ABSTRACT
Approximately 1,600 km of seismic reflection profiles from the eastern Salt Range and Potwar Plateau (SR/PP) of Pakistan is integrated with available magnetostratigraphic, surface geologic, and well data to classify structural styles, determine the timing of deformation, and estimate the amount of telescoping of the sedimentary cover. The eastern SR/PP are similar to other fold-and-thrust belts underlain by evaporites in that (1) it is part of a zone of overthrusting that extends considerably farther over the Himalayan foreland than adjacent areas not underlain by evaporites, (2) the overall thrust wedge has a narrow cross-sectional taper, (3) structures verge toward the hinterland as well as toward the foreland, and (4) fold trends are long and continuous, consisting of tight salt-cored anticlines separated by broad synclines.
Disharmonic folding of the sedimentary section relative to the underlying basement is due to effective decoupling along the intervening salt layer. Subsurface mapping atop a strongly reflective sequence of Cambrian to Eocene strata reveals that many surface folds are cored by both foreland- and hinterland-dipping blind thrusts, and some are fault propagation folds. In some cases, intersecting thrusts result in local triangle zones; other surface folds have a pop-up geometry.
The dip of the basement toward the inner part of the fold-and-thrust belt is more gentle in the eastern SR/PP (1°-1.5°) than the central SR/PP (2°-3°). Mechanical considerations demonstrate that, unlike the relatively undeformed central SR/PP, a broad deformational zone has developed in the eastern SR/PP to provide a surface topographic slope necessary to maintain a critical taper of the thrust wedge.
Cross section balancing indicates that approximately 24 km of shortening has occurred across the foreland in the eastern SR/PP since 5.5 Ma, approximately 18 km in the last 2.5 m.y. The shortening rate of 7 mm/year for the latter time interval is roughly 15% of the 40 to 50-mm/year convergence rate between the Indian and Eurasian plates.
