Compression- and transpression-related deformation in the Kohat Plateau, NW Pakistan

Abstract

Abstract Detailed surface mapping and recently acquired subsurface data from the Kohat Plateau of northwestern Pakistan show complex structural styles created by the overprinting by transpression-related structures of pre-existing compression-related structures. The Kohat Plateau is roughly divided into two structurally distinct regions; a northwest region (NW) and a southeast region (SE). Surface structures in the Plateau include doublyoverturned folds (both limbs overturned) which incorporate a lower Eocene to Pleistocene stratigraphic succession. Intensely folded with the stratigraphy in the NW region is a relict thrust belt, representing an earlier compressional episode of deformation. In the SE region many folds (related to wrench faulting at depth) have had either their southern or northern limbs faulted out by high-angle reverse faults which flatten near the surface along a Lower Eocene shale and evaporite horizon. In the NW region, lower Eocene evaporites and limestones are replaced by thick shales which served as a fault-flat forming horizon. This is believed to have hindered the surfaceward propagation of faults, leaving detachment folds exposed at the surface. Steeply dipping strata at depths of over 4 km, steeply dipping faults that become shallow near the surface forming positive flower structures, large vertical displacements across faults, and left-stepping folds suggest transpressional rather than purely compressional deformation. Regions adjacent to the Kohat Plateau also contain transpression-related structures, rather than thin-skinned imbricate thrusts. In view of the position of the Kohat Plateau near a major syntaxial bend in the Himalayan orogen, both north-south and east-west convergence played a dominant role in creating structural styles, and the combination of these two convergence directions influenced the development during the Plio-Pleistocene of transpression-related structures.