RELATIONSHIPS AMONG TRANSIENT BRITTLE AND DUCTILE DEFORMATION AND METAMORPHIC REACTIONS IN CRUSTAL SHEAR ZONES (Invited Presentation)

Records from exhumed deep crustal terrains and seismic observations of tectonically active deep crust are increasingly illuminating complex interplays between ductile and brittle deformation below the frictional-plastic transition. Understanding the processes involved in such transient behavior has thus become a first order challenge in lithospheric rheology studies. Here, we summarize and contrast examples from exhumed Proterozoic shear zones in western North America and Tertiary shear zones in the central European Alps, with emphasis on integrating microstructural and petrologic analysis. In the Canadian Shield, multiple generations of healed shear fractures and variably deformed pseudotachylyte indicate episodic brittle deformation in an overall ductilely deforming granulite grade (0.8-0.7 GPa, >700°C) shear zone. Limited evidence for hydrous fluids suggest that relatively dry conditions may be one of the factors contributing to episodic brittle failure, perhaps triggered by downward propagation of slip from shallower ruptures, whereas the finer grained assemblages in the pseudotachylyte veins appear to have localized subsequent plastic shearing. The deformation aided chemical re-equilibration in the veins, allowing quantitative constraints to be placed on the P-T conditions of pseudotachylyte formation. Fracture systems in granites in the Alps and in gabbroic dikes in Montana systematically evolved into outcrop-scale ductile shear zones under amphibolite- to eclogite-facies conditions with strong influences from locally fluxing fluids. Chemical potential gradients set up by synkinematic fluid flow and mass transfer allowed widening of the ductile shear zones from their brittle precursors, accompanied by transitions from dislocation creep and local cataclastic deformation in mylonite to dominantly diffusion-accommodated granular flow in ultramylonite. The brittle mechanisms that dominated early in the shear zone evolution may offer insight into some observations of deep crustal tremor and very low frequency earthquakes. While the examples vary widely in scale and tectonic setting, a common observation is that fluid availability and fluid-rock interaction played critical roles in promoting active deformation mechanisms and transient ductile and brittle behavior.