Dyke reactivation across the frictional-viscous transition: Precambrian deformation of Scourie dyke margins in NW Scotland

Basement-hosted regional dyke swarms are recognised worldwide and represent lithospherescale anisotropies that may significantly influence later deformation. Several Precambrian deformation events have affected the Lewisian Complex of NW Scotland, with the Badcallian (ca. 2.75Ga) and Inverian (ca. 2.5Ga) predating and the Laxfordian (ca. 1.75Ga) and ‘Late Laxfordian’ (ca. 1.55Ga) postdating the regional Scourie dyke swarm (ca. 2.4Ga). Brittle-ductile reactivation and reworking of these dykes is widely reported but has lacked detailed description. The present study focuses on dykes in the Assynt Terrane that show a range of contact relationships and reactivation styles. The dyke at Lochinver (NC087219) preserves undeformed intrusive contacts, with obvious ~1.5cm chilled margins. Two dykes at Achmelvich (NC056250 and NC055248) show different reactivation styles. One preserves local developments (<1.2m) of schistose viscous mylonites with dextral shear sense indicators in mica schists at both contacts. The style of deformation, veining and associated metamorphism is consistent with Laxfordian deformation in the nearby Canisp Shear Zone. The second dyke has ductile dextral deformation on its southern margin, but the northern margin preserves a complex history of localised ductile and later brittle reactivation associated with pseudotachylytes, parallel to the dyke margin and as injections into the surrounding gneisses. Shear sense indicators suggest this deformation, along with later crosscutting zeolite veins, are associated with sinistral shearing. Loch Assynt (NC2125) shows evidence for localised (1-5cm) sinistral ultramylonites and pseudotachylytes at the dyke margins, overprinted by brittle sinistral reactivation; both associated with widespread epidote and localised copper mineralisation – there is little evidence for schistose mylonites. Re-Os geochronology of the copper mineralisation yields an age of 1.55Ga. Thus, field and microstructural observations show at least three phases of brittle-ductile reactivation along dyke margins broadly correlated with Laxfordian and ‘Late Laxfordian’ events, both are kinematically coherent and distinct. Favourable regional tectonic stresses can therefore lead to tectonic inheritance within the geological record.