Fluid-driven cyclic reorganization in shallow basaltic fault zones

Abstract Faults represent a critical heterogeneity in basaltic sequences, yet few studies have focused on their architectural and hydromechanical evolution. We present detailed, multi-scale characterization of passively exhumed fault zones from the layered basalts Faroe Islands, which reveals cyclic stages Outcrop-scale structures rock distribution within were mapped field 3-D virtual outcrop models, with detailed microstructure obtained optical scanning electron microscopy. The record deformation localization decameter-wide Riedel shear into meter-wide cores that contain multiple cataclastic bands low-strain lenses organized around central slip zone. Shear zone consist (ultra-) cataclasites zeolite-smectite assemblage replacing original plagioclase-pyroxene host composition. Low-strain are breccias weakly altered or reworked rocks. Slip zone-proximal show significant late-stage dilatation form hydrothermal tabular veins up to decimeter apertures. interpret these as evolving alternating shear-compaction dilation through hydrofracture. core preserves reworking, is interpreted indicate repeated locking migration. styles suggest episodic changes mechanisms driven by transient overpressure release. mechanical properties thus governed combined effects permanent chemical weakening fluid-mediated weakening, cementation healing. model presented for evolution should apply widely shallow, basalt-hosted zones.