages abruptly become much younger. The juxtaposition of ages across the MCT require several km of slip in the past 2 Myr. Moreover, these data define a striking acceleration in erosion rates within the MCT hanging wall beginning ~2.5 Ma. Whereas rapid erosion rates at all time scales and Quaternary motion on the MCT are all consistent with predictions of focused deformation where rainfall is highest, the fission-track data indicate no significant change in erosion rates for ~50 km farther north, deep into the rain shadow. How then does erosion remain rapid despite 10-fold less rainfall? First, hillslopes steepen as rainfall lessens. Second, large storms that drive the most erosion penetrate farther into the range, such that the storm rainfall gradient is only ~4-fold. Third, whereas glaciers today are retracted and eroding slowly, large expansions of glaciers as recently as 8 and 12 ka promoted rapid erosion in the dry hinterland. Ultimately, climate reigns as the pacemaker of Himalayan erosion. At annual scales, it induces landslides; at millennial scales, it drives phases of impulsive aggradation and erosion; and at still longer scales, it flips the switch between the dominance of fluvial versus glacial erosion.