Problematic assumptions and curious coincidences associated with the Iapetus Ocean and pre-Scandian palaeogeography: origin of terranes in the Scandinavian Caledonides

The Caledonian Orogen was the final product of a sequence of processes initiated by extension and continental rifting, followed by translation of detached terranes across the intervening oceans, and eventual amalgamation during the final closing of the oceans and collisional orogenies. The Iapetus, Tornquist, Asgard oceans and various minor seaways developed at different times in response to rifting and dispersion of continental plates and their detached fragments, and were major sites of subduction and related magmatism during sequential orogenic events. The combination of these processes led to complex assemblies of diverse lithological elements in the Appalachians, and the Scottish-Irish and Scandinavian Caledonides. The interpretation and synthesis of the results from many studies on the provenance and histories of the constituent parts of these orogens over time has generated composite and complex tectonic models, which explain many of the observed features, but which are also partly compromised by problematic assumptions and interpretations. This paper summarizes evidence on the geological processes that accompanied the evolving Iapetus Ocean and the surrounding continents during these events. We focus especially on some of the most problematic assumptions in current models of the Scandinavian Caledonides, hinting at possible alternatives.