EVOLUTION OF OFF-FAULT DEFORMATION ALONG ANALOG STRIKE-SLIP FAULTS

Strike-slip faults evolve to accommodate more fault slip, resulting in less off-fault deformation. In analog experiments, the measured fault slip to off-fault deformation ratios are similar to those measured in crustal strike-slip systems, such as the San Andreas fault system. Analog models allow the temporal evolution of this ratio to be analyzed as systems, with and without pre-existing fault surfaces, evolve. Established planar faults have the largest fault slip to off-fault deformation ratio of ~0.98. These faults also have the highest efficiency, which we measure as the strike-slip along the fault divided by the applied plate displacement. In systems without a pre-existing fault surface, crustal thickness and basal detachment conditions affect shear zone width and roughness. However, once the applied plate displacement is 1-2 times the crustal thickness, partitioning of deformation between fault slip and off-fault distributed shear is >0.90, regardless of the basal boundary conditions. In addition, at any moment during the evolution of the analog fault system, the ratio of fault slip to off-fault deformation is larger than the cumulative ratio. We also find that the upward and lateral propagation of faults as an active shear zone developing early in the experiments has greater impact on the system’s strike-slip efficiency than later interaction between non-collinear fault segments. Established, non-planar faults that contain restraining bends also exhibit decreasing off-fault deformation over time. For bends with stepover distance of twice the crustal thickness, the fault slip to off-fault deformation ratio increases up to ~0.80-0.90, after applied plate displacement exceeds twice the crustal thickness. Propagation of new oblique-slip faults around sharp restraining bends reduces the overall off-fault deformation within the fault system and the pre-existing fault segment within the bend is abandoned. In contrast, fault segments within gentle restraining bends continue to slip and the propagation of new oblique-slip faults have less effect on the system’s efficiency than for sharp restraining bends.