Wrench Faulting in the Northern Pakistan Foreland

Abstract

The acquisition, processing, and interpretation of reflection seismic, gravity, and magnetic data, in conjunction with detailed geological mapping throughout the Kohat Plateau, have resulted in a reinterpretation of the structural style of this portion of the northern Pakistan foreland. Our interpretation of the recently acquired geologic and geophysical data, as well as a critical reevaluation of previously acquired data and regional geology, indicates that the Kohat Plateau has undergone major episodes of strike-slip faulting, basement-involved reverse faulting, and minor thrust faulting consistent with transpressional tectonics. Our interpretation contrasts with previous models that invoked only southward-directed thrust faulting. The principal key to the reinterpretation of seismic data was depth migration before stack (MBS) seismic processing, which was performed on five seismic lines to resolve the structural geology. MBS-processed seismic data show complex deformation and steep dips more accurately than other methods, and allowed us to identify the high-amplitude structures in the Kohat Plateau. Surface geological mapping in the Kohat Plateau and adjacent areas indicates tight anticlines, consistent steep dips, horizontal motion fabrics, and a conspicuous lack of low-angle thrust faults. Aeromagnetic and gravity data indicate that Precambrian basement is involved in the deformation. Modern seismicity indicates that faults mapped at the surface and subsurface are dominated by strike-slip displacement. The Kohat Plateau is located near the intersection of the transcurrent Chaman fault system, a 1000-km-long system of north-south-striking sinistral strike-slip faults, and east-west-striking contractual structures that form the grain of the Himalayan mountain chain in this area of northwest Pakistan. Surface and subsurface structures fit the expected regional patterns that result from transpressional deformation in this complex zone of intersection. Our reinterpretation has applicability to other deformed belts worldwide that exhibit analogous characteristics and criteria of transpressional tectonics, but are interpreted as being dominated by low-angle thrust tectonics.