THE BRITTLE-PLASTIC TRANSITION IN EXPERIMENTALLY DEFORMED QUARTZ AGGREGATES

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 tr ansition in nonporous quartzite involves at least three transitions in deformation mechanism that occur with increasing temperature and/or p ressure. First there is a transition from cataclastic faulting to semi brittle faulting; microstructural observations indicate that this tran sition occurs due to the activation of dislocations. In addition, faul ting is more stable in the semibrittle faulting regime due to the blun ting 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 distr ibuted deformation. Microstructural observations indicate that microcr acks nucleate in response to stress concentrations at dislocation pile ups in the semibrittle flow regime. We conclude that the transition to semibrittle flow occurs when the stress intensity at crack tips is in sufficient to allow propagation across grain boundaries. Third, there is a transition from semibrittle flow to dislocation creep. Microstruc tural 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 fro m dominantly mode I (axial) to mode II (shear) microcracking occurs wi th an increase in confining pressure from 0.4 to 0.8 GPa, regardless o f temperature. The differential stresses supported by the experimental ly deformed samples are higher than those expected under geologic cond itions. However, a comparison of the experimentally produced microstru ctures to those reported from natural fault zones suggests that simila r processes are operative in the laboratory and in the Earth. The resu lts of this study provide further evidence to indicate that the brittl e-plastic transition in the continental crust occurs over a relatively wide range of conditions.