Fluid-mediated, brittle-viscous deformation cycles at the brittle-ductile transition

Crustal deformation at the brittle-ductile transition may take place by a combination of competing brittle fracturing and viscous flow processes. This competition may be triggered by variations of properties (e.g., T, P, density) of the fluid hosted by the fault at the time of deformation. Hence, a joint (micro)structural and geochemical approach to the study of the deformation processes and of the assisting fluid phase(s) provides insights into the dynamic evolution of strength in the seismogenic crust. Fault zones in the Olkiluoto nuclear waste disposal site in SW Finland exhibit a mixed brittle-ductile deformation style, and represent excellent targets to study coexisting ductile and brittle deformation in quartz-rich systems. We present results from a fault zone characterized by complex kinematics and reactivation history, as demonstrated by at least two types of synkinematic quartz-chlorite veins (types 1 and 2). Microstructural analysis documents a variety of fully ductile quartz deformation features (WEB’s, bulging and subgrain rotation recrystallization) overprinting and overprinted by mixed brittle-ductile textures (recrystallized cataclasites and healed fractures). Fluid inclusion microthermometry and chlorite geothermometry constrain deformation within the fault to 250-350°C, which is the typical T range defining the greenschist facies metamorphism at the base of the seismogenic crust. We propose that the repeated rheological switches from frictional to viscous deformation mechanisms were controlled by transient and repeated changes of fluid properties. If these variations took place at seismic velocities, the exhumed microstructural record would witness seismic cycles, with type 1 and type 2 veins reflecting discrete failure events during the shocks. The brittle-ductile deformation microstructures would represent instead the interseismic creep.