Subduction and eduction of continental crust: major mechanisms during continent-continent collision and orogenic extensional collapse, a model based on the south Norwegian Caledonides

During continental collision in the middle Silurian, the thickness of the lithosphere under the Caledonides of S. Norway was doubled by subduction of the western margin of Baltica, including the Western Gneiss Region, under Laurentia. Crustal rocks of the Baltic plate reached sub-Moho depths of near 100 km or more as inferred from the presence of coesite in eclogites. Isostatic calculations indicate an average elevation of the mountain chain of about 3 km at this stage. The subducted lithosphere experienced vertical constrictional strains as a result of slab-pull by its heavy and cold root. Eduction of the deeply buried crustal material was initiated by decoupling of the Thermal Boundary Layer in the subducted lithosphere. Isostatic rebound resulted in very rapid uplift (1–2 mm yr^-1), and the deep crust was exhumed, mainly by tectonic extensional stripping over a period of 30–40 Myr. The eduction was probably related to a rolling hinge, footwall uplift mechanism, and the early high-pressure coaxial fabrics were overprinted by extensional simple shear as the deep crust reached middle and upper crustal levels. The model explains the present-day normal crustal thickness under the exhumed deep rocks without necessarily invoking large-scale lateral flow of material in the lower crust or igneous underplating.