Coseismic fault-propagation folding on the Sulaiman Fold and Thrust belt

The continental collision at the western boundary of the Indian continent formed the tectonically complex transpressional zones of the Sulaiman Fold Thrust (SFT) and Kirthar Fold Thrust (KFT) belts. Seismic hazard around the SFT is considered elevated, but shortening across its eastern boundary is poorly understood because of the scarcity of moderate-sized earthquakes in the last few decades. Here, we use Sentinel-1A ascending and descending interferometry to analyze the coseismic crustal deformation associated with the 2015 moment magnitude (Mw) 5.7 Dajal earthquake that occurred on the boundary thrust in the SFT belt. The surface displacement was caused by slip on a blind thrust and coseismic folding in the hanging wall. We use kinematic inversions to determine the distribution of slip on the frontal ramp and flexural slip along active axial surfaces for two end-member models of fault geometry. We first consider a double fault-bend fold system involving two sub-horizontal décollements separated by a ramp. Second, we consider a fault-propagation fold system where the frontal ramp terminates below a thick sediment layer. The fault-bend fold model includes slip on the décollement-ramp-décollement and flexural slip on two active axial surfaces initiated at the fault bends. For the fault-propagation fold, the model includes slip on the décollement-ramp system and flexural slip on the lower axial surface. In a preliminary step, the geometry of the ramp is optimized using a Monte Carlo method using a single asperity model. The geometry of the décollement and active axial surfaces is inferred based on balanced cross-sections, whereby the fold axes bisect the sediment layers across a fault bend. In both end-member models, a shallow décollement branches into a shallow ramp at approximately 7 km depth. However, the undeformed sediment of the overlying floodplain indicates that a ramp is the recent geomorphic feature. We conclude that the Dajal earthquake propagated along the base of the ramp, representing coseismic ramp failure over fault-propagation folding.