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)

Francesca Prando,Giulio Viola,Luca Menegon,Mark Anderson,Barbara Marchesini,Jussi Mattila

doi:10.5194/se-11-489-2020

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.

Highlights

The change from fracturing and frictional sliding to dominant thermally activated creep processes accommodating viscous flow in mylonitic rocks occurs at the brittle–ductile transition zone (BDTZ; e.g. Kohlstedt et al, 1995; Handy et al, 2007)
This study shows that deformation microstructures can be used to evaluate the stress history of a shear zone deforming across the brittle–ductile transition in the continental crust and to reconstruct the cyclical brittle–ductile deformation history of fault zones
The fault zone BFZ045 exploited a mylonitic precursor in the Paleoproterozoic basement in SW Finland and records transient brittle deformation in the form of syn-kinematic quartz veins emplaced during ongoing mylonitic creep at ∼ 450 ◦C and 3.5 kbar, in response to transiently high fluid pressure

Summary

Introduction

The change from fracturing and frictional sliding to dominant thermally activated creep processes accommodating viscous flow in mylonitic rocks occurs at the brittle–ductile transition zone (BDTZ; e.g. Kohlstedt et al, 1995; Handy et al, 2007). Cyclical switches in deformation style during the evolution of mid-crustal shear zones 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). 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. It is important to assess whether evidence of cyclical fluctuations of those parameters is preserved in the geological record and whether the extent of such variations can be estimated by examining natural fault rocks

Methods

Results

Discussion

Conclusion