TRANSITION FROM FORELAND BASIN TO PIGGYBACK BASIN DEPOSITION, PLIOPLEISTOCENE UPPER SIWALIK GROUP, SHINGHAR RANGE, NW PAKISTAN

Plio-Pleistocene synorogenic deposits of the Upper Siwalik Group in th e Shinghar Range (Trans-Indus Salt Ranges) of north-western Pakistan r ecord the transition from foreland-basin to piggyback-basin deposition on the hangingwall of the Salt Range thrust. The Siwalik and Upper Si walik Groups are over 4 km thick in the Shinghar Range. The lower 3 km consists of the Miocene Siwalik Group, which was deposited by a south -flowing foreland trunk stream, the palaeo-Indus River. The upper 1.5 km consists of the Upper Siwalik Group, which is herein divided into t hree members. The lowest member includes deposits of the south-flowing palaeo-Indus River and is distinguished from the underlying Siwalik G roup by the first appearance of conglomerate. The transition from the lower member to the middle member is interpreted as recording uplift o n the Salt Range thrust. As the Salt Range thrust was active, the pala eo-Indus River was bifurcated to the east and west around the embryoni c Shinghar Range and overbank and lacustrine deposition occurred, repr esented by the middle member. When the Shinghar Range achieved signifi cant topography, the upper member was deposited by streams transportin g gravel and sand that flowed north and west out of the range and into a piggyback basin that formed on the hangingwall of the Salt Range th rust. New and previously published palaeomagnetic stratigraphy and fis sion-track ages from volcaniclastic deposits within the Upper Siwalik Group provide tight constraints on the chronology of sedimentary-facie s transitions and timing of uplift of the Shinghar Range. The integrat ion of sedimentological and geochronological data indicates that motio n on the Salt Range thrust and repositioning of the Indus River began at similar to 1.0 Ma.