Fluvial incision and tectonic uplift across the Himalayas of central Nepal

The pattern of fluvial incision across the Himalayas of central Nepal is es timated from the distribution of Holocene and Pleistocene terraces and from the geometry of modem channels along major rivers draining across the rang e. The terraces provide good constraints on incision rates across the Himal ayan frontal folds (Sub-Himalaya or Siwaliks Hills) where rivers are forced to cut down into rising anticlines and have abandoned numerous strath terr aces. Farther north and upstream, in the Lesser Himalaya, prominent fill te rraces were deposited, probably during the late Pleistocene, and were subse quently incised. The amount of bedrock incision beneath the fill deposits i s generally small, suggesting a slow rate of fluvial incision in the Lesser Himalaya. The terrace record is lost in the high range where the rivers ar e cutting steep gorges. To complement the terrace study, fluvial incision w as also estimated from the modem channel geometries using an estimate of th e shear stress exerted by the flowing water at the bottom of the channel as a proxy for river incision rate. This approach allows quantification of th e effect of variations in channel slope, width, and discharge on the incisi on rate of a river; the determination of incision rates requires an additio nal lithological calibration. The two approaches are shown to yield consist ent results when applied to the same reach or if incision profiles along ne arby parallel reaches are compared. In the Sub-Himalaya, river incision is rapid, with values up to 10-15 mm/yr. It does not exceed a few millimeters per year in the Lesser Himalaya, and rises abruptly at the front of the hig h range to reach values of similar to4-8 mm/yr within a 50-km-wide zone tha t coincides with the position of the highest Himalayan peaks. Sediment yiel d derived from the measurement of suspended load in Himalayan rivers sugges ts that fluvial incision drives hillslope denudation of the landscape at th e scale of the whole range. The observed pattern of erosion is found to clo sely mimic uplift as predicted by a mechanical model taking into account er osion and slip along the flat-ramp-flat geometry of the Main Himalayan Thru st fault. The morphology of the range reflects a dynamic equilibrium betwee n present-day tectonics and surface processes. The sharp relief together wi th the high uplift rates in the Higher Himalaya reflects thrusting over the midcrustal ramp rather than the isostatic response to reincision of the Ti betan Plateau driven by late Cenozoic climate change, or late Miocene react ivation of the Main Central Thrust.