High strain rates generated by moderate to large earthquake rupture cycles cause wall rock fragmentation, brecciation, and in-situ shattering, and enhance fluid flow in damage zones of continental thrust faults. This process reduces the rigidity of the hanging wall and raises the question of how hanging wall blocks slide on low angle faults without significant internal structural thickening. Our mapping shows that the Muddy Mountain Thrust slid for 10s-100s km on a low angle basal plane, which suggests the fault was weaker than the hanging wall wedge, an inference which requires significant hanging wall healing. To test this hypothesis, we conducted a uniaxial dynamic compressive loading test using an 8.5 m-long Split Hopkinson Pressure Bar apparatus to observe fracturing behavior at extreme stressing rates similar to conditions in wall rocks next to a seismic fault. We tested samples of healed breccias from the Sevier Muddy Mountain Thrust (MMT) in southern Nevada, USA, and their intact dolostone equivalent. By comparing the natural vs. laboratory deformation features, we constrained the strength recovery associated with breccia healing mechanisms in nature. The uniaxial compressive strength of the pristine dolostone ranges from 0.27 to 0.56 GPa at strain rates of 100 - 110 s-1. In contrast, the healed breccia is significantly weaker, with a uniaxial compressive strength of approximately 0.116 to 0.118 GPa, and deformed at strain rates up to 225 - 240 s -1. The pristine samples developed thicker (> 2.5 mm) throughgoing fractures during lab experiments, than in healed breccia (< 1.5 mm), and micro-cracks mainly concentrated around grain boundaries are more prominently seen in the pristine samples than in the healed breccia. The strength of the brecciated samples constrains an upper limit to the strength of the fault and our new data provide valuable insights into the strength recovery associated with breccia healing mechanisms in nature.
