Pre-Alleghanian terrane tectonics in the British and Irish Caledonides

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Abstract
In the British Isles, 11 major Caledonian (pre-Variscan) terranes can be identified. The northern (Laurentian) miogeocline is composed of two terranes that were probably originally basement to the Cambro-Ordovician passive margin. These were deformed, uplifted, and amalgamated by across-strike shortening in the Grampian Orogeny (ca. 500 Ma) and later between 460 and 410 Ma. The southern miogeocline (Midland platform) is composed of Gondwanaland continental crust and shelf sediments, and appears to contain at least two terranes that were amalgamated prior to the Early Cambrian. Between these two miogeoclines lie a number of terranes that were finally docked in the site of the closed Iapetus by middle Devonian time. Docking deformation was apparently by sinistral transpressional shear, and it occurred at about 400 Ma. This may be equated with the Acadian orogeny of North America. Analysis of these terranes indicates that they are fragments of: (a) the northern miogeocline; (b) the Laurentian continental foreland; (c) arcs, back arcs, and fore arcs of Lower and Middle Ordovician age; and (d) a Silurian successor basin. They appear to indicate that initial closure and cessation of oceanic crustal subduction occurred in Iapetus by Late Ordovician time. The end-Silurian to pre–Middle Devonian sinistral strike-slip may be interpreted as either the oblique convergence of Gondwanaland/Cadomia into the angle formed by the British and mid-European Caledonide trends or as the clockwise rotation and convergence of Gondwanaland/Cadomia against this Laurasian landmass. The latter model may mean that a clockwise-rotating Gondwanaland drove the Caledonian orogeny in the three arms of Iapetus from at least Middle Ordovician times onward. The subsequent counterclockwise rotation of Gondwanaland in the Variscan implies that the Caledonian and Variscan/Alleghanian orogenies are the results of fundamentally distinct plate-tectonic events.

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