The Laurentian Caledonides of Scotland and Ireland

Abstract

AbstractThe Caledonides of Britain and Ireland are one of the most intensively studied orogenic belts in the world. This review considers all the tectonic events associated with the development and closure of the Iapetus Ocean. It first summarizes the tectonic evolution of each segment involved in the Scottish–Irish sector of the Caledonides and then reviews the temporal evolution of the Caledonian Orogeny. Three main tectonic phases are recognized in the Scottish–Irish Caledonides: an Early–Middle Ordovician (475–465 Ma) phase termed the Grampian Orogeny; a phase of Silurian (435–425 Ma) tectonism restricted to the Northern Highland Terrane of Scotland termed the Scandian Orogeny; and an Early Devonian (395 Ma) phase termed the Acadian Orogeny. The Grampian Orogeny was caused by the collision of the Laurentian continental margin with an oceanic arc terrane and associated suprasubduction zone ophiolites during the latest Cambrian–Early Ordovician. Following the Grampian arc–continent collision event, there was a subduction polarity reversal. This facilitated continued subduction of Iapetan oceanic lithosphere and an Andean-type continental margin developed on and adjacent to the Laurentian margin in the Middle Ordovician along with a substantial thickness of accretionary prism sediments (the Southern Uplands–Longford Down Terrane). The Iapetus Ocean is believed to have disappeared by the Late Silurian based on the faunal record and a continent–continent collision ensued. The absence of significant regional deformation and metamorphism associated with the Late Silurian collision between Avalonia and the Scottish–Irish margin of Laurentia suggests that the continental collision in this sector of the Caledonian–Appalachian orogen was ‘soft’ or highly oblique. The exception is the Northern Highlands Terrane of Scotland that was believed to have been situated 500–700 km to the north along orogenic strike. This terrane records evidence for significant Silurian regional deformation and metamorphism attributed to the collision of the Laurentian margin of East Greenland with Baltica (the Scandian Orogeny). Current controversies in the Laurentian Caledonides of Scotland and Ireland are discussed at the end of this review.