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

Abstract. The dynamic evolution of fault zones at the seismogenic brittle-ductile transition zone (BDTZ) expresses the delicate interplay of numerous physical and chemical processes that occur at the time of strain localization. Deformation and flow of aqueous fluids in these zones, in particular, are closely related and mutually dependent during cycles of repeating, transient frictional and viscous deformation. Despite numerous studies documenting in detail seismogenic faults exhumed from the BDTZ, uncertainties remain as to the role of fluids in facilitating deformation in this zone, particularly with regard to the mechanics of broadly coeval brittle and ductile deformation. We combine here structural analysis, fluid inclusion data and mineral chemistry data from synkinematic and authigenic minerals to reconstruct the temporal variations in P, T and bulk composition of the fluids that mediated deformation and steered strain localization in a strike-slip fault from the BDTZ. This is a fault formed within the Paleoproterozoic migmatitic basement of southwestern Finland, hosting in its core two laterally continuous quartz veins formed by two texturally distinct quartz types – Qtz I and Qtz II, where Qtz I is demonstrably older than Qtz II. Veins within the diffuse damage zone of the fault are infilled by Qtz I. Multi-scalar structural analysis indicates recurrent cycles of mutually overprinting brittle and ductile deformation. Fluid inclusion microthermometry and mineral pair geothermometry indicate that both quartz types precipitated from a fluid that was in a homogeneous state during the recurrent cycles of faulting, and whose bulk salinity was in the 0–5 wt % NaCleq range. The temperature of the fluid phase involved with the various episodes of initial strain localization and later reactivation changed with time, from c. 240 °C in the damage zone to c. 350 °C in the core during Qtz I precipitation to Our analysis implies that fluid overpressure at the brittle-ductile transition can play a key role in the initial embrittlment of the metamorphic basement and strain localization mechanisms.