Two types of brittle fracture of silicate rocks under confining pressure and their implications in the earth's crust

Abstract

Four silicate rocks (granite, gabbro, dunite and eclogite) were triaxially compressed up to a confining pressure of 3 GPa at room temperature and were found to exhibit brittle fracture behaviour. From the measurements of the compressive and frictional strengths and the activity of acoustic emission (AE), and the observations of the microstructure of fractured specimens, the pattern of fracture was found to change when the compressive strength became equal to the frictional strength. Two types of fracture mechanism, “low-pressure” and “high-pressure” types, were inferred. The low-pressure type fracture exhibits features of brittle fracture equivalent to those recorded by previous workers. The features of high-pressure type fracture are similar to those observed between brittle fracture and ductile creep in high-temperature triaxial experiments: microcracks are not concentrated close to the main fault, and the main fault is sharp and oriented at approximately 45° to the stress direction. This suggests that the high-pressure type fracture might correspond to the transitional fracture type between the brittle and ductile regimes at room temperature. For granite and gabbro samples of a few centimeters size, high-pressure type fracturing begins at ∼ 0.8 GPa confining pressure, while for dunite and eclogite it begins at ∼ 1.0 and ∼ 2.0 GPa respectively. The size effect on compressive strength under confining pressure was estimated based on the suggestion that the uniaxial strength decreases with increasing specimen size and ceases to decrease for specimens > 1 m. This estimation suggests that the compressive strength for a granite specimen a few meters in size would become equal to the frictional strength at confining pressures as low as 60 MPa. The significance of this is that it may be necessary to model earthquakes using the high-pressure type fracturing, because in the absence of pore pressure 60 MPa confining pressure corresponds to a depth of 3 km. Faulting processes in the crust, including both the initial faulting and the movement of existing faults, were inferred using the experimental results and the estimated size effect on strength.