WRENCH FAULTING IN THE NORTHERN PAKISTAN FORELAND

The acquisition, processing, and interpretation of reflection seismic, gravity, and magnetic data, in conjunction with detailed geological m apping throughout the Kohat Plateau, have resulted in a reinterpretati on of the structural style of this portion of the northern Pakistan fo reland. Our interpretation of the recently acquired geologic and geoph ysical data, as well as a critical reevaluation of previously acquired data and regional geology, indicates that the Kohat Plateau has under gone major episodes of strike-slip faulting, basement-involved reverse faulting, and minor thrust faulting consistent with transpressional t ectonics. Our interpretation contrasts with previous models that invok ed only southward-directed thrust faulting. The principal key to the r einterpretation of seismic data was depth migration before stack (MBS) seismic processing, which was performed on five seismic lines to reso lve the structural geology. MBS-processed seismic data show complex de formation and steep dips more accurately than other methods, and allow ed us to identify the high-amplitude structures in the Kohat Plateau. Surface geological mapping in the Kohat Plateau and adjacent areas ind icates tight anticlines, consistent steep dips, horizontal motion fabr ics, and a conspicuous lack of low-angle thrust faults. Aeromagnetic a nd gravity data indicate that Precambrian basement is involved in the deformation. Modern seismicity indicates that faults mapped at the sur face and subsurface are dominated by strike-slip displacement. The Koh at PIateau 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 tha t form the grain of the Himalayan mountain chain in this area of north west Pakistan. Surface and subsurface structures fit the expected regi onal patterns that result from transpressional deformation in this com plex zone of intersection. Our reinterpretation has applicability to o ther deformed belts worldwide that exhibit analogous characteristics a nd criteria of transpressional tectonics, but are interpreted as being dominated by low-angle thrust tectonics.