STRUCTURAL STYLE OF THE KOHAT PLATEAU, PAKISTAN: TRANSPRESSION TECTONICS, MULTIPLE DETACHMENT FAULTING, AND GRAVITY SPREADING

Fold-and-thrust belts form the geological backbone of Pakistan, and strike-slip faults are subordinate to the folds and thrust faults, with the possible exception of the Kohat Plateau where the main E-W structures have been recently explained as the result of transpression tectonics (Sercombe, Pivnik, et al, Bull.AAPG, 1998). Fault structures with a vertical offset of a few km were described as ‘flower’ structures. Seismology and faults with subhorizontal striae indeed indicate the presence of E-W strike-slip faults.

An overview of the structural style of the Kohat Plateau was derived from satellite imagery analysis, and near the village of Gandiali a 120 km2 area was mapped 1:25,000. Detachment occurred below the Jurassic in the Tolanj Anticlinorium, below the Ghazij shale (Paleocene) and in the Kuldana shale (Eocene). N-S shortening in the study area, mainly by tight upright folds, is 10-15 km min. whereas offset resulting from E-W strike-slip faults appears absent. The overturning of E-W oriented anticlines towards both N and S could be considered as the result of transpression tectonics (flower structure) but ‘collapse’ of a fold during its growth is an alternative explanation. The termination of minor folds at the Algad River in the Tolanj anticlinorium is also suggestive of ‘morphotectonics’.

The major structural features of the Kohat Plateau include a regional detachment fault below the Jurassic and local detachments in Eocene formations resulting in thrust sheets and folds. These detachment features indicate crustal compression, although the smallest klippen may have been emplaced gravitationally. The doubly-overturned folds and other small structural features probably resulted from gravity tectonics. Transpression tectonics is evident in the area of the N-S oriented Kalabagh fault.