Abstract

Few studies have focused on the geological characterization of exhumed subsurface faults and fractures within continental flood basalt provinces. We present field and microstructural observations of basalt-hosted fractures and faults from the Faroe Islands, NE Atlantic margin. For a given displacement, the thickness of these highly mineralized faults varies by over three orders of magnitude. Fault-zone thickness and displacement data from the Faroe Islands span nearly four orders of magnitude in displacement, but there is no strong positive correlation between fault-zone thickness and displacement. Fault-rock characterization reveals important breccia distinctions, including collapse/infill, crush/wear/abrasion, and implosion breccias, each with a respective increase in sealing potential. Collapse/infill breccias indicate sustained fluid-migration pathways, as they require open, subterranean cavities that are formed faster than mineral precipitation can seal them. Crush/wear/abrasion and implosion breccias record crack-seal behavior during successive slip events. Despite having distinctly different fault-rock assemblages, fault-zone thickness and displacement data from basalt-hosted faults are indistinguishable from comparable data obtained from sediment-hosted faults. This observation suggests that the first-order controls on fault development are the same in layered basalts and sediments, namely, fault surface bifurcation and linkage, asperity removal, and the accommodation of geometrically necessary strains in the wall rocks.

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