Pg. Andreasson et al., Dawn of Phanerozoic orogeny in the North Atlantic tract; Evidence from theSeve-Kalak Superterrane, Scandinavian Caledonides, GFF, 120, 1998, pp. 159-172
Despite their Early Phanerozoic age, the Scandinavian Caledonides provide a
remarkably rich record of the continental break-up and development of the
continent-ocean transition as well as the ensuing subduction and imbricatio
n of the pristine plate margin, all emplaced on land and accessible. We fir
st organize the evidence for Baltoscandian rift basin formation and magmati
sm, now scattered in two major nappe complexes (and by semantics) in terms
of a superterrane, the Seve-Kalak Superterrane. Extensive (1000 km) mafic d
yke swarms and partly sheeted dyke complexes are interpreted as fragments o
f a Large Igneous Province. While attempted break-up and some tholeiitic ma
gmatism took place already at c. 800 Ma, successful rifting occurred first
in connection with intense, partly alkaline dyking and emplacement of ultra
mafic complexes between c. 620 and 550 Ma. This late magmatism is markedly
enriched as compared to MORE, interpreted to reflect mixing between an enri
ched mantle source component and depleted mantle. The evidence for Early Or
dovician imbrication and subduction of the rifted and intruded margin is th
en reviewed. In order to explain the preservation of parts of the continent
-ocean transition and rift basins, we suggest early detachment and thrustin
g towards the foreland, by analogy with the emplacement of the Semail Ophio
lite. Intercalated garnet peridotites require incorporation of subcontinent
al mantle fragments during imbrication. Structures and fabrics of eclogites
and their host rocks suggest that extensional tectonics was important duri
ng their Early Ordovician exhumation. If so, how much of Scandian extension
al tectonics is instead inherited Finnmarkian?