Experimental reactivation of faults in marble across the brittle ductile transition

Abstract

The reactivation and stability of faults depend on a number of parameters, like rock composition, (effective) normal pressure, fault roughness, and loading rate. However, not much in known about the impact of temperature on faulting behavior. Using a Paterson-type gas deformation apparatus, triaxial compression experiments were conducted on dry Carrara marble samples containing a saw-cut oriented at about 40° to the axial stress direction. The tests were performed at constant axial strain rate of 1x10-5 s-1, confining pressures, P, between 30 and 150 MPa, and temperatures, T, in the range of 20 to 600°C. Under these conditions, intact Carrara marble deforms mainly in the semi-brittle regime and brittle, localized, deformation associated with strain weakening occurs only at room temperature and P < 100 MPa. At similar temperature, saw cut samples show formation of a new fracture zone inclined at 30° to the loading direction at P = 30 MPa and fault reactivation with stable sliding on the preexisting fault at P = 50 MPa. At higher temperatures up to 400°C and pressures < 100 MPa, we observed a mixture of matrix deformation and unstable (stick-slip) sliding on the fault. The peak stress at the onset of fault reactivation increased with P and T, resulting in higher associated peak strain. Also, the peak stress drop increased with increasing peak stress. At high T (>400°C) and P (>100 MPa) the fault remained locked and samples revealed ductile matrix creep with strain hardening, where the strength is almost similar to the strength of intact sample deformed under similar conditions. Microstructural observations reveal intense microcracking at the lowest P-T conditions. Samples exhibiting stick-slip behavior show a thin, discontinuous gouge layer and high twin density in specimens with late (high stress) fault reactivation. Bulk creeping samples reveal less damage and the fault appears to be partially sealed. Electron backscatter diffraction measurements suggest a slightly increasing crystallographic preferred orientation in the direct neighborhood to the fault compared the matrix under most conditions. Our results indicate that the fault reactivation stress of marbles increases with both, pressure and temperature, limited by the frictional strength. Above the brittle-ductile transition, the strength is limited by the bulk flow strength, which depends on total strain due to strain hardening, eventually leading to failure at high strain.