The Caledonian thrust front and palinspastic restorations in the southern Norwegian Caledonides

The Osen-Røa thrust sheet of the southern Norwegian Caledonides comprises the coarse clastic late Precambrian Sparagmite region, and the folded and imbricated Cambro-Silurian rocks of the Oslo region. Ramp-flat geometries occur in the hangingwall of the Osen-Røa thrust in the Mjøsa district. Two major ramps are recognized. One coincides with the strike of the Ringsaker inversion, while the other coincides with the traditional thrust front in the Gjøvik area. The Osen-Røa thrust cuts up section in the transport direction (south), eventually cutting out all late Precambrian rocks, to lie as a 150 km long flat in the Cambrian Alum shales of the Oslo region. The now eroded detachment termination probably died out horizontally in the Alum shales to end as a buried thrust front in the southern Oslo region. Restoration of hanging- and footwall cutoffs allows the amount of overthrusting to be calculated; the Sparagmite region/Oslo region boundary restores to a minimum of 130 km to the NNW. This displacement estimate agrees with estimates of 135 km NNW transport calculated from balanced cross-section restorations through the Oslo region.