Tariq Majeed Jaswal,² Robert J. Lillie,³ and Robert D. Lawrence³

Very gentle basement dip and almost zero topographic slope in the NPDZ suggest that the Eocambrian salt provides effective decoupling at the present position of the NPDZ. The strong deformation in the NPDZ appears to have developed farther north, in an area where the evaporites may be lacking. Since 2 Ma, the NPDZ moved farther south over the evaporites without further deformation, whereas erosion removed any former topographic slope. Restoring a balanced cross section suggests that the minimum shortening across the NPDZ is more than 55 km. Assuming that this shortening occurred between 5.1 and 2 Ma, the shortening rate is about 18 mm/yr.

INTRODUCTION

Crustal shortening due to northward underthrusting of the Indian plate beneath Eurasia continues to create active tectonic features on the northern fringes of the Indian craton since major collision began in the Eocene (Figure 1A). In the Himalayan foreland of Pakistan, thin-skinned tectonic features are developing in the Salt Range and Potwar Plateau (SR/PP) as a wedge of sediments is contracted and thrust southward along a décollement in Eocambrian evaporite beds (Figure 1B). The Dhurnal oil field structure is a product of this ongoing collision, located at the leading edge of an old deformation front in the northern Potwar deformed zone (NPDZ; Figure 2). 

About 3000 km of industry seismic reflection profiles from the SR/PP were provided to Oregon State University by the Oil and Gas Development Corporation (OGDC) and the Ministry of Petroleum and Natural Resources of Pakistan. These data, along with surface geologic, well, and gravity data, were used to construct the regional cross sections of the western, central, and eastern SR/PP (Baker, 1987; Leathers, 1987; Pennock, 1988) (Figure 1B). The same data set was used by Jaumé (1986) to study the mechanics of thrusting in the SR/PP, and by Duroy (1986) to analyze lithospheric flexure in 

The northern Potwar deformed zone (NPDZ) is part of the active foreland fold and thrust belt of the Salt Range and Potwar Plateau in northern Pakistan. About 500 km of seismic reflection profiles are integrated with surface geologic and drilling data to examine the deformation style and structure of the NPDZ with particular emphasis on history of deformation of the Dhurnal oil field. The seismic lines suggest that the overall structure of the eastern NPDZ is a duplex structure developed beneath a passive roof thrust. The roof thrust is generated from a tipline in the Miocene Murree Formation, and the sole thrust is initiated from the same Eocambrian evaporite zone that extends 
80 km southward beneath the Soan syncline and Salt Range. The Dhurnal oil field structure is a pop-up at the southern margin of the NPDZ, and developed beneath the passive roof thrust. The passive roof thrust crops out just north of Dhurnal on the steep, northern limb of the Soan syncline. An overstep passive roof thrust (Sakhwal fault) is interpreted west of Dhurnal; this fault developed due to