Experimental Insights Into Fault Reactivation and Stability of Carrara Marble Across the Brittle-Ductile Transition

Little is known about the impact of pressure (P) and temperature (T) on faulting behavior and the transition to fault locking under high P-T conditions. Using a Paterson gas-medium apparatus, triaxial compression experiments were conducted on Carrara marble (CM) samples containing a saw-cut interface at similar to 40 degrees to the vertical axis at a constant axial strain rate of similar to 1 x 10(-5) s(-1), P = 30-150 MPa and T = 20-600 degrees C. Depending on the P-T conditions, we observed the complete spectrum of deformation behavior, including macroscopic (shear) failure, stable sliding, unstable stick-slip, and bulk deformation with locked faults. Macroscopic failure and stable sliding were limited to P < 100 MPa and T = 20 degrees C. In contrast, at P >= 100 MPa or T >= 500 degrees C, faults were locked, and samples with bulk deformation experienced strain hardening at strains <= 8.8%. At T = 100-400 degrees C and P <= 100 MPa, we observed unstable stick-slip behavior, where both fault reactivation stress and subsequent stress drop increased with increasing pressure and temperature, associated with increasing matrix deformation and less fault slip. Microstructures indicate a mixture of microcracking, twinning and dislocation activity (e.g., kinking and undulatory extinction) that depends on P-T conditions and peak stress. The transition from slip to lock-up with increasing pressure and temperature is induced by an enhanced contribution of crystal plastic deformation. Our results show that fault reactivation and stability in CM are significantly influenced by P-T conditions, probably limiting the nucleation of earthquakes to a depth of a few kilometers in calcite-dominated faults. Plain Language Summary The nucleation depth of natural earthquakes is often limited to a certain depth range that depends on lithology and environmental conditions (e.g. T and P). Here, we performed an experimental study on Carrara marble with saw-cut and polished faults in a triaxial deformation apparatus at pressures up to 150 MPa and temperatures up to 600 degrees C to investigate the conditions under which the fault is reactivated or already locked. Due to an increasing amount of plastic deformation with increasing temperature and pressure, the deformation of the pre-faulted rock partitions increasingly into enhanced matrix deformation and less fault slip, associated with higher reactivation stress and larger stress drops. Extrapolating our laboratory results to natural calcite-dominated faults suggests that earthquakes may occur at depths of a few kilometers. At greater depths, faults are likely locked, and calcite rocks dominantly deform in the ductile regime.