STEADY STATE BETWEEN UPLIFT AND DENUDATION IN AN ELEMENTARY CATCHMENT AFFECTED BY EPISODIC UPLIFT EVENTS:THE CASE OF THE HIMALAYAN FRONT

The aim of this paper is to describe the counterbalance of tectonic uplift and erosional denudation for an elementary catchment located at the front of a very active tectonic belt. The Holocene shortening rate at the front of Himalaya is estimated from terraces uplift above active structures. It is 14 mm/year along the Main Frontal Thrust of the western Nepal, and the motion along the MFT probably occurs episodically during major earthquakes. The Koilabas river has a drainage area of 17 km2, and its slope varies from 1.8% to 4.9% and this area is affected by a ~10 mm/yr uplift. Nine encased strath surfaces are found along this river to the north of the MFT. They have a slope very close to the river slope (nearly 2%), and they have been incised 3.1 m for T1, 5.8 m for T2, 7.2 m for T3, and 13.2 m for T4. Older terraces lie between 17 and 35 m above the river. The abandonment of parallel strath surfaces suggests incision at a rate similar to that of uplift and could represent sudden movement along the thrust fault. It is inferred that the ~ 1.5 m separation, that is observed very frequently could be related to uplift during the ~M8 earthquakes that affect the Himalayas. A strath surface located 4 m above the river is covered by a 1.8 m thick fluviatile level that overlaps and seals the MFT. This level corresponds to the 1984 catastrophic high flow of the river during the monsoon. The stream power river incision model is used to precise: 1) how an equilibrium profile develops in response to such a transient tectonic input; 2) the shear stress value that affect the flooded strath surfaces and the cause of their preservation.