The role of fluids on strain localization at the base of the seismogenic crust: a case study from the Olkiluoto deep nuclear waste repository, southwestern Finland

This study focuses on the detailed investigation of the role played by fluids in triggering and controlling transient and repeated fluctuations between the frictional and viscous deformation end-members at the brittle-ductile transition zone (BDTZ) of the continental crust. Faults exhumed from the BDTZ and exposed in the Olkiluoto high-grade nuclear waste repository were studied combining field and microstructural observations with fluid inclusions and mineral pair geothermometry on fault minerals to reconstruct the temporal variations in fluid pressure, temperature and composition of the synkinematic fluids. Combined LA-ICP-TOFMS and EBSD analysis were also applied on authigenic sulphides to gain insights into their role upon strain accommodation and deformation-induced elemental transport at the microscopic scale. Faulting initiated as a consequence of a first event of fluid overpressure (Pf > 210 MPa) with the formation of a diffuse network of joints and hybrid–shear fractures. Cyclical brittle and ductile shearing followed, triggered by repeated hydrofracturing induced by a fluid pressure up to 210 MPa under overall ductile environmental conditions, demonstrated by mutually overprinting veining, crystal-plastic deformation and cataclasis. Later exhumation and cooling of the fault system to fully brittle conditions was aided by reactivation triggered by a distinct fluid ingress at lower pressure (140-180 MPa) and temperature (≤ 300° C). Deformation was accommodated at that stage by the interplay of brittle fracturing and low-temperature crystal-plasticity in sulphides. Strain and fluid flow created high diffusivity pathways within the pyrite crystal lattices contributing to- and enhancing the net transport of a significant range of heavy elements (e.g. Co, Ni, Cu, Sn, Ag, As, Sb, Pb). These data indicate that the studied fault acted as a chemically open system and fault valve. Hydrofracturing and their association with pseudotachylite-bearing faults suggest that fluid-mediated deformation may represent the record of the seismic cycle in the studied fault system.