Fracture size, frequency, and strain in the Cambrian Eriboll Formation sandstones, NW Scotland

We investigated fracture intensity of the Cambrian Eriboll Formation sandstone exposures west of the Palaeozoic Moine Thrust Belt (MTB) in NW Scotland, by measuring the number and size of fractures encountered per unit length along lines of observation orientated perpendicular to fracture strike. Eriboll Formation sandstones have at least five fracture sets, each of which includes penetrative arrays of opening-mode microfractures (quartz-filled micro-veins). The size distributions of the fractures can be described by power laws over more than three orders of magnitude. The observed power-law equation parameters suggest that strain is dominated by the largest fractures present in each set; however, field evidence suggests that opening-mode fractures with apertures greater than 1 cm are rare. Strain from microscopic fractures is heterogeneous compared with strain caused by fractures that are sufficiently wide (0.1 mm and above) to be measured on exposures, reflecting greater variation in fracture size apparent at microscopic scales. Although size patterns are from sets formed at different times, consistent cumulative frequency versus aperture slopes suggests that a common mechanism governs organization of fracture sizes. The penetrative sets of natural fractures within the Eriboll Formation record a strain history spanning burial, Caledonian orogenesis, and uplift.