DEVELOPMENT OF THE BALTOSCANDIAN FORELAND BASIN DURING CLOSURE OF THE IAPETUS OCEAN AND BALTICA-LAURENTIA COLLISION

The Baltoscandian foreland basin is preserved in the southern half of the Scandinavian Caledonides in the Jämtlandian nappes of western Sweden and related thrust sheets of the Lower Allochthon in southern Norway. The investigated successions contain hinterland-derived turbidites of the early to mid Ordovician and early to mid Silurian age, separated by an interval of shallow marine deposition, including regressive and transgressive strata reflecting Late Ordovician – Early Silurian climatic and possibly tectonic changes.

On-going studies of detrital zircons in these successions provide strong evidence of earliest Neoproterozoic to latest Paleoproterozoic source terrains, with the prominent Sveconorwegian Orogen(c 950-1150Ma) and Transscandinavian Igneous Belt (1650-1850 Ma) components, with the former particularly dominant in the hinterland-derived sandstones. Recycling from the Neoproterozoic sandstones of the Middle Allochthon should also be taken in account.

Isotope-age investigations of the uppermost thrust-sheets of the Middle Allochthon, the Seve Nappe Complex (granulite facies migmatites, amphibolites and, locally, eclogites) have yielded early Llandovery (Aeronian) zircon ages of peak metamorphism. In the foreland basin in the mid Llandovery (early Telychian) carbonate deposition gave away to a black shale basin, with influx of distal turbidites in the late Telychian. An overlying, coarser turbidite dominated formation is Wenlock in age.

Interestingly, the late Ordovician to Early Silurian successions in the overlying Iapetus-derived nappes of the Upper Allochthon, show facies changes similar to those in the Lower Allochthon , but starting a few million years earlier than in the foreland basin.