| Abstract
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Compressional and transpressional tectonics affected the northern Pakistan foreland region during the Miocene, Pliocene and Pleistocene. Sequences of the Upper Siwalik Group, exposed in the Peshawar Basin, Campbellpore Basin, Shinghar Range, Soan Syncline and Hazara Basin record the sedimentary response to the various deformational events that shaped the foreland. Three main episodes of deformation are recorded. The first, at 3.0 My, corresponds to uplift of the Attock-Cherat Range which initiated alluvial-fan and fluvial sedimentation in the Peshawar Basin. Also at 3.0 My, the Riwat fault in the Potwar Plateau altered drainage patterns of the previously south-flowing Jhelum River and created an angular unconformity within the Upper Siwaliks. An influx of conglomeratic facies in the Upper Siwaliks in the Shinghar Range at 2.5 My may reflect uplift of regions to the north. The second episode, from 2.0 to 1.5 My corresponds to uplift of a series of pressure ridges in the southern Peshawar Basin, creating an angular unconformity within the Upper Siwaliks; uplift of one or more pressure ridges in the Campbellpore Basin, which initiated alluvial-fan and fluvial deposition; uplift of Khair-i-Murat (and other structures?) in the Potwar Plateau, which created an angular unconformity within the Upper Siwaliks; and an influx of conglomeratic facies in the Upper Siwaliks exposed in the Shinghar Range. The third episode, at 1.0 My, corresponds to uplift of the Trans-Indus Salt Range. As uplift along the Salt Range thrust occurred, deposition by the south-flowing paleo-Indus River was replaced by deposition in a piggyback basin that formed on the hanging wall of the thrust. Detailed mapping and seismic-data acquisition in the Kohat Plateau show that many of the structures in the Kohat Plateau, Potwar Plateau and the mountain ranges related to the Main Boundary thrust (Kohat, Attock-Cherat, Kala Chitta and Hazara-Margala Hill Ranges) are related to transpressional wrench faulting. Previous structural interpretations have used thin-skinned, imbricate thrust models to explain structures in the region. Instead, the foreland is a complex hybrid of compression-related and transpression-related structures. It is suggested that as the Himalayan deformational front impinged upon the foreland, thin-skinned thrusting along the Main Boundary thrust occurred in the Miocene, with the sedimentary wedge decoupled from basement. As convergence progressed, basement became involved in the deformation and wrench-related features overprinted previously-developed compressional structures. At present, the youngest deformation in the region, uplift of the Trans-Indus Salt Ranges, is characterized by thin-skinned, imbricated thrusts which are offset by younger strike-slip faults.
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