NORTHWEST-VERGING FOLDS AND THE NORTHWESTWARD MOVEMENT OF THE CALEDONIAN JOTUN NAPPE, NORWAY

A family of asymmetric, northwest-verging back folds and crenulations occur on a regional scale in the Caledonian decollement zone in southwestern Norway. They are the result of reversal of the emplacement movement of the Jotun and other nappes in Early Devonian time. The back folds may have formed progressively by two different mechanisms: actively by buckling of the layering, and more passively by transfer of slip across the layering (reverse-slip crenulations) to balance the thinning of the zone caused by foliation-parallel slip and shear band formation. The former mechanism is considered to be most important during the initial stages of fold development, gradually giving way to the latter mechanism. The hinge lines of these folds appear to have remained stationary during deformation. This is explained by the observation that the hinge lines lie within or close to the shear plane, i.e. the plane of no rotation in simple shear deformation. Integration of strain across the deformation zone indicates a magnitude of back movement in the order of ca. 20-36 km, and it is argued that the deformation was close to simple shear.