Dunk tectonics: a multiple subduction/eduction model for the evolution of the Scandinavian Caledonides

The evolution of the Caledonides of Scandinavia included at least four events that resulted in high-pressure (HP) and ultrahigh-pressure (UHP) eclogite facies metamorphism and the introduction of peridotite from the mantle into the crust. Two are the classic Finnmarkian orogeny at ∼ 500 Ma and the Scandian orogeny between 400 and 425 Ma. We have dated a third HP metamorphism in the Seve Nappe Complex in Jämtland, central Sweden, at ∼454 Ma. Identical Upper Ordovician ages from eclogite of the Troms region, northern Norway, date a simultaneous fourth HP event that occurred in a different tectonic setting, probably western Iapetus. Recently reported Late Silurian ages from eclogite of the Bergen Arcs District, southern Norway, suggest a possible fifth event. We propose that all HP/UHP events in the Scandinavian Caledonides occurred through the subduction of crustal slabs into the mantle and their subsequent buoyancy-enhanced exhumation toward the surface (eduction). The slabs are recognized as intensely deformed, high-grade (eclogite and HP granulite facies) metamorphic rocks although HP metamorphism is not necessarily penetrative and retrogression to medium grade assemblages is common. The HP/UHP metamorphic terranes are bounded by thrust faults at the base and low-angle normal detachments at the top and are separated by these shear zones from less deformed terranes of lower pressure metamorphic grade. The peridotite bodies were introduced from the overlying mantle wedge during subduction or eduction. The evolution of metamorphic terranes in other orogens might be better understood through successive subduction/eduction events, a process we provisionally call "dunk" tectonics.