Post-emplacement kinematics and exhumation history of the Almora klippe of the Kumaun–Garhwal Himalaya, NW India: revealed by fission track thermochronology

Abstract

Tectonically transported crystalline thrust sheet over the Lesser Himalayan meta-sedimentary zone along the Main Central Thrust (MCT) is represented by Almora, Baijnath, Askot and Chiplakot crystalline klippen. The Almora–Dadeldhura klippe in the Kumaun–Garhwal and western Nepal Himalaya is the witness and largest representative of these crystalline klippen, south of MCT. Here, we investigate the post-emplacement kinematics and exhumation history of the Almora klippe. The newly derived zircon fission track (ZFT) ages combined with published apatite fission track (AFT), 40Ar–39Ar ages from the Almora–Dadeldhura klippe and Ramgarh thrust sheet to quantify the temporal variation in cooling ages and exhumation rates. Using 1-D numerical modelling approach, we calculate the transient exhumation rates with respect to different time intervals. New ZFT cooling ages along ~50-km-long orogeny perpendicular transect across the Almora klippe range between 13.4 ± 0.6 and 21.4 ± 0.9 Ma. Published AFT ages and 40Ar–39Ar ages from the Almora–Dadeldhura klippe range 3.7 ± 0.8–13.2 ± 2.7 and 18.20–25.69 Ma, respectively. AFT ages reported from Ramgarh thrust sheet range 6.3 ± 0.8–7.2 ± 1.0 Ma in Kumaun region and 10.3 ± 0.5–14.4 ± 2.2 Ma in western Nepal. The linear age trend along with youngest ZFT age (~14 Ma) close to the North Almora Thrust (NAT) in its hanging wall suggests rapid uplift close to the NAT due to its reactivation as back thrust. The transient exhumation rates of Almora klippe agree that the erosion rate was rapid (0.58 mma−1) close to the NAT in its hanging wall and relatively slow (0.31 mma−1) close to the SAT in its hanging wall during 15–11 Ma. We interpret that the fission track ages and transient exhumation rate pattern of crystalline klippen show a dynamic coupling between tectonic and erosion processes in the Kumaun–Garhwal Himalaya. However, the tectonic processes play the major role in controlling the exhumation pattern.