Himalayan Geology, Vol. 43 (1B), 2022, pp. 253-261, Printed in India
5.5 Ma Reactivation of the Main Boundary Thrust in Kumaun Himalaya: Evidence from Fission-Track Dating
RASOUL SORKHABI1,2*, EDMUND STUMP1
1University of Utah, Energy & Geoscience Institute and Department of Geology & Geophysics, Salt Lake City, UT 84-18, USA
2Arizona State University, School of Earth and Space Exploration, Tempe, AZ 85287, USA
*Email (Corresponding author): rsorkhabi@egi.utah.edu
Abstract: The Main Boundary Thrust (MBT) is an orogen-scale compressional structure responsible for the shortening and thickening of the Himalayan crust as it has transported the Proterozoic Lesser Himalayan rock over the Neogene Siwalik sediments of the Sub-Himalaya. A Middle Siwalik sandstone sample collected from the contact zone with the MBT was analyzed for fission track (FT) dating of apatite crystals. Analysis of three slide mounts of apatite grains (48 in total) gave an FT age of 5.46 ± 0.69 Ma (with chi-squared test at 98%), recording the closure temperatures of ~110 °C, and corresponding to the brittle levels of the MBT's vertical profile. The FT age probably indicates the resetting of the FT clock in the rock by thrust loading (tectonic burial) and/or shear heating by the slip of the MBT at the Miocene-Pliocene boundary. The result and interpretation are consistent with various lines of evidence suggesting a major shift in the Sub-Himalayan sedimentation at 6-5 Ma as the Lesser Himalayan rocks on the hangingwall of the MBT supplied abundant detritus.
Keywords: Fission-track dating, Main Boundary Thrust, Siwalik Sandstone, Himalaya, Kumaun
