The Svecofennian Orogen: a tectonic model

Abstract

Recent geological studies have provided new concepts concerning the timing of the final break-up of the Archaean craton. Age data on detrital zircons from sedimentary rocks in Sweden and Finland show marked similarities and indicate pervasive homogenization of accumulated material. Geochemical and isotopic studies in Sweden and Finland suggest the existence of two volcanic belts, separated by an area dominated by sedimentary rocks. However, the ages of metamorphism and plutonism differ; in particular, the late (post-1.85 Ga) granitoid magmatism was more voluminous in Sweden than in Finland. The Svecofennian crust in Finland is anomalously thick (up to 65 km) and is characterized by a high-velocity lower crust of variable thickness. The evolution of the Svecofennian Orogen is modelled by opening of the pre-Svecofennian ocean at 1.95 Ga and progressive accretion of two arc complexes to the Archaean craton between 1.91-1.87 Ga. Initial accretion at 1.91 Ga led to thrusting of the slices of an island arc complex and Proterozoic marine strata onto the thickening craton margin. Subduction reversal with simultaneous anti-clockwise rotation of the converging plate, or convergence of another plate, resulted in subduction below the newly accreted arc complex. The tectonic style at the craton margin changed from thrusting to lateral displacement in the east (in Finland). In the west (in Sweden), the subduction zone retreated along a transform fault. A large sedimentary basin closed in the east 1.89 Ga ago when another arc complex collided against the accreted one. Collision resulted in lithospheric thickening, subsequent delamination of the thickened lithosphere and compensation by a hot mantle underplate, and large-scale granitoid magmatism as a result of mixing between mantle magma and remelted arc complex material. The thick, dense layer that formed in the lower crust as the result of underplating kept the thickened crust in isostatic balance. Thus, the exhumation rate in this area was similar to the rate in areas of thinner crust. Subduction and sediment accumulation continued in the west until at least 1.87 Ga when large areas in the east were already stabilized. An intracratonic transpressional zone developed as the result of continued convergence. Subsequent extensional collapse along the transpressional zone caused melting of Svecofennian sedimentary rocks, migmatization and generation of the 1.84-1.82 Ga anatectic granites of southern Finland and central Sweden. The voluminous post-1.85 Ga granitoid magmatism in Sweden is tentatively associated with an east-west extension that is not related to the extensional collapse.