Laurentian palaeostress trajectories and ephemeral fracture permeability, Cambrian Eriboll Formation sandstones west of the Moine Thrust Zone, NW Scotland

Abstract: Cambrian Eriboll Formation sandstones of the Ardvreck Group that crop out west of the Moine Thrust Zone contain joints and quartz-filled or quartz-lined fractures that resemble cemented joints. Of the fractures containing quartz, five sets strike north, NW to WNW, NE, west and north; according to crosscutting relations this is a progression from the oldest to the youngest set. Sets include opening-mode microfractures, partly visible in transmitted light as fluid-inclusion planes and sharply defined as microveins using SEM-based cathodoluminescence (CL). Dating the oldest north-striking set, using inferred quartz accumulation rates, fluid inclusions and burial history, suggests that these fractures mark a Palaeozoic east–west least horizontal stress trajectory in Laurentia. The youngest two sets of porous fractures are associated with faults that cut and postdate the Moine Thrust Zone. Data indicate that at depth in basins, pervasive fracture systems arising from discrete loading events are ephemeral owing to fracture porosity destruction by cementation. Exhumed rocks from deeply eroded basins can be used to study fracture patterns arising from past crustal loading conditions, with the purpose of constraining palaeostress trajectories and the growth and decay of potential fracture fluid conduits in deep-basin settings. In NW Scotland, Cambro-Ordovician rocks exten