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Abstract. The dynamic evolution of fault zones at the seismogenicbrittle–ductile transition zone (BDTZ) expresses the delicate interplaybetween numerous physical and chemical processes. Deformation and fluid flowat the BDTZ are closely related and mutually dependent during repeating andtransient cycles of frictional and viscous deformation. Despite numerousstudies documenting in detail seismogenic faults exhumed from the BDTZ,uncertainties remain as to the exact role of fluids in facilitating broadlycoeval brittle and ductile deformation at that structural level. We combinestructural analysis, fluid inclusion, and mineral chemistry data fromsynkinematic and authigenic minerals to reconstruct the temporal variationsin fluid pressure ( Pf ), temperature ( T ), and bulk composition ( X ) ofthe fluids that mediated deformation and steered strain localization alongBFZ300, a strike–slip fault originally active at the BDTZ. BFZ300 deformsthe Paleoproterozoic migmatitic basement of southwestern Finland and hostsin its core two laterally continuous quartz veins formed by two texturallydistinct 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 andmicrostructural analysis combined with fluid compositional data indicaterecurrent cycles of mutually overprinting brittle and ductile deformationtriggered by oscillations of fluid pressure peaking at 210 MPa. Fluidinclusion microthermometry and mineral pair geothermometry indicate that thetwo documented quartz types precipitated from different fluid batches, withbulk 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 withinitial strain localization and later fault reactivation evolved throughtime from > 350  ∘ C during Qtz I precipitation to ∘ C at the time of Qtz II crystallization. The peakfluid pressure estimates constrain pore pressure oscillations between 80 and210 MPa during the recorded faulting episodes. Our results suggestvariability of the physico-chemical conditions of the fluids steeringdeformation ( Pf , T , X ), reflecting the ingress and effects ofmultiple batches of fluid in the fault zone. Initial fluid-mediatedembrittlement generated a diffuse network of joints and/or hybrid–shearfractures in the damage zone; subsequent strain localization led to morelocalized deformation within the fault core. Localization was guided bycyclically increasing fluid pressure and transient embrittlement of a systemthat was otherwise under overall ductile conditions. Our analysis suggests that fluid overpressure at the BDTZ can play a keyrole in the initial embrittlement of the deforming rock and steer subsequentstrain localization.

solid earth

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)