Impact of Material Strength on Releasing Bend Evolution

Releasing bends along active strike-slip faults display a range of fault patterns that may depend on crustal strength. Scaled physical experiments allow us to directly document the evolution of established releasing bend systems under differing strength conditions. Here, we use a split-box apparatus filled with wet clay of differing strengths to run and analyze releasing bend evolution. Precut vertical discontinuities within the clay slip with right-lateral displacement of the basal plate followed by the development of oblique-slip secondary faults. In contrast to the weaker clay experiment, which produces left-lateral cross faults that facilitate major reorganization of the primary slip pathway, the stronger clay experiment produces negligible cross faults and has a persistent primary slip pathway. Within both experiments, the dip of initially vertical faults shallows due to lateral flow at depth and left-lateral slip develops along normal fault segments that have highly oblique strike. The experiments show that fault systems within weaker strength materials produce greater delocalization of faulting, with both greater number of faults and greater off-fault deformation that can impact hazard. For example, the hot, thin and weak crust hosting the Brawley Seismic Zone accommodates slip along many distributed faults, which is in sharp contrast to the more localized fault network of the Southern Gar Basin in cooler, thicker and stronger crust. The fault patterns observed in the experiments match patterns of crustal examples and may guide future models of fault evolution within relatively strong and weak crust that have differing heat flux and thickness.