Fold and Thrust Propagation in the Western Himalaya Based on a Balanced Cross Section of the Surghar Range and Kohat Plateau, Pakistan (1)

Abstract

Layer-parallel slip is dominant in a wedge tapering about 2 degrees across the 70-km-wide Kohat foreland fold and thrust belt between the Main Boundary thrust and the Surghar Range of northern Pakistan. Balanced cross sections show over 50% line length shortening by blind or partially emergent folding and thrusting, of which only about half is documented cover sequence deformation. Folds with 1 to 5-km wavelengths, thrust faults, and backthrusts occur in the Siwalik clastic wedge over buried tip lines and a zone of decoupling. Decollement thrusting advanced across the Kohat Plateau to the southernmost frontal thrust ramp of the Surghar Range about 2 Ma, before thrusting 12-20 km to the north of the Surghar Range in the northern Kalabagh fault zone. This out-of-sequence th usting is interpreted as a taper-building response to an increased decollement dip above the basement. The low critical taper of the Kohat-Surghar wedge is explained by the nearly flat basement and low basal shear traction along Eocene and Eocambrian evaporite layers.