Abstract

Surface studies of structure and stratigraphy have been combined with seismic reflection, drill hole, gravity and paleomagnetic data to examine the general form, timing and mechanics of foreland thrusting in the sub-Himalaya of Pakistan. The reflection profiles confirm earlier interpretations that the zone of decollement lies within an Eocambrian evaporite sequence, and reveal many of the gross features predicted in mechanical studies of thrust belts developed over salt, including: 1) an overthrust wedge of narrow cross-sectional taper; 2) a zone of overthrusting that extends farther over the foreland than adjacent areas not underlain by salt; 3) tight, salt-cored anticlines separated by broad synclines; and 4) structures concentrated along the edges of the salt basin.

Beneath the decollement, the crystalline basement of the Indian shield has a gentle upward convexity, with dips increasing northward from about 1° beneath the central Salt Range to about 4° beneath the northern Potwar Plateau. The upward convexity is associated with north-dipping normal faults that offset the basement, and appears to be a result of flexure of the Indian plate in response to the load of Himalayan thrust sheets. At the toe of the overthrust wedge, surface relief and the exposure of Mesozoic and Paleozoic strata in the Salt Range can be attributed to two factors: 1) the low-strength evaporites allow the decollement to extend far out over the foreland; and 2) basement relief on a normal fault with down throw to the north deflected the decollement, forming a ramp through the lower part of the section.

North of the Salt Range, late Tertiary synorogenic deposits (Rawalpindi and Siwalik groups) thicken to 5.5 km beneath the axis of the Soan Syncline. Dating of these deposits by means of magnetic polarity reversal stratigraphy documents the southward migration of the foredeep and the timing of local episodes of deformation. Integration of the structural and timing data, together with mechanical considerations, help to explain differences in structural style and evolution observed within the fold-and-thrust belt. In the central and western parts of the belt, the deformation front was abruptly transferred, about 2 Ma, from a deformed zone in the northern Potwar Plateau to the Salt Range front; the 100 km wide Soan Syncline/Salt Range region then moved southward as a coherent slab over the thick section of evaporites. Mechanical modelling suggests that the 2° to 3° dip of the basement surface in this region is sufficient to provide a critical taper to the wedge; no topographic slope is required and, hence, there is little internal deformation. By contrast, in the eastern Potwar Plateau and Pabbi Hills area, the observed basement slope is less than 1°. The overthrust section there is being deformed in order to build up an adequate topographic slope for thrusting to continue; paleomagnetic studies show that, over the last 3 My, the deformation front has moved from north to south at a more uniform rate.