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Structural Evolution of Kohat Potwar Fold Thrust Belt of Pakistan

Abstract

The Kohat and Potwar in Pakistan represent the external zone of the Himalayan fold and thrust belt. The Main Boundary thrust marks their northern boundary yet they show significant variations in structural style along strike. In order to better understand the sequence of deformation and the kinematics of fold and thrusts in the Kohat and Potwar, we have conducted Apatite U-Th-Sm/He (AHe) and Apatite fission track (AFT) dating. We have systematically sampled Cambrian to Pliocene sandstones from different stratigraphic sections in the Salt, Surghar and Kohat Ranges. The AHe and AFT data from the Kohat suggest totally reset AHe and AFT ages. AHe data from the Salt Range show ages between 2 to 150 Ma; however, the majority of ages are less than 10 Ma. AFT ages are clustered around 200 Ma but show younger ages near the ramp and the northern part of the study area where the thickness of the stratigraphic wedge increases. Combining our thermochronology data with our structural reconstructions, we interpret the deformation history as follows: 1) synchronous movement on the MBT and out of sequence movement on the SGT in the Surghar Range at ~12 Ma. 2) To the south of the MBT in the Kohat, AHe and AFT ages date cooling while uplifting above the duplex: at 7 Ma on the Lachi thrust, synchronous with out-of-sequence deformation on the SRT, in the Salt Range. 3) The formation of Shakardarra-Karak thrust at 3 Ma is synchronous with the deformation in the Kalabagh Fault Zone and formation of the lobe in the western Salt Range. The spatial distribution of AHe and AFT ages in combination with the structures shown in balanced sections suggest that deformation south of MBT did not uniformly propagate towards the south. Instead, deformation was active on different structures at different time, causing an out of sequence evolution of the Kohat and Potwar fold and thrust belts.