The early-Sveconorwegian orogeny in southern Norway: Tectonic model involving delamination of the sub-continental lithospheric mantle

Abstract

The Sveconorwegian orogeny is a more than 250 Myr-long multiphase orogenic event that contributed to the amalgamation of Fennoscandia into Rodinia at the end of the Mesoproterozoic. We propose a new lithospheric-scale tectonic model for the pre- to early Sveconorwegian orogenic evolution between 1280 and 1080 Ma. This evolution is recorded in southern Norway by two c. 25 km wide belts of up to granulite-facies rocks, the Bamble and Kongsberg Lithotectonic Units, and a c. 80 km belt in the Telemarkia Lithotectonic Unit, which contains lower-grade rocks and evidence of widespread plutonism. New zircon U–Pb geochronological data from 54 samples confirm magmatic activity between 1575 and 1485 Ma, and 1210 and 1140 Ma. The new data constrain peak amphibolite to granulite-facies metamorphism in Bamble and Kongsberg to between 1145 and 1130 Ma. New geological mapping shows that the Kongsberg Lithotectonic Unit is characterized by a regional, N–S trending highly transposed lithological banding reflecting E–W orthogonal shortening as expressed by tight to isoclinal folds, and a penetrative subvertical foliation bearing a steep peak-metamorphic lineation. Telemarkia is instead characterized by crustal extension-related bimodal magmatism and intermontane basin sedimentation between 1280 and 1080 Ma. Paired and in part coeval extension and compression are interpreted as reflecting asthenospheric upwelling, orogenic plateau development and delamination of the sub-continental lithospheric mantle beneath Telemarkia, followed by foundering (dripping-off) of the lithospheric mantle slab under the Kongsberg and Bamble Lithotectonic Units. This model stresses the pull effect of a foundering mantle slab in an actively shortening orogen to facilitate highly localised compression and granulite-facies metamorphism in the crust. It accounts for the well-known, first-order geological characteristics of the orogen and does not require the accretion of exotic terranes to explain the early-Sveconorwegian orogenic evolution.