The role of fluids in low temperature, fault-related deformation of quartz arenite

The microstructural and fluid histories in quartz arenite deformed in a regional-scale fault zone were investigated to determine the role of fluids in the selection of grain-scale deformation mechanisms. Using cathodoluminescence, fluid inclusion microthermometry, and stable isotope geochemistry of quartz cements, three chemically distinct, aqueous fluids were identified that affected the rock at different times during deformation. The earliest deformation microstructures are distributed cataclasites, which were cemented by reddish-brown luminescing quartz precipitated from an aqueous, CaCl2 brine (Fluid I). Once cemented, brittle deformation continued, but localized along discrete shear surfaces, in narrow zones 20–1000 μm wide, and on discrete microfractures. Microveins associated with this phase were cemented with bluish-green luminescing quartz precipitated from an aqueous, CaCl2 brine (Fluid II). Water, channelized along dissolution surfaces and in pervasive microfracture networks that formed at this time, promoted hydrolytic weakening and crystal-plastic deformation, especially near fluid pathways. Temperatures during deformation associated with Fluids I and II were 225–235 °C as determined from fluid inclusion data. In the later stages of deformation, an influx of iron-bearing aqueous fluid (Fluid III) greatly accelerated the rate of quartz dissolution and filled fractures, stylolites, and vugs with goethite. The progression of deformation mechanisms indicated by the microstructures and their relationship to the different fluids show that the selection of grain-scale deformation processes is affected not only by the presence of fluids, but also by their chemistry.