Sandbox models of relay ramp structure and evolution

Relay ramps are a common feature during the growth of normal fault systems. We performed analogue experiments to investigate the structure and evolution of extensional relay ramps. An extending rubber sheet induces extension at the base of a sand pack (brittle crust analogue). Silicone bars between the rubber and the sand control the location of fault nucleation. We tested the role of fault spacing, length, overlap and strike. The experimental ramps evolved in three stages, characterized by the growth of the normal faults, their interaction and linkage. Interaction and linkage occurred only when the total length of the faults was larger than 8 times their spacing. The length to width ratio of the relay ramps during the interaction stage showed preferred geometries, clustering around 3. The propagation of the fault tips was observed both before and after the linkage stage, even though the final deformation pattern was identical. Overlap and spacing relations of the experimental ramps are similar to those in nature, at different scales, and can be explained using existing mechanical models. Nevertheless, the further propagation of the fault tips after the linkage was never described previously.