Experiments on fault stability of Carrara marble across the brittle-ductile transition

To explore the mechanical behavior and slip stability of faults, we performed frictional experiments on saw-cut samples of dry Carrara marble using a gas medium triaxial apparatus. The temperatures (T) were 70 similar to 400 degrees C in the experiments and the confining pressure (P-c) ranged from 30 to 100 MPa, and the slip velocity (V) was switched among 0. 08 mu m . s(1), 0. 4 mu m . s(1) and 2 mu m . s(1), respectively. The experiments reveal that the temperature dependence of the marble friction varies with the confining pressure in the case of P-c = 30 MPa, frictional coefficient increases first and then decreases with temperature in the range of 70 to 400 degrees C; for P-c >= 70 MPa, frictional coefficient shows an opposite trend. The marble friction shows a transition from initial velocity-strengthening behavior to velocity-weakening behavior at 100 similar to 300 degrees C, and then back to velocity-strengthening behavior at around 400 degrees C. Macroscopic morphology and microstructure analysis of postmortem fault surfaces show that stable sliding tends to produce mirror-like surfaces with clear striations; and stick-slip seems to be associated with shiny but bumpy surfaces. In contrast, no visible striation can be found on the slip surfaces of the creeping faults under high confining pressure. These results suggest that crystal plastic deformation process activated by temperature plays a key role in the activation of unstable slip, while high confining pressure may suppress the unstable slip. Our work may shed light on how to identify the slip behaviors of faults (stick slip or creep) in the field and provide new insights into mechanisms of earthquake nucleation at depth.