Extensional tectonics of the Outer Hebrides Fault Zone, South Uist, northwest Scotland

Abstract

The Outer Hebrides Fault Zone is a major ESE-dipping reactivated structure within Lewisian basement gneisses of the Laurentian craton, northwest Scotland. Detailed mapping in South Uist reveals important new evidence that contributes to a better understanding of the kinematic evolution of the fault zone. Large quantities of pseudotachylite which characterize the fault zone on South Uist may in part be lithologically controlled, and therefore of little value in determining areas of greatest deformation and displacement. Only limited evidence is preserved for ductile and brittle thrust-sense movements along this portion of the fault zone. The tectonics of the fault zone on South Uist are dominated by structures associated with several episodes of pervasive top-down-to-the-SE to -ENE brittle extensional deformation, which are progressively overprinted by protophyllonitic and phyllonitic fabrics associated with top-down-to-the-E to -ENE extension. A series of late-stage high-angle normal faults record top-down-to-the-ESE to -ENE extension and cut the phyllonites. Fluid inclusion studies from syntectonic quartz veins constrain the conditions of phyllonite formation at 370 ± 20 °C. Field evidence suggests that this section of the Outer Hebrides Fault Zone may have been largely unaffected by sinistral strike-slip reactivation as reported along-strike to the north, suggesting both a varied and compartmentalized tectonic and evolutionary history along the length of the Outer Hebrides Fault Zone.