AAPG Bulletin, V. 82 (1998), No. 11 (November 1998), P. 2003-2030.

Wrench Faulting in the Northern Pakistan Foreland¹

William J. Sercombe,² David A. Pivnik,² Wayne P. Wilson,³ Martin L. Albertin³, Richard A. Beck,⁴ and Margaret A. Stratton⁵

©Copyright 1998.  The American Association of Petroleum Geologists.  All Rights Reserved

¹Manuscript received June 20, 1996; revised manuscript received October 10, 1997; final acceptance April 23, 1998.
²Gulf of Suez Petroleum Co. (GUPCO), P.O. Box 4381, Houston, Texas 77210.
³Amoco E & P Sector, P.O. Box 3092, Houston, Texas 77253.
⁴Department of Geology, Miami University, Oxford, Ohio 45056.
⁵Amoco Canada, Calgary, Alberta T2P 2H8, Canada.

We are indebted to the entire Amoco team that contributed to the exploration effort in Pakistan. We wish, in particular, to thank Jeffrey Barndt and the staff of Amoco Pakistan Exploration Company in Islamabad, and Javed Khan, Ahmed Hussain, and Rashid Tahirkheli, who unselfishly shared the incredible geology of their spectacular country with us. Potential-field data processing was completed by Earthfields Technology, and interpretations were assisted by Robert E. VanNieuwenhuise of PGS SERES Inc. Military protection in the North-West Frontier Province was provided by the Khyber Rifles, Tochi Rifles, and South Waziristan Scouts. Insightful reviews were provided by M. Coward, R. Raynolds, K. Biddle, and an anonymous reviewer. Some graphics were completed by David Orchard and the staff of Manzanita Graphics, Houston, Texas. Special mention is reserved for Michael J. Griffin, Ian R. Halliday, and Robert F. Wilson for their initial consultations. 

ABSTRACT

The acquisition, processing, and interpretation of reflection seismic, gravity, and magnetic data, in conjunction with detailed geological mapping throughout the Kohat Plateau, have resulted in a reinterpretation of the structural style of this portion of the northern Pakistan foreland. Our interpretation of the recently acquired geologic and geophysical data, as well as a critical reevaluation of previously acquired data and regional geology, indicates that the Kohat Plateau has undergone major episodes of strike-slip faulting, basement-involved reverse faulting, and minor thrust faulting consistent with transpressional tectonics. Our interpretation contrasts with previous models that invoked only southward-directed thrust faulting. The principal key to the reinterpretation of seismic data was depth migration before stack (MBS) seismic processing, which was performed on five seismic lines to resolve the structural geology. MBS-processed seismic data show complex deformation and steep dips more accurately than other methods, and allowed us to identify the high-amplitude structures in the Kohat Plateau. Surface geological mapping in the Kohat Plateau and adjacent areas indicates tight anticlines, consistent steep dips, horizontal motion fabrics, and a conspicuous lack of low-angle thrust faults. Aeromagnetic and gravity data indicate that Precambrian basement is involved in the deformation. Modern seismicity indicates that faults mapped at the surface and subsurface are dominated by strike-slip displacement. The Kohat Plateau is located near the intersection of the transcurrent Chaman fault system, a 1000-km-long system of north-south-striking sinistral strike-slip faults, and east-west-striking contractual structures that form the grain of the Himalayan mountain chain in this area of northwest Pakistan. Surface and subsurface structures fit the expected regional patterns that result from transpressional deformation in this complex zone of intersection. Our reinterpretation has applicability to other deformed belts worldwide that exhibit analogous characteristics and criteria of transpressional tectonics, but are interpreted as being dominated by low-angle thrust tectonics.