GEOMORPHIC EVIDENCE OF RECENT TECTONIC ACTIVITY: MCT ZONE, BHAGIRATHI RIVER VALLEY, GARHWAL HIMALAYA, INDIA

The Himalayan Main Central Thrust (MCT) is a 10 km thick shear zone that separates the Lesser Himalaya from the High Himalaya. The MCT zone had been thought to be inactive since the early Miocene (approx. 22 Ma), but recent earthquake activity (magnitude 6.6 quakes in 1991 and 1996) in the Garhwal Himalaya of northern India have caused us to reexamine this claim. This study reports some preliminary findings on the spatial-temporal distribution of slope failures and other geomorphic evidence of rapid landscape change within the MCT zone of the Garhwal Himalaya.

Slope failures were mapped in the Bhagirathi River drainage using IRS-1C geocoded false color composite images with a 23.5-meter resolution and Landsat7 panchromatic images with a 15-meter resolution. Topographic profiles were created for several valley sections from 40-60 meter DEMs. A topographic profile of four major river terraces on the downstream edge of Uttarkashi was drawn from a field GPS survey. The age of each terrace is being determined with cosmogenic isotope dating of rocks collected near the scarp of each terrace. Channel unit types were mapped for the section of the Bhagirathi River traversing the MCT zone, between Dabrani and Sainj. In mountainous terrain, the abundance of various channel unit types is generally related to channel gradient and the proximity of hillslopes.

Evidence to date confirms that the MCT zone is geomorphically active in recent times. At a given slope angle, slope failures occur more frequently inside the MCT zone than outside. Valley topographic profiles are more convex inside the MCT zone than outside of the zone. Within the MCT zone, channel units types in the Bhagirathi River show a poor relation with channel gradient, indicating the river is unable to adjust to sizes and amounts of sediment delivered from hillslopes. Cosmogenic isotope dating will be used to derive ages for large landslides, lateral moraines, and the river terraces to help determine the timing of geomorphic activity.