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

Abstract

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.