The Physical and Mechanical Properties of Natural Fault Zones in Basaltic Rocks

Faults represent a critical heterogeneity in basaltic sequences, which are gaining importance as potential sites for geothermal heat extraction and carbon storage. The processes controlling the behaviour of such faults at depth remain poorly constrained. The purpose of this research is to develop an integrated model for upper-crustal fault evolution in basalts, based on detailed structural, petrological, and hydromechanical characterisation of passively exhumed fault zones in the Faroe Islands.

Faults record strain localisation from incipient decametre-wide zones comprising networks of Riedel shears, into high strain, metre-wide fault cores. These contain multiple cataclastic shear bands enclosing low-strain lenses of hydrothermal breccias and/or tabular veins, indicating significant late-stage dilatation. Abundant overprinting fault rock assemblages record episodic fault zone reorganisation and reworking of fault rocks. Fluid-mediated alteration leads to replacement of the tholeiitic basalt protolith, dominated by plagioclase, pyroxene, and volcanic glass, by zeolite-smectite-pyroxene assemblages in and around the fault core. Alteration and mineralisation increase velocity-strengthening behaviour within the fault core, while frictional strength and permeability are reduced by alteration and enhanced by mineralisation. The damage zone is relatively strong and permeable, and encloses a fault core comprising weak and impermeable cataclastic shear bands and lenses of strong and more permeable hydrothermal breccias. Strength recovery between slip events is minimal. In such a configuration, shear on impermeable cataclasites may lead to transient fluid pressurisation within the slip zone, promoting episodic hydrofracture and triggering slow slip events. Acceleration to seismic slip rates is inhibited by velocity-strengthening behaviour in the fault core. Strengthening by cementation following hydrofracture may drive slip zone locking and migration into weaker structures.

Shallow basalt-hosted faults in the Faroe Islands record hydrofracture events punctuating periods of aseismic cataclastic creep within a widening slip zone. These findings highlight the importance of constraining the natural heterogeneity and distribution of fault rock properties, as they have significant implications for fault behaviour.