The mechanics of the Salt Range-Potwar Plateau, Pakistan : qualitative and quantitative aspects of a fold-and-thrust belt underlain by evaporites Public Deposited

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  • The collision of the Indian subcontinent with Asia beginning 40 million years ago produced the Himalayan orogenic belt, the largest continental collision belt active today. The foreland fold-and-thrust belt in northern Pakistan consists of the Salt Range-Potwar Plateau area. In this region the distance from the Main Boundary Thrust(MBT) to the front of the fold-and-thrust belt is very wide (100-150 km) because a thick evaporite sequence forms the zone of décollement. Recent studies have combined seismic reflection profiles, petroleum exploration wells, Bouguer gravity anomalies, and surface geology to construct cross sections in the eastern, central, and western Salt Range-Potwar Plateau areas. In this study the sections are compared with a previous model that considers the mechanics of a fold-and-thrust belt to be analogous to that of a wedge of snow or soil pushed in front of a bulldozer (Davis et al., 1983; Dahlen et al., 1984; Dahlen, 1984), and a later model (Davis and Engelder, 1985) which suggests that fold-and-thrust belts underlain by salt will have: a) narrow (< 1°) cross-sectional tapers, b) larger widths than areas not underlain by salt, c) symmetrical structures, and d) changes in deformational style at the edge of the salt basin. The section across the eastern Potwar Plateau most closely resembles this latter model, having: a) a taper of 0.8° ± 0.1°, b) a width of 100-150 km, c) thrust faults that verge both to the north and south, and d) structures rotated 30° counterclockwise with respect to the Salt Range. From the observed taper and pore fluid pressures of the eastern Potwar Plateau, estimates of the values for the yield strength of the evaporites (τ₀) and the coefficent of internal friction (μ) are calculated as τ₀ = 1.33-1.50 MPa and μ = 0.95-1.04, which are then applied to the other cross sections. In the central and western sections a basement uplift, the Sargodha High, interferes with the front of the fold-and-thrust belt. This feature causes the ramping of the Salt Range Thrust and produces a relatively steep basement slope (2°-4°) beneath the Potwar Plateau. This dip, together with the weak evaporite layer, allows the thrust wedge of the southern Potwar Plateau to be pushed over the décollement without undergoing internal deformation. In detail, the Salt Range ramping is caused by a large normal fault in the basement in the central section and the basement upwarp of the Sargodha High in the western section. The northern Potwar Plateau is strongly folded and faulted, yet the topographic slope remains flat. Although the deformation suggests that salt is not present there, the observed taper in the northern Potwar Plateau is best fitted by the model with salt at the décollement. Combining this with published paleomagnetic and geologic constraints, a model for the evolution of the northern Potwar Plateau suggests that the area deformed as a steeply tapered (3.5°-5.5°) thrust wedge until approximately 2 million years ago, when the décollement encountered the Salt Range formation. Between 2 m.y.a. and the present, the northern Potwar Plateau has been pushed along the salt décollement without deformation, and erosion has reduced its original steep topographic slope to a nearly level surface. The success of the mechanical model in predicting the observed features in the Salt Range-Potwar Plateau suggests that salt may lie beneath other fold-and-thrust belts in Pakistan. Two areas, the Sulaiman Lobe and the Karachi Arc, are possible candidates. Although published subsurface information is lacking in these areas, surface observations show that they both: a) extend far across the foreland, b) exhibit low topographic slopes, c) display symmetrical structures, and d) show a change in structural orientation along what is believed to be the edge of the salt basin.
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