Evidence for Caledonian sinistral strike-slip motion and associated fault zone weakening, Outer Hebrides Fault Zone, NW Scotland

The Outer Hebrides Fault Zone is a major ESE-dipping reactivated fault within the Lewisian gneisses of the Laurentian craton in NW Scotland. Early thrust structures are overprinted by a network of retrograde ductile shear zones in which fluid channelling has hydrated the pre-existing basement rocks at low temperatures, forming chlorite-white mica phyllonites in regions of highest strain. Strike-parallel mineral lineations and shear-sense indicators suggest sinistral displacements that are thought to be late Caledonian based on isotopic data and regional considerations. The phyllonites have focused extensional movements that overprint strike-slip fabrics and may control the location of the Mesozoic Sea of Hebrides and Minch basins.