FAULT ZONE ROCK PULVERIZATION INHIBITED BY EXHUMATION OF ANISOTROPIC ROCK: INSIGHTS FROM RIDGECREST, CALIFORNIA

During dynamic rupture along faults, high stress and strain-rate loading can produce pulverized rock in the near field. Fault rock pulverization has largely been observed within isotropic rocks including granite, tuff, sandstone, and carbonates. In contrast, pulverized rock fabric in foliated metamorphic rocks that are brought to the surface through tectonic exhumation has been difficult to interpret and link to rupture processes. It remains unclear how anisotropic fault rocks will evolve over multiple earthquake cycles and subsequently impact earthquake nucleation and propagation. Here we use successive dynamic transversely isotropic tension experiments conducted with a Split Hopkinson Pressure Bar to explore how anisotropic rock influences rock pulverization at loading conditions produced in country rock during seismic fault rupture. The decrease in fragment size with increasing number of loading events is well described by a power law. However, in testing samples from exhumed meter-scale chlorite-epidote mylonitic shear zones, we find that anisotropy reduces fault-rock fragmentation over multiple earthquake cycles. Fracture orientations become more anisotropic in foliated rocks with increasing numbers of loading events compared to isotropic crystalline rocks. The bulk properties of strike slip faults may reflect a difference between exhumed anisotropic versus isotropic upper crustal wall rocks. To investigate this further we analyze fault zone properties of numerous faults activated during the 2019 Ridgecrest, CA earthquake sequence. Coseismic displacement fields from InSAR are used in conjunction with static stress change models for the Ridgecrest earthquakes to estimate the elastic modulus variations between the country rock and fault zones. Our results suggest that fault zones formed in reactivated exhumed chlorite-epidote mylonitic shear zones may be stiffer (i.e., higher elastic modulus) than fault zones in pure granite. This implies that brittle fault damage evolution for actively exhuming fault zones will be less intense and more anisotropic than for faults with no exhumation leading to stiffer fault zones. Furthermore, this suggests that there may be variations in earthquake behavior and off-fault energy dissipation through brittle fragmentation depending on the long-term exhumation history of a fault zone.