Analog models of restraining stepovers in strike-slip fault systems

Scaled sandbox models have successfully simulated the geometries and progre ssive evolution of antiformal pop-up structures developed in a weak sedimen tary cover above restraining stepovers in offset sinistral strike-slip faul t systems in rigid basement. Models were run both with and without synkinem atic sedimentation, which was added incrementally to cover the growing anti formal structures. Vertical and horizontal sections of the completed models permit the full three-dimensional (3-D) structure of the pop-ups to be ana lyzed in detail. Three representative end-member experiments are described: 30 degrees underlapping restraining stepovers; 90 degrees neutral restrain ing stepovers; and 150 degrees overlapping restraining stepovers. The experimental pop-ups are typically sigmoidal to lozenge-shaped, antifor mal structures having geometries that are dependent on both the stepover an gle and stepover width in the underlying basement faults. Underlapping rest raining stepovers typically form elongate lozenge-shaped pop-ups; 90 degree s neutral restraining stepovers produce shorter, squat rhomboidal pop-ups; and overlapping restraining stepovers produce sigmoidal antiformal pop-ups. Trans pop-up cross fault systems are characteristic at large displacements on the basement fault system. Above the offset principal displacement zone s, the pop-ups are commonly small, narrow, positive newer structures, where as in the stepover region, they widen out and become markedly asymmetric. T his pop-up asymmetry switches across the center of the stepover, where the pop-ups are largely symmetical. Maximum rotations measured within the centr al highly uplifted region of the pop-ups increase from 7 degrees counterclo ckwise for the underlapping (30 degrees) stepovers, to 14 degrees countercl ockwise for the neutral (90 degrees) stepovers, to 16 degrees counterclockw ise for the overlapping (150 degrees) stepovers. In models where no synkinematic sediments were added during deformation, th e pop-up structures are bound by convex, flattening-upward, oblique-slip re verse fault systems that link downward to the offsets in the basement fault system. In contrast, in the experiments where synkinematic sediments were added incrementally during deformation, the pop-ups are formed by oblique-s lip reverse faults that steepen upward into the synkinematic strata with th e formation of fault-propagation growth folds. The analog models are compared with natural examples of pop-up structures a nd show strong similarities in structural geometries and stratal architectu res. These models may provide structural templates for seismic interpretati on of complex contractional structures in offset strike-slip fault systems.