Microscopic Study on Mechanisms of Brittle Fracture and Ductile Deformation in Rocks under High Confining Pressures.

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Microstructures of four brittle-fractured rocks and a cataclastically-deformed rock were studied. They were compared with the mechanical properties which had been observed in the triaxial experiments up to 3 GPa confining pressure at room temperature.Fracture mechanism of four low-porosity rocks (granite, gabbro, dunite and eclogite) changed from the low- to high-pressure types when the compressive strength became equal to the frictional strength. The low-pressure type exhibited the equivalent features to the well-documented brittle fracture. The high-pressure type was different from it: the strength increases linearly at lower increasing rate with increasing confining pressure; the AE activity does not increase rapidly before fracture but stays constant followed by a sudden final fracture; and microcracks are not concentrated close tothe main fault being sharp and oriented at 45 to the maximum stress direction. The high-pressure type fracture might correspond to the transitional one between brittle and ductile regimes, considering the similarity in the faulting features to those found in high temperature experiments. Estimated size effect on strength under pressure gave the suggestion that fracture in the crust might be of the high-pressure type which earthquakes should be modeled upon. Possible faulting processes in the crust including the first faulting and movement of exisiting faults were modeled.A dry fine-grained basalt with 7% porosity, in the ductile regime, exhibited cataclastic deformation behavior. In the progressive stage of granulation at higher stress levels, the flow mechanism was found to change from the power-law creep with stress exponent of 3 to 1 at 2 GPa confining pressure. It was found to results from the remaining-pore closure and pervasion of highly granulated portions. The viscosity drop of -10^5 Pa・s was estimated associated with the mechanism change at the strain rate corresponding to that for the crust and mantle.