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Tectonic Wedge Below the Riwat Thrust, Himalayan Foreland, Pakistan: Implications
for Hydrocarbon Exploration1
Ishtiaq A. K. Jadoon and W.
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
1997. The American Association of Petroleum Geologists. All rights reserved.
received August 28, 1995; revised manuscript received
May 6, 1996;
final acceptance October 17, 1996.
of Earth Sciences, Quaid-i-Azam University, Islamabad, Pakistan.
Tübingen, Sigwartstrasse 10, 72076 Tübingen, Germany. (E-mail:
I. A. K. Jadoon
thanks the Alexander von Humboldt Foundation, Germany, for a postdoctoral
research fellowship (1994-1996) and Tübingen University for use of
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.
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