Out of Sequence Thrusting in Coulomb Wedges

Saad Haq, Purdue University

The theory of Critical Coulomb wedges in conjunction with scaled analog models has been successful in explaining the pattern of deformation in accretionary prisms and thin-skinned fold and thrust belts under a variety of mechanical conditions. In this study we use frictional analog models, along with detailed kinematic analysis of developing structures in cross-section (using image correlation and particle tracking) to investigate the mechanism for the development of out-of-sequence (OSS) thrusting in contractional systems. We examine the influence that a hinterland-ward increase in bulk mechanical strength, normal to the convergence direction, which occurs as the wedge becomes mechanically consolidated with time, has on OSS faulting. While some OSS thrusting is expected in a wedge to maintain a tapered topography and a force balance, a hinterland-ward increase in strength can lead to significant OSS slip on both older and newly formed thrust structures. In nature the observation of major OOS thrusting is frequently associated with the development of mega-splay faults in accretionary prisms. These faults frequently produce large tsunami due to their steep dip. We investigate the mechanical conditions that can enhance or decrease OOS fore and back-thrusting in frictional wedges, including the role of fault orientation, basal friction, and sedimentation. We find that significant OOS thrusting is an inevitable process in frictional wedges even under constant mechanical conditions, that small variations in wedge strength will enhance the need for OOS thrusting, and that this process is cyclical in behavior driven by ongoing frontal thrust accretion with periodic recovery of taper and followed by additional OSS faulting.