Extensional tectonics in the Caledonides: Synorogenic or postorogenic?

Extensional tectonism may form during as well as after the contractional hi story of a collisional orogeny. Kinematic studies combined with various age constraints in the southern Scandinavian Caledonides have revealed an exte nsional history which started by hinterland-directed transport of the oroge nic wedge above the basal decollement zone (Mode I extension) and proceeded by the development of hinterland-dipping shear zones (Mode II) and subsequ ent brittle faults (Mode III). The top-to-hinterland kinematics of the basa l decollement zone during Mode I extension indicate that this extensional h istory was entirely postorogenic already from the start. Any synorogenic ex tensional deformation must have occurred prior to the onset of this extensi onal history, which is dated to circa 405 Ma. Although synorogenic extensio n is likely to have occurred, large-scale synorogenic extensional collapse models in the Caledonides are at present difficult to prove, whereas impres sive postorogenic multi-stage extension is beautifully portrayed.