Timing of Caledonian Evorogenic Activity in the Scandinavian Caledonides

Abstract

RECENT discussion of the precise timing, extent and intensity of late Cambrian and early Ordovician movements and metamorphism in the Caledonian orogen and in Scandinavia in particular, has concentrated on graptolite faunal studies1–5 and radiometric age determinations6,7. In Norway, interest has centred on the West Finnmark and Trondheim regions following the revelation of an early Ordovician evorogenic event in Finnmark; in compiling evidence correlatable with this period of folding, intrusion and metamorphism, Sturt et al.6 tended to overemphasize the significance of the Trondhjem (Trondheim) disturbance8 so that later workers3,5,9 have mistakenly promoted this parorogenic episode to the status of an important deformation and metamorphism. Concern has been voiced in Scandinavian circles over this repeated misuse of Holtedahl's term8, which was originally used to define a local unconformity where a conglomerate overlies a greenstone sequence. Similar crustal movements recognized higher up in the stratigraphy, the Ekne and Horg disturbances of Vogt10, have analogues in minor breaks occurring in the Appalachian11 and British Caledonian12 Lower Palaeozoic successions. As the evolution of the Caledonian-Appalachian orogen is now being reassessed in the light of the plate tectonics philosophy13,14, it is timely to review aspects of the Norwegian evidence before further models are proposed.