Faults, fault rocks and fractures in basalts: a macro- to micro-analysis of fault rock evolution on the NE Atlantic Margin

Many upper crustal fault zones contain significant volumes of brecciated wall rock formed in the vicinity of dilational jogs, which can form permeability pathways for the migration of mineralising hydrothermal fluids or hydrocarbons. Such fault-breccia formation is commonly assumed to be a geologically instantaneous process, resulting from a sudden differential in fluid pressures between a dilational jog and its surrounding country rock, which leads to inward implosion. However, at shallow crustal depths (0-2km, and potentially deeper with increased fluid pressures) mechanically strong rocks (e.g. crystalline/carbonate rocks) may be able to support dilational jogs as persistent, high permeability, open subterranean cavities that become more gradually filled by fragments of the surrounding wall rocks through time. Understanding the development of fault breccias is therefore scientifically and economically important, as the two breccia models have markedly contrasting sealing and fluid flow histories. The Faroe Islands - the location of the present study - sit above the Jurassic-Palaeogene-age Faroe-Shetland basin on the European NE Atlantic margin. The islands are largely made up of Palaeocene-age basaltic lava units (the Faroe Islands Basalt Group: FIBG; part of the North Atlantic Igneous Province: NAIP) that were emplaced as a precursor to continental break up, and sea-floor spreading in the NE Atlantic. Deformation structures developed on the islands include variously oriented fault-sets (relating to anticlockwise rotation of the extension direction through time) and broad anticlines that form a trilete pattern centred on the islands. These deformation structures were formed and evolved immediately before, during and following continental break-up. This study documents the development of regionally syn-magmatic fault arrays, and contrasts these with later post-magmatic fault-reactivation at shallow burial depths and the development of, potentially, very high-permeability pathways (fault voids and their infills) through the FIBG during the latter event. We find that, in particular, faults in basalts are in many ways comparable to faults formed at shallow crustal depths in carbonate rocks and crystalline basement, most likely reflecting the similarities in their mechanical properties under near surface pressures and temperatures. The nature and style of the fault infills provides compelling evidence to suggest that subterranean voids associated with faults were persistent features within the FIBG, and if structurally linked to faults cutting the underlying basin fill sediments, could facilitate significant hydrocarbon migration from deep reservoirs.