The Hebridean terrane
The Hebridean terrane consists of that part of northwest Scotland, lying west of the Moine Thrust Zone, which formed part of the stable foreland of the Caledonian orogenic belt during the Palaeozoic era. The rocks making up this piece of crust comprise a crystalline, mainly gneissose, Archaean to Palaeoproterozoic basement (the Lewisian complex) overlain by an undeformed or gently tilted sedimentary cover of Neoproterozoic (the Torridonian) and Cambro-Ordovician age.
The present eastern margin of the terrane is formed by the Moine Thrust Zone, of Caledonian age, which itself conceals an earlier inferred boundary of Grenvillian age (~1.1 Ga) separating Lewisian crust with only minor Grenvillian effects in the west from gneisses with a high-grade (eclogite/granulite facies) overprint in the east (cf. Sanders et al. 1984). It also constitutes the western margin of the Knoydartian (~800 Ma) ‘orogenic’ event (cf. Vance et al. 1998). The nature of this major, long-lived crustal boundary clearly implies that the Hebridean terrane is at least parautochthonous, and may possibly be allochthonous, with respect to the adjacent Northern Highlands terrane.
The crystalline Lewisian basement of the Hebridean terrane was welded together during the Palaeoproterozoic at ~1.9Ga when it formed part of a large continental assemblage, including much of North America and Scandinavia. Further amalgamation and re-organization of crustal fragments around 1.0 Ga resulted in the supercontinent of Rodinia, which existed during the Neoproterozoic, and during this time the Torridonian deposits formed in intracontinental basins. After the break-up of this sector of Rodinia at the end of