Crustal shortening and tectonic evolution of the Salt Range in Northwest Himalaya, Pakistan Public Deposited


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  • The Salt Range is clearly the active participant in the scenario of the progressive southward migration of the Himalayan thrust front. It extends approximately 180 km ENE along strike and is underlain by salt. This is manifested by its very narrow (<1°) cross-sectional taper and great (150 km) width. Integration of approximately 450 km of seismic reflection data with available surface geologic, magnetostratigraphic, and exploration well data help in delineating different tectonic features in the Salt Range. These studies reveal a concealed duplex structure under the roof sequence, help to determine the footwall and hangingwall geometries of the leading edge at different successive evolutionary stages, estimate the lateral extent of a basement normal fault, constrain the ages of different structural features, and define lateral variations in the deformational style within the leading edge. The above mentioned features have been synthesized to document an out-of-sequence evolutionary model of the Salt Range. The newly recognized, concealed duplex structure extends more than 40 km along the strike and gradually progrades southward along a décollement that first ramps within the Salt Range Formation and then across the platform sequence and follows the shaley horizons of overlying Murree Formation near the contact. This duplex structure is terminated along the two lateral ramps in the east and west. The northern ramp in the footwall of the roof sequence is localized by a basement normal fault in the central Salt Range, and changes its position and characteristics in the eastern and western Salt Range. In the western Salt Range, it is located 15 km farther south and is entirely within the sedimentary sequence. These two segments are linked by a lateral ramp that developed over the western culmination wall of the lateral ramp associated with the underlying duplex structure. In the eastern Salt Range, however, the northern ramp first continues within the sedimentary sequence beyond the end of the basement normal fault and farther east it changes into an oblique ramp. This oblique ramp is truncated by another N-S trending lateral ramp farther to the east. The monoclinal structure of the Chambal Ridge marks the southernmost extension of this lateral ramp. Along this lateral ramp the roof sequence steps down and joins the basal décollement. Due to the down stepping of the roof sequence the structural style also changes from fault-bend fold to fault propagated fold geometry. Because in fault-bend fold geometry the major component of shortening was accommodated across the northern ramp, very little shortening occurred within the roof sequence. In contrast, all the shortening in the east has been distributed over a region in a prograde fashion. Therefore, the thrust wedge is internally deformed into a fault propagated fold geometry to provide a surface topographic slope necessary to maintain a critical taper. The concealed duplex structure is the earliest structure of the Himalayan thrust front that was formed during 9-7 m.y., and further suggests that out of sequence thrusting has occurred over a region of 150 km during the past 9 m.y. Due to the development of a basement normal fault at 7 m.y., the thrust acquired a high friction front and was unable to move forward. Crustal shortening was then taken up by the Main Boundary Thrust zone in the north, which was quite active during this time. Between 5-6 m.y., the thrust wedge started to ramp over the basement normal fault, facilitated by the development of a thick salt pad on the down-thrown side, during 7-6 m.y. The newly built topography due to the ramping of the thrust wedge resisted the southwards propagation of the roof sequence and caused further out-of-sequence thrusting in the north but was not sufficient to stop its southward progradation. It was followed by the major horizontal translation of the roof sequence over the roof sequence flat. This study also suggests that 13° counter clockwise rotation has occurred along the northern ramp and the concealed duplex structure. Recognition of the concealed duplex structure and better understanding of the footwall geometry of the roof sequence also generates new prospects of oil exploration in the Salt Range.
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