A Caledonian plate tectonic model

A plate tectonic model for the Caledonides of the British Isles with three non-parallel lower Ordovician-lower Devonian subduction systems is presented. In the north-west, the Caledonian thrust front is interpreted as the site of multiple, slow ESE.-directed subduction (Sd1) of Lewisian-bearing continental foreland plate beneath continental crust containing an ensialic Dalradian basin. South of the continental crust of the Southern Uplands zone lay a ENE.–WSW.-trending plate boundary (Sd2) where Iapetus oceanic crust was subducted to the NNW. The convergence of subducted lithosphere beneath the Moine-Dalradian plate is linked to its prolonged history of diachronous deformation, metamorphism, intrusion and uplift. The oblique subduction on Sd1 and Sd2 caused collision below the Moine-Dalradian outcrop generating sinistral strike-slip faults with downthrown south-east sides, spreading north-east with time. Sinistral displacement on the Great Glen Fault system, and of the Connemara inlier is linked to this model. A third (Ordovician-Silurian) subduction system (Sd3) is defined to the north-west of a volcanic arc from the Lake District in England, through southern Ireland. Iapetus oceanic crust was subducted to the south-east here. Evidence is presented for oblique collision between Sd2 and Sd3 from late Ordovician to early Devonian, followed by an unstable triple point migrating south-westwards across the British Isles region, and subsequent dextral strike-slip movement on the suture. Triple-point migration to the south-west is measured using diachronous ending of volcanism and plutonism on Sd 2 and Sd 3 Opposed lower Devonian collision points now appear to be dextrally offset by about 900 km. Calculated closure rates on Sd 2 and Sd 3 give minimum Arenig widths for Iapetus of 600–800 km. About 1270 km of late Ordovician-Devonian dextral displacement on the Iapetus suture could have generated 570 km of foreland convergence in the East Green­land-Scandinavian Caledonides.