Polyphase kinematics and geochronology of the late-Caledonian Kollstraumen detachment, north-central Norway
pp. 299-316

The condensed tectonostratigraphic section from the Central Norway basement window, across the Devonian Kollstraumen detachment zone, and in to the Helgeland Nappe Complex provides an excellent opportunity to assess the timing and kinematics of polyphase Caledonian deformation in central Norway. The Helgeland Nappe Complex, at present preserved in the hanging-wall of the Kollstraumen detachment zone (in the southwest) and the Nesna shear zone (north and east), provides evidence of Ordovician, Taconic deformation succeeded by emplacement of voluminous calc-alkaline plutons. These events pre-date the eastward thrusting of the Helgeland Nappe Complex across subjacent rock units during Scandian contractional deformation.

Structurally below the Kollstraumen detachment zone, Palaeoproterozoic orthogneisses and overlying supracrustal rocks of the Skjøtingen Nappe form the northern Central Norway Basement Window. During the Scandian event, these rocks were metamorphosed at medium to high grade and acquired a penetrative ductile fabric folded by roughly orogen-parallel, NE-SW-oriented regional folds. U-Pb dates on dykes of tonalite and granite that intruded supracrustal rocks of the Skjøtingen Nappe have yielded ages of 436 Ma and 430 Ma, respectively. Combining geochronological and structural data, we infer that the granite dykes may overlap in time with earliest Scandian contractional deformation. Later deformation of the dykes may have occurred during continued Scandian contraction as well as extension parallel to the regional fold axis. Titanite and monazite U-Pb dates from the dykes (c. 401-402 Ma) coincide in time with numerous pegmatites in central Norway and are interpreted to date metamorphism during latest Scandian exhumation of the Central Norway Basement Window. The contrasts in latest- to post-Scandian structural and metamorphic development suggests a major structural break between the Central Norway basement window and the Helgeland Nappe Complex, across which Middle and Upper Allochthonous units are excised along the Kollstraumen detachment zone.