Geometry of a Concealed Trap Below the Riwat Thrust, Himalayan Foreland, Pakistan
JADOON, ISHTIAQ A. K.
The foreland-directed Riwat thrust is one of the major faults in the Himalayan foreland of north Pakistan. Based on seismic-reflection data, the Riwat thrust developed as a roof thrust of a hinterland-vergent tectonic wedge (triangle zone) underlain by evaporites. The Riwat thrust propagates up-section from a depth of about four km at the base of the Siwalik Group. At this depth, it merges into a hinterland-vergent blind thrust that propagates up-section as a ramp from Eocambrian evaporites covering the basement at a depth of about six km. Bounded between this set of conjugate faults, a tectonic wedge of Eocambrian (evaporites) to Neogene strata is thrust towards the hinterland to form a triangle zone.
The roof thrusts of triangle zones were widely mapped as back-thrusts. Recognition of hinterland motion of tectonic wedges as in the Riwat thrust triangle zone with a trap forming geometry below a foreland-directed thrust is important for the interpretation of particular fold belts and for hydrocarbon exploration. These structures could be predicted by the surface geology data where hinterland vergence of a fold below a thrust is apparent. However, seismic reflection data apears to be critical for their recognition.