Deformation in the Caledonides of England, Ireland and Scotland

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Abstract
Summary: The past few years have witnessed a remarkable change in our view of the British Caledonides resulting from radiometric work, application of plate tectonic models as well as basic field work. Overshadowing all else has been the acceptance of a later Proterozoic (probably Grenville) orogeny in the Moine rocks occurring between the Moine thrust and the Great Glen fault. The northern Moines are clearly polymetamorphic in character, but several matters are still controversial, for example the separation of Proterozoic and Caledonian fabrics and the areal extent of the older orogeny. This northern block was subjected to intense and repeated tectonism and, above all, large thrust translation during the Caledonian orogeny and the rocks are now juxtaposed with the older Proterozoic and Archaean gneisses of the Caledonian foreland. In Ireland small areas of Grenville basement are in slide contact with the younger Moine and Dalradian rocks. South of the Great Glen fault above a postulated Grenville basement, are late Proterozoic sediments (Central Highland Granulites and Dalradian) which were orogenically deformed in lower Ordovician times. In this area the essence of the structural pattern established some years ago stands unquestioned for the most part, but significant modifications have been made, notably the recognition of a slice of ‘Cadomian’ basement in the Dalradian pile of NE Scotland, and possible pre-Caledonian basement in W Ireland. In Ireland the structural pattern in the Scottish Highlands may be recognised in the north, but becomes considerably modified in the west where the strike becomes E-W. The picture of the Scottish Highlands as a metamorphic Alpine-type orogen, dating from the early Ordovician, and undergoing uplift during Ordovician and later times, is complemented by the recent recognition of the Southern Uplands zone (later, non-metamorphic Caledonides) as being an accretionary prism, on the Aleutian model, in which imbricate thrusts developed in series towards the south above the subducting oceanic crust of Iapetus. The Iapetus suture lies between the Southern Uplands and northern England and passes through central Ireland. The pattern of Ordovician vulcanicity in northern England, Wales and SE Ireland has been attributed to southerly directed subduction of the Iapetus crust beneath the opposing continental margin. Final closure of Iapetus in Silurian and Devonian times, seems to have been marked by relatively mild tectonic activity in the ‘metamorphic Caledonides’, but by considerable compression across the slate belts of the Lake District, Wales and SE Ireland with polyphase structural evolution.