Contrasting modes of reactivation in the Outer Hebrides Fault Zone, northern Barra, Scotland

The gently ESE-dipping Outer Hebrides Fault Zone may be traced for c. 170 k m through the Lewisian gneisses of northwestern Scotland and defines broad, arcuate swings in strike from NE-SW to almost north Youth. This study eval uates the kinematic evolution of the fault zone within the hinge region of such a strike swing exposed in the small isles north of Barra. New structur al mapping of this area has concentrated on the geometry, kinematic signifi cance and the relative age of different structural elements of the fault zo ne, and provides a comparison with the better known tectonics of the northe rn portions of the Outer Hebrides Fault Zone. Hitherto unrecognized top-to- the-NW ductile thrusting in this southern portion of the fault zone, couple d with abundant evidence of extensional reactivation and lack of sinistral strike-slip in phyllonites as reported further north, suggests that the pre sent segmented and arcuate geometry is a primary feature of the fault syste m. The original thrust template governed the subsequent reactivation and re lative importance of both late Caledonian strike-slip and extensional colla pse components. Inherited structural architectures may thus provide geometr ic constraints on the nature and patterns of subsequent reactivation and te ctonism.