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 du ctile shear zones in which fluid channelling has hydrated the pre-exis ting basement rocks at low temperatures, forming chlorite-white mica p hyllonites in regions of highest strain. Strike-parallel mineral linea tions 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 tha t overprint strike-slip fabrics and may control the location of the Me sozoic Sea of Hebrides and Minch basins.