Extensional tectonics in the Caledonides: Synorogenic or postorogenic?

Extensional tectonism may form during as well as after the contractional history of a collisional orogeny. Kinematic studies combined with various age constraints in the southern Scandinavian Caledonides have revealed an extensional history which started by hinterland‐directed transport of the orogenic wedge above the basal décollement zone (Mode I extension) and proceeded by the development of hinterland‐dipping shear zones (Mode II) and subsequent brittle faults (Mode III). The top‐to‐hinterland kinematics of the basal décollement zone during Mode I extension indicate that this extensional history was entirely postorogenic already from the start. Any synorogenic extensional deformation must have occurred prior to the onset of this extensional history, which is dated to circa 405 Ma. Although synorogenic extension is likely to have occurred, large‐scale synorogenic extensional collapse models in the Caledonides are at present difficult to prove, whereas impressive postorogenic multi‐stage extension is beautifully portrayed.