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

Abstract

The gently ESE-dipping Outer Hebrides Fault Zone may be traced for c . 170 km through the Lewisian gneisses of northwestern Scotland and defines broad, arcuate swings in strike from NE–SW to almost north–south. This study evaluates 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 structural mapping of this area has concentrated on the geometry, kinematic significance and the relative age of different structural elements of the fault zone, and provides a comparison with the better known tectonics of the northern portions of the Outer Hebrides Fault Zone. Hitherto unrecognized top-to-the-NW ductile thrusting in this southern portion of the fault zone, coupled with abundant evidence of extensional reactivation and lack of sinistral strike-slip in phyllonites as reported further north, suggests that the present segmented and arcuate geometry is a primary feature of the fault system. The original thrust template governed the subsequent reactivation and relative importance of both late Caledonian strike-slip and extensional collapse components. Inherited structural architectures may thus provide geometric constraints on the nature and patterns of subsequent reactivation and tectonism.