The Riwat thrust, with a surface trace of over 50 km, is one of the major faults in the footwall of the main boundary thrust in the Himalayan foreland of Pakistan. Surface geology shows that the Riwat thrust is a foreland-vergent thrust along which lower to middle Siwalik molasse strata are thrust southward over upper Siwalik strata. Seismic reflection interpretation shows that the Riwat thrust developed as a roof thrust of a hinterland-vergent tectonic wedge (triangle zone) underlain by evaporites. The Riwat thrust propagates upsection from a depth of about 4 km at the base of the Siwalik Group. At this depth, it merges into a hinterland-vergent blind thrust that propagates upsection as a ramp from Eocambrian evaporites covering the basement at a depth of about 6 km. Bounded between this set of conjugate faults, a tectonic wedge of Eocambrian (evaporites) to Neogene strata is thrust toward the hinterland to form a triangle zone.

The roof thrusts of triangle zones have been widely mapped as backthrusts in deformed mountain fronts. Hinterland motion of tectonic wedges as in the Riwat thrust triangle zone may be a feature of the fold-and-thrust belts underlain by evaporites acting as an extremely weak decollement layer. Their recognition, with a trap-forming geometry below a thrust, is important for interpreting particular fold 

©Copyright 1997. The American Association of Petroleum Geologists. All rights reserved.

¹Manuscript received August 28, 1995; revised manuscript received 
May 6, 1996; final acceptance October 17, 1996.

²Department of Earth Sciences, Quaid-i-Azam University, Islamabad, Pakistan.

³Eberhard-Karls-Universität Tübingen, Sigwartstrasse 10, 72076 Tübingen, Germany. (E-mail: [email protected])

I. A. K. Jadoon thanks the Alexander von Humboldt Foundation, Germany, for a postdoctoral research fellowship (1994-1996) and Tübingen University for use of its facilities.

We gratefully acknowledge A. Kemal (Oil and Gas Development Corporation of Pakistan; now Pakistan Oil Limited), N. K. Siddique (Pakistan Petroleum Limited), and A. Bhatti (Geological Survey of Pakistan) for their support in various ways, and S. M. Ali and R. Ahmed (Hydrocarbon Development Institute of Pakistan) for continuous encouragement, discussion, and guidance. S. Mitra, R. Raynolds, and C. Kluth were AAPG reviewers. Their critical comments were supportive and helpful in improving the first version of this manuscript.