Modeling of thrust fronts above ductile and frictional detachments: Application to structures in the Salt Range and Potwar Plateau, Pakistan

Series of scaled sandbox models are used to simulate the development of thin-skinned simultaneous shortening above adjacent ductile and frictional substrates. These models simulate the evolution of the Potwar Plateau and Salt Range in Pakistan, where Paleozoic to Holocene sediments are shortened partly above a ductile substrate of the Salt Range Formation. In this study, variations in the initial thickness of the ductile substrate and the influence of prekinematic and synkinematic overburden wedge have been systematically investigated. Model results confirm that forward-vergent imbricates forming relatively steep wedges develop above a frictional substrate, whereas low-taper wedges with both foreland and rearwardvergent imbricates develop above ductile substrates. Furthermore, deformation propagates farther and more rapidly above a ductile substrate than above a frictional substrate. The differential rate of propagation of a detachment between adjacent areas with ductile and frictional substrates generates an inflection subparallel to the shortening direction. Transpression develops across this lateral inflection boundary due to differential rates of propagation. Folding of the overburden carries the ductile substrate along the inflection boundary, creating a conduit along which salt walls and stocks develop, which are potentially diapiric. Model results show structures similar to those observed in the Salt Range-Potwar Plateau. Pop-up and/or pop-down structures are characteristic features of the Potwar Plateau, and are observed in all model series. On the basis of model results, it is suggested that the anomalous thickness of salt beneath the hanging wall of the Salt Range thrust is a consequence of buttressing by a basement fault and the southward evacuation of salt resulting from differential sedimentary and tectonic loading, a feature clearly displayed by the models.