THE ROLE OF FLUIDS ON THE BRITTLE-DUCTILE TRANSITION IN THE CRUST

The role of fluids on the processes responsible for the brittle-plastic transition in quartz-rich rocks has not been explored at experimental conditions where the kinetic competition between microcracking and viscous flow is similar to that expected in the Earth. Our initial analysis of this competition between these brittle and ductile processes suggests that the effective pressure law for fracture and sliding friction should not work as efficiently near the brittle-plastic transition (BPT) as it does at shallow conditions. Our motivation comes from two observations. First, extrapolation of viscous creep laws for quartzite indicates the brittle-plastic transition (BPT) occurs at a temperature of ~300^oC at geologic strain rates for conditions where fault strength is controlled by a coefficient of friction (μ) of 0.6 with a hydrostatic pore-fluid pressure gradient. Second, by considering the influence of pore-fluids on brittle deformation, we suggest that the preservation of relatively high stress viscous microstructures indicates that the effective pressure law must sometimes evolve rapidly near the BPT - for example - from highly efficient to zero efficiency with increasing depth. To illustrate this point, first consider the abundant evidence for the presence of fluids during viscous deformation of mylonites (e.g., recrystallization and redistribution of micas, dissolution and reprecipitation of quartz). Furthermore, analyses of fluid inclusions preserved in mylonitic rocks indicate near lithostatic pore fluid pressures. Based on the simple interpretation of the strength-depth diagrams, maintenance of high pore fluid pressure is incompatible with viscous creep stresses in the range of 100-200 MPa. A similar “paradox” is evident at experimental conditions where we study viscous creep in the laboratory. In this case, the presence of fluid (which should produce low effective stress) does not promote localized brittle failure, even though these experiments are conducted under undrained conditions. Indeed, the introduction of fluids is actually required to inhibit brittle processes at experimental strain rates, through the effects of hydrolytic weakening.