FAULTS, FAULT ROCKS AND FRACTURES IN BASALTS: THE FAROE ISLANDS, NE ATLANTIC MARGIN

Few detailed field studies to date have characterised the faults, fractures and associated fault rocks within continental flood basalt provinces. The Faroe Islands are situated on the Faroe platform on the NE Atlantic margin and form part of the extensive Palaeogene flood basalts that cover much of the Faroe-Shetland basin (FSB). Seismic and potential field data suggest that the faults and fractures in the Faroes are related to NW-SE trending lineaments, previously interpreted as broad transfer-zones. The exceptional exposure of the Faroe Islands provides an opportunity to study sub-seismic scale brittle deformation within the Faroe Islands Basalt Group (FIBG), which is likely to be analogous to deformation in the FSB offshore. The FIBG displays significant vertical and lateral variation, and is interspersed with sedimentary marker horizons (up to 25 m thick) laid down during periods of volcanic quiescence, providing an excellent opportunity to study the geometry and growth of faults, fractures and joints within both clastic and volcanic units.

Our detailed analysis of the individual faults and fault zones – the first attempted in this area - has revealed new constraints on the generation and relative timings of faults within the FIBG. Field observations reveal three distinct fault types: 1) individual fault surfaces with shear hydraulic fractures/veins; 2) broader fault zones comprising multiple fault clusters that display brecciation and tensile hydrofracture veins filled with euhedral zeolites; and 3) open faults with shear hydraulic fractures/veins, bounding clastic fills, some of which have experienced fluidization and injection events. These different types are interpreted as forming at different depths and times, likely under varying differential stresses and pore-fluid pressure conditions. Future study will use microstructural, geochemical and fluid inclusion studies to verify preliminary interpretations of fault formation within the FIBG.