DEPOSITIONAL RESPONSE TO PLIOCENE-PLEISTOCENE FORELAND PARTITIONING IN NORTHWEST PAKISTAN

Pliocene-Pleistocene synorogenic deposits its of the Upper Siwalik Gro up in the northwest Pakistan foreland record changes in dispersal patt erns, provenance, and sedimentary facies in response to Himalayan cont raction. 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 northwa rd-prograding alluvial fans and gravelly braided streams deposited at ca, 3.0 Ma was folded by en echelon pressure ridges bounded by high-an gle reverse faults between 1.7 and 1.4 Ma. The alluvial fan and fluvia l deposits were entrenched and infilled by a second generation of nort hward-flowing braidplain deposits. In the Campbellpore Basin, southwes tward-directed fluvial and alluvial-fan deposition in response to upli ft of the Kawa Ghar hills began at ca. 1.8 Ma. In the Soan syncline de pocenter, deposition occurred at ca. 2.0 Ma as southeastward-flowing b raided streams emanating from the Khair-i-Murat uplift, and a southwes tward-flowing, gravelly, meandering river derived from highlands to th e northeast. Previous stratigraphic and structural interpretations of this part of the Himalayan foreland have included north-dipping, imbri cated thrust faults and duplex structures to characterize the deformat ion patterns in the region. Recently acquired seismic and other geophy sical and surficial structural data from the western part of the forel and indicate that the foreland has experienced strike-slip as well as contractile deformation. The structures that controlled depositional p atterns of the Upper Siwalik Group are localized uplifts bounded by hi gh-angle reverse faults and not regional thrust faults. The pattern of Pliocene-Pleistocene uplift and resultant deposition across the forel and does not indicate a progression of deformation in either the forel and or hinterland directions, typical of fold-and-thrust belts.