Analog Models of Restraining Stepovers in Strike

Scaled sandbox models have successfully simulated the geometries and progressive evolution of antiformal pop-up structures developed in a weak sedimentary cover above restraining stepovers in offset sinistral strike-slip fault systems in rigid basement. Models were run both with and without synkinematic sedimentation, which was added incrementally to cover the growing antiformal structures. Vertical and horizontal sections of the completed models permit the full three-dimensional (3-D) structure of the pop-ups to be analyzed in detail. Three representative end-member experiments are described: 30° underlapping restraining stepovers; 90° neutral restraining stepovers; and 150° overlapping restraining stepovers.

The experimental pop-ups are typically sigmoidal to lozenge-shaped, antiformal structures having geometries that are dependent on both the stepover angle and stepover width in the underlying basement faults. Underlapping restraining stepovers typically form elongate lozenge-shaped pop-ups; 90° 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 zones, the pop-ups are commonly small, narrow, positive flower structures, whereas in the stepover region, they widen out and become markedly asymmetric. This pop-up asymmetry switches across the center of the stepover, where the pop-ups are largely symmetical. Maximum rotations measured within the central highly uplifted region of the pop-ups increase from 7° counterclockwise for the underlapping (30°) stepovers, to 14° counterclockwise for the neutral (90°) stepovers, to 16° counterclockwise for the overlapping (150°) stepovers.

In models where no synkinematic sediments were added during deformation, the pop-up structures are bound by convex, flattening-upward, oblique-slip reverse 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-slip reverse faults that steepen upward into the synkinematic strata with the formation of fault-propagation growth folds.

The analog models are compared with natural examples of pop-up structures and show strong similarities in structural geometries and stratal architectures. These models may provide structural templates for seismic interpretation of complex contractional structures in offset strike-slip fault systems.