Intermontane basins and bimodal volcanism at the onset of the Sveconorwegian Orogeny, southern Norway

Age, sedimentological, and geochemical data for the volcanosedimentary units in the Bandak succession record an episode of intracontinental extension, associated with formation of a series of intermontane basins. We hypothesize that these localized basins were formed at different stages of extensional collapse following a pulse of regional orogenesis. The Ofte and Røynstaul formations accumulated during rifting and were followed by bimodal volcanism within the Morgedal, Dalaå, and Gjuve formations. The Eidsborg Formation, at the top of the succession, records post-extension transgressive sedimentation. This tectonic scenario represents a pre-Sveconorwegian stage of orogenesis that can be related to convergent margin processes. The surface expression of crustal growth during this time is relatively small; however, the volume of a postulated mafic underplate that accreted and intruded the lower crust in the Telemark region at this time is unknown. Crustal growth during syn-convergent extension in a retro-arc region may be significant if volumes of accreted mafic underplate are large.