STRUCTURAL ANALYSIS OF THE TRANS-INDUS RANGES: IMPLICATIONS FOR THE HYDROCARBON POTENTIAL OF THE NW HIMALAYAS, PAKISTAN

Abstract

The Trans Indus ranges constitute the western end of the frontal ranges that border the active foreland fold-and-thrust belt of the Himalaya in North Pakistan. Over 200 line km of 2D seismic data and sub-surface information of seven wells drilled in the surrounding areas have been integrated with surface geological information in order to understand the structural evolution of the Trans Indus ranges. Cambrian to Eocene platform succession unconformably overlain by a thick pile of fluviatile molasse deposits outcrop along the Trans-Indus ranges. The Trans-Indus ranges display pronounced structural swings in plan view, giving rise to a pair of re- entrants known under the name of Kalabagh and Tank. Both these re-entrants are flanked by wrench zones in the east and west that have produced contrasting styles within the Trans-Indus ranges. The structural styles include south-verging anticlinal- monoclinal ridge above a frontal fault, wrenching and compression related en-echelon anticlines and fault bend folds. The current investigations have led to the understanding that the structural evolution of the east-west and east-north-east oriented segments of the Trans Indus ranges is attributed to a south directed ramping whereas the north- south/northwest trending segments are related to wrenching concomitant with compression. The frontal bounding fault in the Trans-Indus ranges is less significant as compared to that along the Salt Range in terms of its level of decoupling and the magnitude of southward translation. The Himalaya-related deformation that shaped the Trans-Indus ranges is distinguished into three discrete episodes including pre-molasse, syn-molasse and post-molasse. The pre-molasse deformation includes the extensional tectonic activity resulted in the development of the normal basement faults with down thrown side to the north. The syn- molasse deformational episode is well demonstrated by the gradual south-younging sequence of molasse deposits in the Kohat Basin. The post-molasse deformational episode started in the region at the time when the macroscopic thrust slab underneath the Kohat-Bannu Basin ramped up section at the site of Trans Indus ranges. This ramping led to the creation of the present day Trans-Indus ranges and the internal deformation of the thrust slab which is still continuing. xvThe arcuate nature of the Trans Indus ranges is interpreted to be original, partly controlled by some pre-existing basement irregularities and possible strain partitioning. It is interpreted that the onset of compression at the northern and southern Surghar Range and Manzai ranges was synchronous caused by north-south and east-west compression induced by proto Main Boundary Thrust and Kurram Fault respectively. At the time when Proto MBT was transmitting south directed stresses at the site of the northern Surghar Range, left lateral wrenching along the Proto Kurram zone was transmitting east directed compressive stresses resulting in the north-south oriented folds in the Manzai and southern Surghar Range. The onset of thrusting is believed to be the earliest at the site of Surghar and Manzai ranges followed by wrenching along Pezu, Kundal and Makarwal faults. This wrenching episode was subsequently followed by thrusting at the site of Khisor and Marwat ranges. The stratigraphic succession and structural styles observed within the Trans Indus ranges offer a complete petroleum system comprised of multiple reservoir, source, seal rocks and traps including structural as well as stratigraphic for the accumulation of hydrocarbons.