Fluid-driven Cyclic Reorganisation in Shallow Basaltic Fault Zones

Faults represent a critical heterogeneity in basaltic sequences, yet their architectural and hydromechanical evolution is poorly constrained. We present a detailed multi-scale characterisation of passively exhumed fault zones from the layered basalts of the Faroe Islands, which reveals cyclic stages of fault evolution. Outcrop-scale structures and fault rock distribution within the fault zones were mapped in the field and in 3D virtual outcrop models, with detailed characterisation of fault rock microstructure obtained from optical and SE-microscopy. The fault zones record localisation from decametre-wide Riedel shear zones into metre-wide fault cores, containing multiple cataclastic shear bands and low strain lenses organised around a central principal slip zone (PSZ). Shear bands and the PSZ consist of (ultra-) cataclasites with a zeolite-smectite assemblage replacing the original plagioclase-pyroxene host rock composition. Low-strain lenses are hydrothermal breccias of weakly altered host rock, or reworked fault rocks. PSZ-proximal zones show significant late-stage dilatation in the form of hydrothermal breccias or tabular veins with up to decimetre apertures. We interpret these structures as evolving from alternating shear-compaction and dilation through hydrofracture. The fault core preserves PSZ reworking, evidencing repeated shear zone locking and migration. The alternating deformation styles of shear compaction and dilatation suggest episodic changes in deformation mechanisms driven by transient overpressure and release. The fault zone mechanical properties are thus governed by the combined effects of permanent chemical weakening and transient fluid-mediated mechanical weakening, alternating with cementation and healing.