The Scottish paratectonic Caledonides

Abstract

Synopsis The southern margin of the Laurasian continent has been destructive or strike-slip from at least 576 Ma to c. 360 Ma. There may have been a pre- or early Arenig obduction of ophiolite onto it, and this ophiolite now provides the base to the Ordovician part of the Highland Border Complex and could have been responsible for regional metamorphism in Dalradian or related rocks. At c. 480 Ma there was certainly final obduction of the Ballantrae Complex, which formed in an oceanic setting some distance south of its present location. This ophiolite was initially obducted over olistostromes and formed the basement to a later fore-arc basin, the proximal part of which is now exposed at Girvan. The Midland Valley, a Palaeozoic plutonic—volcanic arc, founded on a metamorphic basement, supplied sediments to the fore-arc—accretionary prism on its southern margin. This metamorphic basement is unlikely to have been of local Dalradian type. Growth of the paratectonic zone involved thrusting and probably strike-slip accretion of possibly 6 terranes. Ophiolites and the accretionary prism of the Southern Uplands were thrust NW onto Midland Valley basement. The whole of the paratectonic zone was initially juxtaposed against the Moine—Dalradian terrane by either strike-slip or thrust movements, but the latest interaction between the Midland Valley and Dalradian terranes involved thrusting. Future investigations into the timing and extent of thrust and strike-slip activity will greatly increase our understanding of continental growth in this region.