Balanced structural cross section of the western Salt Range and Potwar Plateau, Pakistan : deformation near the strike-slip terminus of an overthrust sheet Public Deposited

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  • The Salt Range and Potwar Plateau of northern Pakistan are part of the thin-skinned, active thrust system related to the ongoing collision of the Asian and Indian continental blocks. Platform rocks and orogenic molasse of the Indo-Pakistani shield are deformed in south-verging thrusts and folds, relative to a northward-converging basement. This results in three different styles of deformation along a north-south, balanced cross section in the western Potwar Plateau and Salt Range. This cross section, as well as structure and isopach maps for the entire Salt Range/ Potwar Plateau region, were constructed using surface geologic maps, seismic reflection, drillhole, and gravity data. In the Northern Potwar Deformed Zone (NPDZ), shortening of 68% is accomodated by a trailing imbricate stack of high ad low-angle, south-verging faults. These faults are rooted in a master decollement within an Eocambrian evaporite sequence (Salt Range Fm.), which overlies a north dipping basement surface at a depth of approximately 8 km. Southward progression of regional deformation with time appears to be interrupted by overstep, or back-break, thrusting. The low-angle faults, or a possible basement uplift, may represent the ramp for a protracted interval of imbricate faulting that began in middle to late Miocene ([approximately] 15-10 mybp). Later, the basal detachment propagated rapidly, transporting the NPDZ sequence and the developing Soan Syncline (southern Potwar Plateau) southward, without major internal deformation within the Syncline. Within the last 2.0 my, the older units overlying the Salt Range Fm. have been exposed along the Salt Range, where the basal detachment ramped to the surface. A north-facing, subsurface normal fault, that offsets basement and controls frontal ramping along the central portion of the Salt Range, is not present in the western Salt Range. Rather, the older platform rocks and overlying molasse are carried southward up a basement monocline. This monocline is a protrusion on the northeast flank of the northwest-trending Sargodha Ridge, a buried basement arch that may represent a flexural response to Himalayan tectonic loading. Although the footwall, underlying the thrust plane, has not been drilled in the Salt Range, seismic control and two-dimensional gravity modeling suggest the presence of autochthonous sedimentary rocks, overlain by a thickened layer of evaporites, In the western Salt Range, the leading edge of the Salt Range Thrust (SRT) overrode its footwall block about 34 km. Additional shortening may have been accomodated by a series of imbricate thrusts in the Salt Range Fm., or the footwall strata may be imbricated. This would add northward taper to the basement dip, allowing the SRT to be exposed at the surface. An average shortening rate of approximately 1.3 cm/yr is comparable to the shortening rate for the NPDZ, if thrusting there occurred over a 7-8 my interval. Overthrust shortening along the SRT in the central Salt Range has been estimated at between 19 and 23 km. The larger amount of overthrusting in the western Salt Range, as compared to the central Salt Range, is consistent with the structurally-salient range front along this western edge. Minimum overall shortening, from the southern edge of the Peshawar Basin to the undeformed Jhelum Plain, is estimated at 213 km, a shortening of about 49%. This is comparable to the amount of fold and thrust shortening in other deformed forelands.
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