Structural analysis and restoration of the Tolanj Anticlinorium, North Eastern Kohat Basin, Khyber Pakhtunkhwa, Pakistan

Abstract

The mapped area includes the northeastern Kohat Fold and Thrust Belt (KFTB) and a part of the Kohat ranges separated by Main Boundary Thrust (MBT) which carries Mesozoic-Paleocene sediments in hanging wall. The rock sequence in KFTB generally consist of Paleocene-Eocene rock assemblages, which are unconformably overlain by the Miocene terrestrial synorogenic foreland basin deposits. This study is focused on understanding the structural kinematics of a portion of the Tolanj and Landiwal anticlinorium within the KFTB incorporating the surface data and construction of viable models using Move suite. The mapped coordinates have three major, east west trending structures comprised of the Bazid Khel anticlinorium in the north, followed by Landiwal and Tolanj anticlinorium in the south. Paleocene to Eocene rock assemblages have occupied the core of these structures. Bazid Khel and Tolanj anticlinorium host Panoba Shale whereas the Landiwal anticlinorium is cored by Kohat Formation as the mapped coordinates covers its western end. Several major thrust faults are mapped which include Tolanj-Tarkhobi Fault, Shahi Khel Fault, Bagh Fault, Shindand Fault and MBT (from south to north) which are mostly detached within Eocene strata except the MBT. Five balanced cross sections were created which helped in predicting the subsurface projection of the surface structures. These cross sections were restored which show about 50% overall shortening of the stratigraphic package involved in the deformation. The tight anticlinal and broad synclinal fold structures mapped within the Eocene rocks have evolved as detachment folds which have been truncated by unique thrust faults. These structures are detached from the deeper structures by incompetent formations which are Patala and Panoba Shale. As a result, these formations have played a major role in the structural evolution and disharmony between surface and subsurface formations of study area. Majority of the regional thrust faults are splays from a regional basal decollement which is translating southwards in response. The sub-surface model illustrates that variations in structures along the trend are result of fickle and variant slip vectors of thrusts along their strike.