THRUST GEOMETRIES AND KINEMATICS IN THE HIMALAYAN FORELAND (NORTH POTWAR DEFORMED ZONE), NORTH PAKISTAN

Hydrocarbon exploration and magnetostratigraphy from the Himalayan for eland (Salt Range/Potwar Plateau) in North Pakistan has provided new s urface/subsurface data and information on the tectonic evolution of an overthrust belt. A N-S balanced cross section constrained by seismic reflection and well data across the eastern North Potwar deformed zone (NPDZ) shows a buried thrust front with a triangle zone and ramp-and- flat structures. The triangle zone is characterized by a core wedge of an irregular geometry bounded between a floor thrust in the Eocambria n evaporites and a roof thrust in the Rawalpindi group strata. A blind thrust propagates upsection from a depth of approximately 7 km above the basement and merges in the roof thrust at a depth of approximately 3.7 km. The core wedge is bounded by the Soan backthrust and the expo sed S-verging Khairi-Murat thrust. The Khairi-Murat thrust soles out i n the main decollement and is overlain by a plane-roofed duplex horse over the hanging wall flat of the former structure. Both the Khairi-Mu rat thrust and the duplex are deformed by out-of-sequence thrusting. T he duplex geometry and out-of-sequence thrusting is responsible for th e absence of the Siwalik strata and complex deformation north of the K hairi-Murat thrust. Nearly all the seismic lines show a step of approx imately 600 m in the basement with hanging-wall block down-to-the-nort h displacement. The basement normal fault seems to provide a buttress for thrusting at the deformation front in the eastern NPDZ. Timing of Siwalik sedimentation (13.1-5.1 Ma) and thrusting(8-5 Ma) indicate sed imentation partly simultaneous to thrusting. We consider the possibili ty of piggyback thrusting and a disconformity prior to the development of a passive-roof duplex with backthrusting and a triangle zone in th e eastern NPDZ. A balanced structural cross section 40 km long is rest ored to an original length of 81 km. Forty one kilometres of shortenin g along balanced cross section added to the 14 km along the Golra thru st provides total shortening of approximately 55 km between 8 and 1.9 Ma in the NPDZ. Approximately 49.5 km of shortening has occurred betwe en 8 and 5 Ma. The shortening rate of 16.5 mm/a is roughly 33% of appr oximately 50 mm/a convergence rate between the Indian and Eurasian pla tes.