The development of cavities and clastic infills along fault-related fractures in Tertiary basalts on the NE Atlantic margin

Field-based geological observations have revealed the hitherto unrecognised development of post-magmatic, brittle deformation structures cutting Tertiary volcanic rocks in the Faroe Islands. These faults and fractures are characteristically associated with different styles of clastic sedimentary infill including: 1) 0.3–1.0 m thick clastic units infilling open fractures formed along pre-existing steeply-dipping to sub-vertical faults; 2) 0.1–0.6 m thick sub-horizontal clastic units displaying internal features consistent with deposition from flowing water passing through complex open subterranean cavity systems within fractured basalts; 3) Anastomosing mm-scale and planar dm-scale clastic intrusion features mobilised and emplaced during transient, fault-related overpressuring events along pre-existing fractures cutting the surrounding volcanic units. The infill features provide evidence for the existence of sustained open cavities in the sub-surface. The clastic materials are commonly internally affected by later fault-related deformation and lack mineralisation, unlike all preceding faulting episodes in the Faroes region, perhaps reflecting their near-surface development. We believe structures equivalent to these features may occur widely in other parts of the NE Atlantic margin, particularly along the outer arcs of gentle regional-scale fold hinges. The uncemented fracture-hosted clastic infills potentially represent important fluid migration pathways within the otherwise low permeability Cenozoic volcanic sequences of the NE Atlantic region.

Research highlights
► The fault-related clastic sediments detailed in the present paper provide evidence for the existence of sustained open cavities in the sub-surface.
► The lack of associated mineralisation within the clastic sediments probably indicates post-burial, near-surface fault movements (<1–2 km depth?).
► Clastic infills may be widespread offshore along the NE Atlantic margin. The unmineralised nature of the clastics may mean that these faults present fluid-flow pathways, particularly at higher levels, but also potentially deeper, within the Faroe-Shetland Basin.
► The open cavities that originally formed would have introduced considerable localized permeability, potentially facilitating rapid cross-fault and cross-stratal migration of fluids, including hydrocarbons.