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Microstructural record of fault rocks active at the brittle ductile transition zone (BDTZ) may retain information on the rheological parameters driving the switch in deformation mode, and on the role of stress and fluid pressure in controlling different fault slip behaviours. In this study we analysed the deformation microstructures of the strike-slip fault zone BFZ045 in Olkiluoto (SW Finland), located in the site of a deep geological repository for nuclear waste. We combined microstructural analysis, electron backscatter diffraction (EBSD), and mineral chemistry data to reconstruct the variations in pressure, temperature, fluid pressure and differential stress that mediated deformation and strain localization along BFZ045 across the BDTZ. BFZ045 exhibits a mixed ductile-brittle deformation, with a narrow (

The microstructural record of fault rocks active at the brittle–ductile transition zone (BDTZ) may retain information on the rheological parameters driving the switch in deformation mode and on the role of stress and fluid pressure in controlling different fault slip behaviours. In this study we analysed the deformation microstructures of the strike-slip fault zone BFZ045 in Olkiluoto (SW Finland), located in the site of a deep geological repository for nuclear waste. We combined microstructural analysis, electron backscatter diffraction (EBSD), and mineral chemistry data to reconstruct the variations in pressure, temperature, fluid pressure, and differential stress that mediated deformation and strain localization along BFZ045 across the BDTZ. BFZ045 exhibits a mixed ductile–brittle deformation, with a narrow ( cm thick) brittle fault core with cataclasites and pseudotachylytes that overprint a wider (60–100 cm thick) quartz-rich mylonite. Mylonitic deformation took place at 400–500 ∘C and 3–4 kbar, typical of the greenschist facies metamorphism at the base of the seismogenic crust. We used the recrystallized grain size piezometry for quartz to document a progressive increase in differential stress, from ca. 50 to ca. 120 MPa, towards the shear zone centre during mylonitization and strain localization. Syn-kinematic quartz veins formed along the mylonitic foliation due to transiently high pore fluid pressure (up to lithostatic value). The overprint of the veins by dynamic recrystallization and mylonitic creep is further evidence of the occurrence of brittle events under overall ductile conditions. We propose a conceptual model in which the ductile–brittle deformation cycle was controlled by transient oscillations in fluid pressure and progressively higher differential stress, possibly occurring in a narrowing shear zone deforming towards the peak strength of the crust at the BDTZ.

Fault zones hosted in granitic gneisses in the Olkiluoto nuclear waste disposal site in SW Finland exhibit a mixed brittle-viscous deformation style, and represent excellent targets to study coexisting ductile and brittle deformation in quartz-rich systems. Here we investigate the microstructural record of mutually overprinting brittle and viscous deformation events in a conjugate set of subvertical strike slip faults, in order to derive a conceptual model of the failure modes experienced by the faults at seismogenic depth. One of the two faults (BFZ045) exploits a mylonitic precursor and contains foliation-parallel veins that are cut by cataclasites and pseudotachylytes. The other fault (BFZ300) manifests itself as multiple generations of subparallel quartz-chlorite veins. Fluid inclusion microthermometry and chlorite and graphite geothermometry constrain the deformation T within the conjugate set to the 250-400 ◦ C range, which is the typical T range defining the greenschist facies metamorphism at the base of the seismogenic crust. Both faults show mutual overprinting relationships between viscous and brittle deformation. BFZ045 shows bulging and subgrain rotation recrystallization of quartz veins emplaced along the mylonitic foliation. The oldest generation of veins in BFZ300 contains a variety of quartz deformation features (extinction bands, bulging and subgrain rotation recrystallization) overprinting and overprinted by mixed brittle-viscous textures (recrystallized cataclasites and healed fractures). Quartz recrystallized grain size piezometry was used to estimate the oscillations in differential stress during the deformation cycles. Mohr failure diagrams were derived to model the switches in slip behaviours experienced by the conjugate faults. We propose that the brittle-viscous deformation cycles were controlled by transient oscillations in fluid pressure and in differential stress during the earthquake cycle. Thus, we suggest that the exhumed brittle microstructural record witnesses discrete failure events during the shocks, while the viscous deformation microstructures instead represent the interseismic creep