Ductile rebound of an orogenic root: case study and numerical model of gravity tectonics in the Western Gneiss Complex, Caledonides, southern Norway

The main Caledonian deformation in the Western Gneiss Complex (southern Norway) has been interpreted as resulting from the gravity-driven ductile rebound of the orogenic root. Detailed field data from the Sognefjord profile, a 60-km long, continuously exposed shore section through the complex, are compared with the results of a numerical scale model of root rebound dimensioned according to reconstructions of the overall tectonic evolution. The strain pattern in the model can be closely tied to the previously recognized structural regimes, from east to west: R1 – little deformed Precambrian basement (stiff orogenic lid, basal part of upper crust); R2 – heterogeneously sheared basement ('simple shear' domain at upper crust/lower crust boundary); R3 – completely overprinted basement with eclogitic remnants ('pure shear' regime in the lower crust of the rebounded root). Subsequent to root rebound the profile remained practically unaffected by later tectonic events, except for tilting/uplift/erosion, providing a unique example of deep-seated gravity tectonics.