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  • Himalayan collision produced frontal and lateral ramps and associated Pliocene to Quaternary tectonic geomorphic features in the foreland fold-thrust belt of northwestern Pakistan. The transpressional right-lateral Kalabagh tear fault zone displaced the emergent Surghar Range frontal thrust from the western Salt Range by 16-19 km since 1.9-2.1 Ma: the age of youngest Siwalik molasse strata erosionally truncated during the southward advance of decollement thrusting. Folds and fanglomerate deposits resulting from decollement thrusting were also cut by the Kalabagh fault. North of the eastern Surghar Range, the N15W-trending Kalabagh fault bends to the west into north-dipping thrust faults that sole out beneath the southern Kohat Plateau. Foreland subsidence associated with the southward advance of thrusting controlled the distribution of Indus River conglomerates during the late Pliocene and Quaternary. Uplift in the northern Kalabagh fault zone diverted the Indus river eastward to its present course. South of the Main Boundary thrust (MBT) and west of the Indus River, decollement thrusting dominated by layer-parallel slip of as much as 32 km on a single thrust fault emplaced blind thrust sheets and fault-bend folds. Balanced cross sections show over 50% line-length shortening in sedimentary strata between the top of the basement and the base of the elastic wedge of the Murree and Kamlial formations and the Siwalik Group. A NNWtrending basement ridge modeled from Bouguer gravity data may have nucleated the similarly-oriented Kalabagh fault lateral ramp along the western Salt Range. A basement fault may have produced the frontal ramp at the Surghar Range. Low friction along the basal decollement thrust, with rapid underthrusting and internal thickening, produced a subcritical wedge tapering approximately 2° across the 70-km-wide Kohat foreland fold-thrust belt between the MBT and the Surghar Range thrust.
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