The provenance and evolution of the Rjukan Rift Basin, Telemark, south Norway: The shift from a rift basin to an epicontinental sea along a Mesoproterozoic supercontinent

Abstract

The Rjukan Rift Basin formed in the Mesoproterozoic (ca. 1.5–1.4 Ga) within the margin of a supercontinent that was formed by the Laurentian and Fennoscandian shields. The basin evolved from a continental syn-rift stage to a marine post-rift stage. Great volumes of bimodal volcanic rocks characterize the syn-rift stage. During the syn-rift and post-rift stages, the Rjukan Rift Basin was filled with ca. 7-km-thick sedimentary succession. The succession started with coarse alluvial sediments that were succeeded by marine incursions and an intervening alluvial sedimentation. At the end of the post-rift stage, the rift basin was completely filled, and a shallow epicontinental sea covered the basin and its surroundings. This area is now represented by quartz arenite successions covering older basement rocks in south Norway. The trend of a vanishing local provenance signal and an increasing dominance of Paleoproterozoic sediment sources occurs throughout the rift infill. This trend supports a period of volcanic quiescence and cratonization before the onset of the Sveconorwegian Orogeny. Detrital zircon U–Pb geochronology and Hf isotopic analyses suggest that the same sources prevailed throughout the history of the basin. Basement rocks for all major age groups, represented by the detrital zircon U–Pb age populations, can be found both in the Fennoscandian and Laurentian shields. However, the existence of ca. 1730-Ma ages in almost all of the sediments and the apparent paucity of 1650–1500-Ma ages support a Laurentian provenance. This suggests that southern Norway was originally closer to Laurentia but later moved to its current location. This finding supports previous ideas of the origin of southern Norway. A similar age pattern and Paleoproterozoic zircon dominance have been reported from other sedimentary rocks outside of the study area. Correlation of these rocks to the final stages of the Rjukan Rift Basin is plausible.