Extensional tectonics in the North Atlantic Caledonides: a regional view

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Abstract Extensional structures characterize significant parts of the North Atlantic Caledonides. Silurian extensional deformation took place, particularly in the heated crust in the southern Greenland Caledonides, but the majority of the mapped extensional structures are Devonian (403–380 Ma). They formed by reactivation of low-angle Caledonian thrusts and by the formation of hinterland-dipping shear zones, of which the largest system is located in SW Norway and related to exhumation of the subducted margin of Baltica. The Devonian extension was concentrated to the central and southern part of the Caledonides, with maximum extension occurring in the area between the Western Gneiss Region of SW Norway and the Fjord Region of East Greenland. Kinematic data indicate that the main tectonic transport direction was toward the hinterland, and this pattern suggests that the main Devonian extension/transtension in the southern part of the North Atlantic region was postcontractional while strike-slip motions and possibly transpression occurred farther north. Late Devonian to enigmatic Early Carboniferous ages from UHP metamorphic assemblages in NE Greenland suggest that intracontinental subduction was going on in NE Greenland at a time when extensional deformation governed the rest of the orogenic belt.