LATE CALEDONIAN EXTENSION IN THE SCANDINAVIAN CALEDONIDES - THE RORAGEN DETACHMENT REVISITED

This paper concerns the interplay of compression and extension during Palaeozoic collisional orogeny in Scandinavia, and focuses on a small Old Red Sandstone basin in the central Scandes at Roragen. Early Devon ian sediments are preserved there in a half-graben located in the west ern limb of a major, late, upright antiform. The latter, the Vigelen A ntiform, exposes Proterozoic crystalline rocks in the core and folds t he entire Caledonian tectonostratigraphy of the Lower, Middle and Uppe r Allochthons. Mylonites related to the eastward-directed thrust assem bly of these allochthons have been mapped over the antiform; they occu r in the western limb of the fold in the footwall of a high-angle W-di pping normal fault, the latter separating the Old Red Sandstones from the Caledonian nappes. These thrust-generated mylonites exhibit a suit e of superimposed down-to-NW shear sense indicators in the footwall of the fault. Displacement on this Roragen Detachment is inferred to be about ten kilometers. Reflection seismic profiling across the northern extension of the Vigelen Antiform, in the Storlien area, indicates th at this major longitudinal fold developed in the hanging wall of a sol e thrust that dips very gently westwards beneath this part of the moun tain belt. It is argued here that the Early Devonian Roragen half-grab en may have developed during thrust stacking of the Vigelen Antiform, providing evidence of Scandian upper crustal extension accompanying th e emplacement of the thrust sheets onto the Baltoscandian Platform. In the southern Scandes, full crustal extension would have occurred ther eafter, leading to the development of the mid-Devonian basins in the h interland of westernmost Norway.