SYN-KINEMATIC STRATA INFLUENCE THE STRUCTURAL EVOLUTION OF EMERGENT FOLD-THRUST BELTS

In ways that are directly comparable with salt allochthons, the first-order structure of emergent thrust systems is strongly influenced by the amount and location of syn-kinematic sedimentation. High syn-kinematic sedimentation at the toe of emergent systems creates ramps, promoting the formation of imbricate fans with thrusts active together across the array. In contrast, low sedimentation allows the frontal thrust to accumulate displacement, increasing the propensity for large-displacement thrust allochthons. These behaviors are illustrated to discuss the evolution of the emergent thrust systems of the the NW Himalayan foothills. Lateral variations in structural style, from a single emergent thrust in the Salt Range (Salt Range thrust, in the W) to a bi-direction fold-thrust system (Jhelum fold belt - in the E) have generally been ascribed to differences in the propensity for slip on the basal detachment. Yet both parts of the system are underlain by halite. It is the thickness and accumulation rates of syn-kinematic sedimentation that varies regionally and offers a better explanation for the change in structural style. The Salt Range Thrust emerged into a distal, weakly developed foredeep with significantly reduced rates of sediment accumulation. Displacements were strongly localized onto this thrust (c 25 km displacement) which activated an upper detachment along the syn-orogenic surface. It is an arrested thrust-allochthon. Lateral variations into the adjacent, ramp-dominated but still salt-detached, Jhelum fold-belt is marked by increases in syn-kinematic sedimentation approaching the main foredeep of the Himalayan arc. The imbricate fan at the front of this arc developed within a foredeep that experienced high rates of syn-kinematic sedimentation and therefore the structures are widely spaced and are active together. As sedimentation styles can vary in space and time, individual thrusts and thrust systems can evolve from being allochthon-prone to imbricate-dominated. The eastern Salt Range shows this evolution as the area experiences increasing sedimentation rates. Evaluating patterns of syn-tectonic sedimentation can assist in making appropriate choices of structural style when developing regional tectonic models.