Mechanics of the Salt Range-Potwar Plateau, Pakistan: A fold-and-thrust belt underlain by evaporites

The Salt Range and Potwar Plateau are part of the active foreland fold-and-thrust belt of the Himalaya in northern Pakistan. 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 decollement. 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 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 (Chapple, 1978; Davis et al., 1983; Dahlen et al., 1984; Dahlen, 1984). Models which include the effects of evaporites at the base (Chapple, 1978; Davis and Engelder, 1985) suggest that these thrust belts will have (1) narrow (< 1°) cross-sectional tapers, (2) larger widths than areas not underlain by evaporites, (3) symmetrical structures, and (4) changes in deformational style at the edge of the evaporite basin. The section across the eastern Potwar Plateau most closely resembles this latter model, having (1) a taper of 0.8° ± 0.1°, (2) a width of 100–150 km, (3) thrust faults that verge both to the north and south, and (4) 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 (τo) and the coefficient of internal friction of the overlying wedge (μ) are calculated as τo = 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, in the presence of the weak evaporite decollement, is sufficient to provide critical taper; no topographic slope is necessary, and the thrust wedge of the southern Potwar Plateau is pushed over the decollement without significant internal deformation. The northern Potwar Plateau is strongly folded and faulted, yet the topographic slope remains flat. Although the deformation suggests that evaporites are not present there, the observed taper in the northern Potwar Plateau is best fitted by the model with evaporites at the decollement. 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 southward propagating decollement encountered the evaporites. Between 2 Ma and the present, the northern Potwar Plateau has been pushed along the salt decollement without deformation, and erosion has reduced its original steep topographic slope to a nearly level surface.