Deformation mechanisms in room-dry and water-saturated specimens of Charcoal Granodiorite, shortened at 10-4s-1, at effective pressures (Pe) to 100 MPa and temperatures to partial melting (= 1050ºC) are documented with a view toward providing criteria to recognize and characterize the deformation for geological and engineering applications. Above 800ºC strength decreases dramatically at effective pressures = 50 MPa and water-weakening reduces strength an additional 30 to 40% at Pe = 100 MPa. Strains at failure are only 0.1 to 2.2 percent with macroscopic ductility (within this range) increasing as the effective pressures are increased and in wet versus dry tests. Shattering (multiple faulting) gives way to faulting along a single zone to failure without macroscopic faulting as ductility increases. Microscopically, cataclasis (extension microfracturing and thermal cracking with rigid-body motions) predominates at all conditions. Dislocation gliding contributes little to the strain. Precursive extension microfractures coalesce to produce the through going faults with gouge zones exhibiting possible Riedel shears. Incipient melting, particularly in wet tests, produces a distinctive texture along feldspar grain boundaries that suggests a grain-boundary-softening effect contributes to the weakening. In addition, it is demonstrated that the presence of water does not lead to more microfractures, but to a reduction in the stresses required to initiate and propagate them.
Solutions of rock-mechanics problems associated with energy extraction from the geothermal regime above and in magma bodies and structural studies of exhumed paleomagma bodies require knowledge of the deformation mechanisms operative in rocks at temperatures to partial melting and at low to modest effective pressures. Recognition and documentation of these mechanisms in experimentally deformed rocks (1) serves as a basis for extrapolating laboratory data (strength, ductility, and flow laws) to the field (similar mechanisms must exist in both naturally and experimentally deformed materials for such extrapolations to be valid), and (2) provides the structural petrologist with fabric criteria potentially to recognize and interpret natural deformations. While extensive studies have been made of ductile mechanisms a thigh P and T (e.g., Carter, 1976; Tullis, 1979), comparatively little is known about mechanisms in the semi-brittle regime at temperatures to partial melting and at low to modest pressures and in wet as well as dry rocks (Carter and Kirby, 1978; Friedman et al., 1979; Van der Molen and Paterson, 1979; Bauer et al., 1981; and Bauer, in press). A few studies have detailed the microstructures developed upon experimental deformation of igneous 'rocks at temperatures of partial melting (e.g., Van der Molen and Paterson, 1979; and Auer et al., 1981), but the minimum effective pressure was 300 MPa. In those tests axial (extension) microfractures and grain-boundary partings predominate, crystal-plastic behavior of the quartz and feldspar in slight, early melting of biotite and other hydrated minerals is evident, feldspar melts before quartz, and the melt occurs preferrentially along the axial cracks. The critical melt fraction for the transition from solid creep to viscous flow appears to range between 20 and 40% (Arzi, 1978; Van der Molen and Paterson, 1979; and Auer et al., 1981).
