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 thi n-skinned simultaneous shortening above adjacent ductile and frictional sub strates, These models simulate the evolution of the Potwar Plateau and Salt Range in Pakistan, where Paleozoic to Holocene sediments are shortened par tly above a ductile substrate of the Salt Range Formation. In this study, v ariations in the initial thickness of the ductile substrate and the influen ce of prekinematic and synkinematic overburden wedge have been systematical ly investigated. Model results confirm that forward-vergent imbricates form ing relatively steep wedges develop above a frictional substrate, whereas l ow-taper wedges with both foreland and rearward-vergent imbricates develop above ductile substrates. Furthermore, deformation propagates farther and m ore 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 in flection boundary due to differential rates of propagation. Folding of the overburden carries the ductile substrate along the inflection boundary, cre ating a conduit along which salt walls and stocks develop, which are potent ially diapiric, Model results show structures similar to those observed in the Salt Range-Potwar Plateau, Pop-up and/or pop-down structures are charac teristic features of the Potwar Plateau, and are observed in all model seri es. On the basis of model results, it is suggested that the anomalous thick ness of salt beneath the hanging wall of the Salt Range thrust is a consequ ence of buttressing by a basement fault and the south-ward evacuation of sa lt resulting from differential sedimentary and tectonic loading, a feature clearly displayed by the models.