Tracing the origin of parental fluids and paleo-temperature distribution in fault zones: case studies from the Carboneras Fault (Spain) and Lemnos Island (Greece)

Multiple episodes of brittle deformation tend to increase the structural complexity of fault zones. This commonly results in the development of juxtaposed and non-coeval distinct Brittle Structural Facies (BSF) formed at different times, depths, and temperature. Indeed, these BSFs are characterized by an irregular distribution of inherited, syn-kinematic, and post-kinematic minerals, whose study provides useful information about the temperature conditions of (de)formation, and the origin of fluids circulating within faults. We combined X-ray diffraction (XRD) analyses and polytype determinations of whole-rock and several grain-size fractions (6-10 μm, 2-6 μm, 0.4-2 μm, 0.1-0.4 μm, and <0.1 μm), with H-isotope data of 76 fault rocks and 6 protoliths from two structurally and well-characterized fault zones with different kinematics: the Carboneras strike-slip fault zone (Betic Cordilleras, SE Spain) and the Kornos-Aghios Ioannis extensional fault zone on the Lemnos Island (North Aegean Trough, Greece). Mineralogical and geochemical data allowed us to (1) reconstruct the distributions of syn/post-kinematic minerals in distinct BSFs, (2) constrain their formation temperature, and (3) unravel the origin of fluids involved during faulting. In the Carboneras fault rocks, we distinguished a protolithic mineralogical assemblage consisting of quartz, carbonates, K/Na-micas (2M₁ polytype), chlorite, kaolinite, and Fe/Ti-oxides, a syn-kinematic assemblage composed of mixed layers chlorite-smectite and illite-smectite (1M_d polytype), and a post-kinematic assemblage made up of smectite, chlorides, and sulphates. The coexistence of randomly (R0), short-range (R1), and long-range (R3) ordered illite-smectite in different BSFs indicates contrasting formation temperatures. Bulk samples generally display δ²H values (V-SMOW) between -15‰ and -60‰ (with a few exceptions at -90‰), while their respective <2μm fractions show δ²H values between -10‰ and -60‰. The combination of mineralogical and geochemical data from the Carboneras fault zone depicts a complex history of multiple brittle events occurring at different temperature conditions, wherein parental fluids of mostly meteoric origin infiltrated into the fault zone and interacted with the host rocks at various degrees and depths. In the fault rocks of Lemnos Island, we identified a host-rock mineralogical assemblage composed of quartz, feldspars, carbonates, K-mica (2M₁ polytype), chlorite, R0 illite-smectite, anatase, and Fe-oxides and hydroxides, a syn-kinematic assemblage made up of R3 illite-smectite (1M_d polytype), and kaolinite, and a post-kinematic assemblage characterized by halite and gypsum. Bulk samples display δ²H values (V-SMOW) between -70‰ and -140‰, while their respective <2μm fractions show δ²H values between -60‰ and -85‰. Such results indicate that Kornos-Aghios Ioannis fault rocks formed during a deformation event with predominantly hydrothermal fluids circulating into the fault zone (>160°C). This multidisciplinary approach represents an innovative point of view for studying fluid circulation, mineral crystallization, and temperature evolution in complex fault zones, and it can be applied to both orogen scale faults and smaller fault systems.