Abstract: Structural Style and Evolution of the Sibi Trough, Pakistan

GOFFEY, GRAHAM, and JOHN POTHECARY

The Sibi Trough forms a tapering embayment between foldbelts of the Northern Kirthar Ranges and Sulaiman Lobe. These foldbelts expose Jurassic to Eocene rocks which are many kilometers above their regional level in the adjacent foreland. The trough contains in excess of 7 km of Miocene to Pleistocene molasse resting on an attenuated pre-molasse section and folded into a series of apparently arcuate anticlinoria. Satellite imagery, geological fieldwork, magnetic and gravity modeling and reprocessed seismic data have been integrated in the course of evaluating the hydrocarbon prospectivity.

The trough is shown to be underlain by a basement high, while basement graben underlie the adjacent foldbelts at considerable depth. The graben fill has been shortened and elevated by thin-skinned duplexing. Across the trough, duplex structures show a 90 degrees difference in vergence direction. This can be explained by different modes of transpressional structuration during oblique collision, modified by the pre-existing structural grain.

Monoclinally folded molasse at the margin of the trough marks the limit of duplexing. The roof thrust of these duplexes continues into the trough, causing fault propagation folding at thrust terminations. Minor out-of-syncline detachments originate within the molasse in the fold forelimbs. These detachments form a parasitic break-back sequence and ramp up-section above underlying structures thereby creating shallower folds which obscure the deeper structure. Suites of east-south-east and originally south vergent structures interfere within the trough. The latter suite has apparently been rotated by 40-50 degrees through continued forward translation of the adjacent duplex.

AAPG Search and Discovery Article #90942©1997 AAPG International Conference and Exhibition, Vienna, Austria