Deformation experiments have been conducted to provide constraints on the processes responsible for the brittle-plastic transition in quartz aggregates. A correlation between mechanical behavior and distinctive microstructural characteristics indicates that the brittle-plastic transition in nonporous quartzite involves at least three transitions in deformation mechanism that occur with increasing temperature and/or pressure. First there is a transition from cataclastic faulting to semibrittle faulting; microstructural observations indicate that this transition occurs due to the activation of dislocations. In addition, faulting is more stable in the semibrittle faulting regime due to the blunting of the stresses at the advancing fault tip by dislocation glide. Second, there is a transition from semibrittle faulting to semibrittle flow; this transition corresponds to a change from localized to distributed deformation. Microstructural observations indicate that microcracks nucleate in response to stress concentrations at dislocation pileups in the semibrittle flow regime. We conclude that the transition to semibrittle flow occurs when the stress intensity crack tips is insufficient to allow propagation across grain boundaries. Third, there is a transition from semibrittle flow to dislocation creep. Microstructural observations suggest that this transition occurs as a result of an increase in grain boundary mobility with increasing temperature. In addition, microstructural observations indicate that a transition from dominantly mode I (axial) to mode II (shear) microcracking occurs with an increase in confining pressure from 0.4 to 0.8 GPa, regardless of temperature. The differential stresses supported by the experimentally deformed samples are higher than those expected under geologic conditions. However, a comparison of the experimentally produced microstructures to those reported from natural fault zones suggests that similar processes are operative in the laboratory and in the Earth. The results of this study provide further evidence to indicate that the brittle-plastic transition in the continental crust occurs over a relatively wide range of conditions. ¿ American Geophysical Union 1994