FAULT CORE ARCHITECTURE AND EVOLUTION IN BASALTS: IMPLICATIONS FOR MECHANICAL AND FLUID-MEDIATED WEAKENING

Faults constitute a major source for mechanical and permeability heterogeneity in basaltic sequences, yet their architecture, and mechanical and physical properties remain poorly understood. These factors are, however, critical for geothermal applications and CO₂ storage. Here we present a detailed micro- to outcrop-scale characterisation of mature fault zones in layered basalts in the Faroe Islands. Outcrop scale structures and fault rock distribution within the fault zone were mapped in the field to build 3D virtual outcrop models, with detailed characterisation of fault rock microstructure obtained from microscopy.

The fault zones record progressive localisation from dam-wide shear zones in which deformation is accommodated by Riedel shears, to m-wide fault cores with distributed internal shear. Deformation mechanisms in the core alternate between shear-compaction, evidenced by foliated cataclasite and gouge development, and dilatation through fluid overpressure, leading to hydrofracture and vein formation. Generally, a dam-wide damage zone of Riedel faults is centrally transected by the fault core. The Riedel sets are highly localised, sigmoidal and preferably develop as R- and subordinate P-shears. The fault core is organised around a principal slip surface (PSS) hosted in a dm-wide principal slip zone (PSZ). The PSZ consists of highly mature (ultra‑) cataclasites with a zeolite matrix and varying concentrations of smectite. The transition from PSZ to damage zone occurs either as an immediate shear strain boundary with undeformed and unaltered host rock, or as a more gradual transition through lenticular low strain zones bounded by anastomosing high strain cataclastic bands. Low strain zones and cataclastic bands are distinguished by their degree of comminution and chemical alteration. PSS-proximal zones show significant late stage dilatation by hydrothermal breccias or tabular veins with up to dm-apertures, filled with early zeolite and/or late calcite. These structures are mutually overprinting, evidencing pulsed fault activity and PSS migration. The alternating deformation styles – shear‑compaction and dilatation – suggest changes in deformation mechanism that may be linked to transient permeability decrease within the PSZ, followed by fluid overpressure, and hydrofracture. Overall rock mechanical properties are thus governed by the combined effects of permanent chemical weakening and transient fluid-mediated mechanical weakening, alternating with cementation and healing.