Fluid-mediated, brittle–ductile deformation at seismogenic depth – Part 2: Stress history and fluid pressure variations in a shear zone in a nuclear waste repository (Olkiluoto Island, Finland)

Abstract

Abstract. 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 (<20 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.

Brittle Faulting of Ductile Rock Induced by Pore Fluid Pressure Build‐Up

“…We demonstrate that such pore fluid pressure increases would cause the brittle-ductile transition to migrate to greater depth, possibly leading to shear failure and seismicity at unexpected depths. This result is in agreement with field data suggesting that periods of elevated pore fluid pressures are responsible for ductile shear zones to be overprinted by brittle deformation (e.g., Marchesini et al, 2019;Prando et al, 2020;Wehrens et al, 2016).…”

Cyclic Brittle‐Ductile Oscillations Recorded in Exhumed High‐Pressure Continental Units: A Record of Deep Episodic Tremor and Slow Slip Events in the Northern Apennines

“…Angiboust et al., 2012; Austrheim, 1987; Giuntoli, Brovarone, et al., 2020; Molli et al., 2017). This transient cyclic behavior is triggered and steered by the interplay of different chemical and physical parameters, including, but not limited to, strain rate and stress variations, pore pressure fluctuations between hydro‐ and lithostatic conditions, mineral reactions, and associated strain weakening or hardening of the deforming rock (Brander et al., 2012; Bukovská et al., 2016; Gerald & Stünitz, 1993; Giuntoli, Menegon, et al., 2020; Gueydan et al., 2003; Hyndman, 1994; Mancktelow & Pennacchioni, 2005; Marchesini et al., 2019; Menegon & Fagereng, 2021; Menegon et al., 2013; Prando et al., 2020; Putnis, 2015; Simpson, 1986; Torgersen & Viola, 2014; Viola et al., 2006; Wehrens et al., 2016).…”

Strain-induced trace element mobility in a quartz-sulphide vein system: An example from the ONKALO™ spent nuclear fuel repository (Olkiluoto, SW Finland)

“…During progressive regional exhumation and cooling, the Olkiluoto bedrock was affected by a long brittle deformation history starting at 1.75 Ga (Mattila and Viola, 2014). This involved both the brittle reactivation of pre-existing structures (Viola et al, 2011;Mattila and Viola, 2014;Skyttä and Torvela, 2018;Prando et al, 2020), but also the formation of new faults and joints (Marchesini et al 2019). Marchesini et al (2019) and Prando et al (2020) studied in detail a system of conjugate strike-slip faults, as a representative example of late Svecofennian structures responsible of the initial embrittlement of the basement.…”