Balanced structural cross-sections of the central Salt Range and Potwar Plateau of Pakistan : shortening and overthrust deformation

Graduation date: 1988The Salt Range and Potwar Plateau are part of the currently active\ud foreland fold-thrust belt of the Himalaya in Pakistan. Seismic reflection data have been combined with surface mapping, drillhole and\ud paleomagnetic information to construct balanced structural cross-sections of the current frontal thrust zone (Salt Range), and to estimate\ud the amount of horizontal contraction and timing of deformation for the\ud central Salt Range/Potwar Plateau.\ud The data show that a 1 km offset of the basement acted as a\ud buttress that caused the central Salt Range/Potwar Plateau thrust sheet\ud to ramp to the surface, exposing Mesozoic and Paleozoic strata at the\ud range front. The basement offset may have been formed during Eocambrian\ud rifting; however, an origin related to Neogene crustal flexure is\ud supported. The frontal portion of the thrust sheet was passively\ud folded as it overrode the subthrust surface upon a thick, ductile layer\ud of salt. The proposed ramping mechanism includes deformation prior to\ud overriding the basement offset and suggests a smoother thrust trajectory than the ideal, stairstep profile. Balanced sections do not\ud indicate a lateral change in shortening in the central Salt Range, as\ud would be expected during proposed regional rotation during ramping.\ud Just north of the Salt Range, the thrust sheet is undeformed,\ud except for minor folding related to salt redistibution. Lack of\ud internal deformation in the Soan syncline is due to decoupling of\ud sediments from the basement along the thick Eocambrian salt layer.\ud Still farther north, in the Northern Potwar Deformed Zone (NPDZ), a\ud hinterland dipping duplex is disharmonic with mapped surface structure, suggesting an upper-level detachment. Duplexing ends southward in a\ud "triangle zone" geometry, and an apparent imbricate fan. Beneath the\ud northern limb of the Soan syncline, the duplex and imbricate fan are\ud separated by an oblique ramp. Section balancing suggests about 45 km\ud of shortening across the NPDZ.\ud The timing of structural events is constrained by stratigraphic\ud evidence, paleomagnetic dating of unconformities and sediment accumula\ud tion rates. Early to middle Pliocene (approx. 4.5 Ma) clastic deposition in the southern Potwar Plateau, previously interpreted in terms of\ud compressional deformation, may instead be due to passage of the peripheral bulge (Sargodha High) through the Indian lithospheric plate\ud causing basement normal faulting, uplift, and erosion. Although the\ud timing of deformation in the central Potwar Plateau is poorly constrained, the Salt Range thrust sheet is estimated to have overridden\ud the basement offset from 2.1-1.6 mya. Cross-section balancing gives a\ud minimum of 19 to 23 km of shortening across this frontal ramp. The\ud rate of Himalayan convergence which can be attributed to underthrusting\ud of Indian basement beneath sediments in the Pakistan foreland is there\ud fore 9-14 mm/yr. Deformation front migration appears to be controlled\ud by the distribution of salt and basement buttressing in the central\ud Salt Range/Potwar Plateau. Rapid facies progradation, measured in the\ud Jhelum Reentrant, is thought to be controlled by thrust propagation\ud rates and not indicative of typical foredeep migration. Restorations\ud suggest that the lower and middle Siwalik molasse depocenter was in the\ud southern Potwar Plateau, prior to jumping south of the Salt Range about\ud 2 mya