The significance of the Caledonian Orogeny (sensu stricto) in Britain and Ireland

Paul D. Ryan1, Jack Soper2

1=Geology Department, NUI, Galway, Ireland. ; 2=Gam’s Bank, Riverside, Threshfield, N. Yorkshire BD23 4NP

Paul.ryan@nuigalway.ie

Orogenic names, in modern usage, are associated with the closure of a palaeo-ocean. Orogenic phases are, therefore, related to processes associated with the destruction of oceanic lithosphere within this ocean, such as: arc-collision; micro-continent collision; thermal and topographic pulses at an active margin; or final suturing. The Caledonian Orogeny (sensu lato) is identified as being the result of the closure of the Iapetus Ocean and the Caledonian Orogeny (sensu stricto) with its final suturing. In Britain and Ireland, early events related to Ordovician arc-collision north of the Iapetus Suture are termed the Grampian. A similar metamorphic even, although reported elsewhere, is not recorded south of the Suture in these islands. Late Ordovician and Silurian deformation to the north of the Suture is generally related to dewatering within an accretionary prism. In western Ireland, Ordovician and Silurian strata deposited in South Mayo lay in the hanging wall of the associated subduction zone and only record local titling during this time. The main dewatering cleavage in the rocks of South Mayo developed during the Pridoli. We relate this event to the suturing of Iapetus and, hence, the Caledonian Orogeny (sensu stricto). Rocks of similar age to the south of the Suture record foreland basin style deposition into the early Devonian. They do not develop a cleavage until late Emsian times. We argue that this Emsian cleavage was developed during the Acadian Event of the Variscan Orogeny caused by the closure of the Rheic Ocean to the south of Avalonia and that it should not be termed ‘Caledonian’.