THE ROLE OF EXTENSIONAL TECTONICS IN THE CALEDONIDES OF SOUTH NORWAY

Detailed and regional structural-kinematic analyses of the Caledonides of south Norway show evidence for a crustal extension that is far more widespread and substantial than previously thought. Two different modes of extension are recognized. The first (Mode I) involved kilometer-scale back movement of the Caledonian nappes. The back movement resulted in extensive shear deformation within the decollement zone between the nappe wedge and the basement as seen by abundant asymmetric mylonite structures indicating translation of the Caledonian nappes towards the northwest. A second mode (Mode II) involved extension of the Baltic Shield by the formation of large-scale, normal-sense oblique shear zones. Mode II extension occurred partly during and partly after the back movement of the overlying nappes. The extensional deformation consistently overprints the contractional Caledonian deformation in the whole region, and is therefore interpreted as a distinct (Devonian) event post-dating the Ordovician-Upper Silurian construction of the Caledonian orogenic wedge. The uniform and regional-scale back movement of the orogenic wedge towards the central parts of the orogen indicates that the extension at the end of the Caledonian orogeny was closely related to post-collisional, Lower to Middle Devonian plate divergence.