Balanced and retrodeformed geological cross-section from the frontal Sulaiman Lobe, Pakistan: Duplex development in thick strata along the western margin of the Indian Plate

Abstract

A balanced structural cross-section has been constructed integrating seismic reflection profiles, drillhole, surface geology, and Landsat data across the technically active frontal Sulaiman fold belt in the western Himalayas. Restoration of the section provides information regarding the chronology of structures, structural style, sequence of thrusting, and the amount of shortening. General structural form evidenced by gentle topography and a broad fold belt is similar to that of other mountain belts underlain by a weak detachment. A sequence of about 10 km of dominantly platform (>7 km) and molasse strata thickens tectonically to about 15 km, 129 km north of the southwards verging deformation front. Nearly all of the 10 km thick stratigraphic sequence has been detached at the deformation front. Structural style is that of a foreland-verging duplex separated from the roof sequence by a passive-roof thrust in thick Cretaceous shale. This structure is expressed at the surface by fault-related folds. Toward the northerly hinterland, progressively older rocks are present at the surface in the hinge zones of the anticlines. They have been uplifted by duplexing several kilometres higher than their regional stratigraphic level. The passive-roof thrust has not been cross-cut by backthrusts, and it is present over a distance of 60 km along the line of section. Progressive deformation reveals a series of structural and geometric features including: (1) broad concentric folding at the fault tip; (2) development of a passive roof and duplex sequence by forward propagation of floor and roof thrusts; (3) forward propagation of the duplex as critical taper is achieved; and (4) tear faults and extensional normal faults within the overthrust wedge. A retrodeformed cross-section shows that about 76 km of orogenic contraction in the cover sequence has occurred across the frontal 129 km of the Sulaiman fold belt.