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Kinematic and thermal constraints on the reactivation of the Outer Hebrides Fault Zone, NW Scotland

Abstract The Outer Hebrides Fault Zone is a major easterly dipping reactivated shear zone which displaces Lewisian gneiss of the Laurentian craton, NW Scotland. Despite a number of detailed field studies, the fault zone remains poorly understood with regard to both its age of inception and precise conditions of reactivation. The island of Scalpay in the northern portion of the Outer Hebrides Fault Zone provides exceptional exposures through a variety of reactivated fault rock types and therefore represents an ideal location to investigate fault zone evolution via fluid inclusion studies of syn-tectonic quartz veins. This fluid inclusion study constrains reactivation temperatures more precisely than hitherto possible with top-to-the-NW ductile thrusting occurring at 500 ± 30°C. Subsequent phyllonitization is associated with oblique sinistral top-to-the-NE strike-slip at 230 ± 20°C, followed by a discrete system of top-to-the-NE/SE extensional detachments at 150 ± 20°C. Other recent fluid inclusion studies in the southern portion of the Outer Hebrides Fault Zone constrain phyllonitization associated with top-to-the-E displacement to 370 ± 20°C, with subsequent top-to-the-NE extensional detachments operating at 150–210°C. Thus, late-stage extensional detachment systems record consistent conditions of reactivation along the strike length of the Outer Hebrides Fault Zone. However, our results also clearly emphasize that conditions of earlier fault zone reactivation and phyllonitization were highly heterogeneous between the northern and southern portions, thus suggesting a spatial and temporal variation in the deformation and/or fluid flux system.