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.
Highlights
The physical and chemical properties of fault systems play a fundamental role in controlling the rheological behaviour of the Earth’s crust and in steering channelled fluid flow (e.g. Caine et al, 1996)
We studied 28 fluid inclusion assemblages” (FIAs) entrapped within two distinct generations of quartz forming two different generations of veins and exhibiting the least petrographic evidence of post-entrapment overprinting by later ductile and/or brittle deformation, which provided ca. 800 microthermometric properties
In light of the field observations discussed and the constraints derived, we suggest that BFZ300 behaved in a seismic way at least during the emplacement of the principal Qtz I and Qtz II veins
Summary
The physical and chemical properties of fault systems play a fundamental role in controlling the rheological behaviour of the Earth’s crust and in steering channelled fluid flow (e.g. Caine et al, 1996). The nucleation and development of permeable fault systems and the mechanisms whereby individual faults may weaken and eventually fail are complex functions of a number of processes. In this perspective, the interaction between fluid and mineral phases within fault rocks needs to be studied with a system approach in order to single out the role and importance of all processes involved (Kaduri et al, 2017). An obvious effect of fluid involvement, in crustal volumes that have experienced large deformationcontrolled fluid fluxes, is the precipitation of authigenic and hydrothermal minerals within faults (Oliver and Bons, 2001; Viola et al, 2016) and their immediately adjacent host rock (Mancktelow and Pennacchioni, 2005; Garofalo, 2004). Hydrothermal ore deposits, in which fault networks focus relatively large volumes of ore fluids and precipitate economic minerals (Cox et al, 2001; Boiron et al, 2003; Moritz et al, 2006; Scheffer et al, 2017a), are pertinent examples of significant deformation-controlled fluid ingress
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