Fluid-mediated, brittle–ductile deformation at seismogenic depth – Part 1: Fluid record and deformation history of fault veins in a nuclear waste repository (Olkiluoto Island, Finland)

Abstract

Abstract. The dynamic evolution of fault zones at the seismogenic brittle–ductile transition zone (BDTZ) expresses the delicate interplay between numerous physical and chemical processes. Deformation and fluid flow at the BDTZ are closely related and mutually dependent during repeating and transient cycles of frictional and viscous deformation. Despite numerous studies documenting in detail seismogenic faults exhumed from the BDTZ, uncertainties remain as to the exact role of fluids in facilitating broadly coeval brittle and ductile deformation at that structural level. We combine structural analysis, fluid inclusion, and mineral chemistry data from synkinematic and authigenic minerals to reconstruct the temporal variations in fluid pressure (Pf), temperature (T), and bulk composition (X) of the fluids that mediated deformation and steered strain localization along BFZ300, a strike–slip fault originally active at the BDTZ. BFZ300 deforms the Paleoproterozoic migmatitic basement of southwestern Finland and hosts in its core two laterally continuous quartz veins formed by two texturally distinct types of quartz – Qtz I and Qtz II, with Qtz I older than Qtz II. Veins within the damage zone are formed exclusively by Qtz I. Mesostructural and microstructural analysis combined with fluid compositional data indicate recurrent cycles of mutually overprinting brittle and ductile deformation triggered by oscillations of fluid pressure peaking at 210 MPa. Fluid inclusion microthermometry and mineral pair geothermometry indicate that the two documented quartz types precipitated from different fluid batches, with bulk salinities in the 1 wt % NaCleq–5 wt % NaCleq range for Qtz I and in the 6 wt % NaCleq–11 wt % NaCleq range for Qtz II. The temperature of the fluids involved with initial strain localization and later fault reactivation evolved through time from > 350 ∘C during Qtz I precipitation to < 300 ∘C at the time of Qtz II crystallization. The peak fluid pressure estimates constrain pore pressure oscillations between 80 and 210 MPa during the recorded faulting episodes. Our results suggest variability of the physico-chemical conditions of the fluids steering deformation (Pf, T, X), reflecting the ingress and effects of multiple batches of fluid in the fault zone. Initial fluid-mediated embrittlement generated a diffuse network of joints and/or hybrid–shear fractures in the damage zone; subsequent strain localization led to more localized deformation within the fault core. Localization was guided by cyclically increasing fluid pressure and transient embrittlement of a system that was otherwise under overall ductile conditions. Our analysis suggests that fluid overpressure at the BDTZ can play a key role in the initial embrittlement of the deforming rock and steer subsequent strain localization.

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)

“…This study uses the N-S sinistral strike-slip fault BFZ045 as a natural laboratory to investigate the stress history of the ductile precursor and the possible role of fluids on the deformation processes active at the brittle-ductile transition. The companion paper by Marchesini et al (2019) has thoroughly described the deformation history of the conjugate dextral BFZ300 fault.…”

“…11a). The pressure during mylonitization was estimated using the Si-in-phengite geobarometer (Massonne and Schreyer, 1987). Representative compositions of white mica are listed in Table 1.…”

“…Average R2 ratio (refer to text for explanation) for graphite-rich mylonitic and host rock samples was measured to derive a T estimate using the method by Beyssac et al (2002) and Aoya et al (2010). (b) Estimated P of mylonitization using the Si-in-phengite barometer (Massonne and Schreyer, 1987) in the probed white mica grains is 3.12-3.16. This compositional range indicates a pressure of 2-4 kbar for the mylonitization ( Fig.…”

“…However, cyclical switches in deformation style during the evolution of mid-crustal shear zones (e.g. Pennacchioni and Mancktelow, 2007;Fusseis and Handy, 2008;Wehrens et al, 2016;Melosh et al, 2018) demonstrate that the BDTZ occupies a depth interval that can vary transiently, reflecting changes in, for example, bulk strength of the shear zones (Hirth and Tullis, 1994;Scholz, 1998;Fossen and Cavalcante, 2017;Melosh et al, 2018) and fluid pressure (Cox, 2010;Kjøll et al, 2015;Sibson and Rowland, 2003;Yardley and Baumgartner, 2007;Hirth and Beeler, 2015;Marchesini et al, 2019). Lithology; P -T conditions; and variations in stress, strain rate, and fluid pressure are important factors controlling the occurrence of different deformation mechanisms (dislocation creep, diffusion creep, fluid-assisted veining, dissolution-precipitation creep, fracturing, and cataclasis) that overlap in space and time at the BDTZ.…”

“…Fluids can also play a fundamental role in triggering a transient switch from dominantly ductile to brittle deformation, as demonstrated, for example, by the syn-kinematic emplacement of quartz veins subsequently overprinted by crystal-plastic deformation (Handy et al, 2007;Kjøll et al, 2015;Trepmann and Seybold, 2019;Marchesini et al, 2019). Cyclical ductile-brittle-ductile deformation associated with high fluid fluxes involving a fault-valve behaviour (Sibson, 1990) implies cycles of fluid pressure build-up followed by fluid venting and pressure drop, and it has been related to seismic fault behaviour (Sibson, 1992;Cox, 1995;Nguyen et al, 1998;Viola et al, 2006).…”