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The Scandinavia - Greenland segment of the Caledonian - Appalachian orogen provides an excellent example of continent-continent collision, with evidence for the closure of an ocean (Iapetus) and underthrusting of Laurentia by Baltica involving many hundreds of kilometres of horizontal shortening. Similarity with the on-going India-Asia collision in the Himalaya is notable. Attention is focused here on the remarkable presence, in both orogens, of a central nappe complex, composed of granulite facies migmatites with leucogranites, derived from the continental margin of the underthrust slab and extruded great distances onto the platforms.
In the Scandes, these ductile high grade rocks (the Seve Nappe Complex) originated from the Baltoscandian outer margin and continent-ocean transition zone and were emplaced c. 400 km eastwards onto the platform. Metamorphic grade decreases both upwards into the overlying, ocean-derived Köli Nappes and downwards through underlying allochthons of the Särv and Offerdal nappes and thick mylonites and phyllonites into sedimentary successions of the lowermost thrust sheets. The contact to Köli, Iapetus-derived allochthons is, in many places, a normal fault; sometimes a major extensional detachment.
Existing isotope age data (Claesson 1987) indicate that the granulite facies metamorphism of the Seve nappes of central west Jämtland occurred in the mid Silurian at c. 425 Ma, after which they were extruded eastwards, probably separating leucogranites (c. 415 Ma) and pegmatites en route, and eventually cooled in the foreland in the early-middle Devonian (c. 395 Ma). The present geometry, with westward wedging-out of this vast allochthon and its lenticularity in the hinterland was apparently established during late extensional collapse of the mountain belt.
On-going work seeks to better constrain the provenance of the Seve nappes in Jämtland, the timing of their high grade metamorphism and the conditions during emplacement onto the Baltoscandian platform. New work is also needed in other parts of the Scandes, where the Seve nappes contain eclogites and the high grade metamorphism occurred during Iapetus ocean closure, prior to Scandian collision.
References:
Claesson S., 1987: Isotopic evidence for the Precambrian provenance and Caledonian metamorphism of high grade paragneisses from the Seve Nappes, Scandinavian Caledonides. Contributions to Mineralogy and Petrology 97, 196-204.
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