Wrench faulting in the Northern Pakistan Foreland Region: Rethinking a deformed belt

Abstract

The acquisition, processing, and interpretation of reflection seismic, gravity, and magnetic data in conjunction with detailed geological mapping throughout the entire Kohat Plateau has required a re-interpretation of the structural style for this portion of the Northern Pakistan foreland region. Interpretation of the recently acquired data, combined with a critical re-evaluation of the regional geology and previously acquired data, suggest that the Kohat P la teau has undergone major ep i sodes o f wrench faulting, rather than only uni-directional thrust faulting. The subsurface structures that were imaged by seismic, were steeply dipping, of high amplitude, complexly faulted and cored by basement. Geologic studies of the regional tectonic setting, detailed surface mapping, predominance of horizontal motion fabrics in kinematic observation and focal mechanism solutions for historical earthquakes all indicate strike slip faulting. Gravity and magnetic data indicate basement involvement in the deformation. The data from seismic lines were processed by depth migration before stack (MBS). Our evaluation has resulted in the recognition that models employing low angle imbricate thrust faults and duplex structures may not be appropriate for the northern Pakistan foreland region. We recognized and applied a high amplitude variant of the “flower type” structural model for interpreting this type of deformation. This type of interpretation should be considered, where applicable, instead of a conventional two dimensional imbricate thrust models or classic f lower structures. These conventional models have been derived, in part, from older vintage seismic processing that was only able to image the simpler aspects of structures.