Thrusting sequence in the Western Himalayan foreland basin during the late phase of continental collision defined by low-temperature thermochronology

Abstract

Foreland basins associated with collisional mountain belts undergo postdepositional deformation in response to compressive stress exerted from the evolving orogen. The low thermal regime of such processes requires sensitive methods to unravel the structural evolution of such regions. In this study, we conducted apatite fission track (AFT) and (U-Th)/He thermochronology on the sedimentary succession of the Subathu basin in the Himalayan foreland of the Himachal Pradesh region in India, an area preserving the sedimentary record of the India-Asia collision since the late Paleocene-early Eocene. The results show that the Subathu basin exposes a fossilised apatite partial annealing zone (APAZ) depicted by incompletely reset AFT ages and shortened track lengths in the Paleocene-middle Miocene pre-Siwalik units and unreset ages in the Neogene Siwalik Group. We associate the partial annealing of the pre-Siwalik rocks with thickening due to internal imbrication and nappe stacking triggered by shearing along the Main Boundary Thrust and its splays since the middle Miocene. Thermal modelling of the Siwalik rocks projects a rapid heating up to approximately 90–100 °C reached between 4 and 1 Ma. Following the thermal peak, the Siwalik rocks cooled rapidly, as indicated by 2.6–1.2 Ma AHe ages. We associate this recent cooling event with SW-directed thrusting along the Main Frontal Thrust, which placed the foreland basin over the Gangetic Plains. Our results are similar to those obtained for the foreland basin in Nepal, indicating that the movement along the youngest major discontinuity of the Himalaya is synchronous along the orogen.