Depositional response to Pliocene–Pleistocene foreland partitioning in northwest Pakistan

Pliocene–Pleistocene synorogenic deposits of the Upper Siwalik Group in the northwest Pakistan foreland record changes in dispersal patterns, provenance, and sedimentary facies in response to Himalayan contraction. Three depocenters in the northern Pakistan foreland have been studied: the Peshawar and Campbellpore Basins, situated north of the ranges associated with the Main Boundary fault; and the Soan syncline depocenter, situated in the northern Potwar Plateau, south of the Main Boundary fault. In the Peshawar Basin, an early generation of northward-prograding alluvial fans and gravelly braided streams deposited at ca. 3.0 Ma was folded by en echelon pressure ridges bounded by high-angle reverse faults between 1.7 and 1.4 Ma. The alluvial fan and fluvial deposits were entrenched and infilled by a second generation of northward-flowing braidplain deposits. In the Campbellpore Basin, southwestward-directed fluvial and alluvial-fan deposition in response to uplift of the Kawa Ghar hills began at ca. 1.8 Ma. In the Soan syncline depocenter, deposition occurred at ca. 2.0 Ma as southeastward-flowing braided streams emanating from the Khair-i-Murat uplift, and a southwestward-flowing, gravelly, meandering river derived from highlands to the northeast. Previous stratigraphic and structural interpretations of this part of the Himalayan foreland have included north-dipping, imbricated thrust faults and duplex structures to characterize the deformation patterns in the region. Recently acquired seismic and other geophysical and surficial structural data from the western part of the foreland indicate that the foreland has experienced strike-slip as well as contractile deformation. The structures that controlled depositional patterns of the Upper Siwalik Group are localized uplifts bounded by high-angle reverse faults and not regional thrust faults. The pattern of Pliocene–Pleistocene uplift and resultant deposition across the foreland does not indicate a progression of deformation in either the foreland or hinterland directions, typical of fold-and-thrust belts.