In north-central Norway, 40Ar/39Ar ages from a profile through the Nesna Shear Zone (NSZ) and into the underlying Sjona window, a gneiss-cored culmination, document time of onset of ductile shearing and two episodes of unroofing for the crustal package. A progressive decrease in white mica and biotite ages from 397.8 ± 1.0 to 378.3 ± 0.9 Ma, downward through the tectonostratigraphy, conforms to an overall pattern of cooling and argon retention as the crustal section was progressively unroofed. The age differences between the NSZ and Sjona window micas allow us further to distinguish two kinematically distinct events. White mica and biotite ages within the NSZ fall between 397.8 ± 1.0 and 387.1 ± 0.8 Ma and correspond closely to time of genesis of the ductile, top-WSW stretching fabrics within the shear wne. Biotites within the Sjona window Jack the same ductile fabric and instead record younger cooling largely between 384.3 ± 1.0 and 378.3 ± 0.9 Ma. Sjona window biotites passed through closure temperatures similar to those in the NSZ, but under conditions dominated by positive buoyancy of the Sjona gneisses, aided by shortening normal to the top-WSW stretching direction. Unroofing or 'doming' of the Sjona window was almost certainly promoted, and perhaps accelerated, by earlier, NSZ-related extension in the overlying nappe pile. Extension had waned by latest Mid Devonian time when the shear-wne system was overtaken by activity on younger, steeper, ductile-to-brittle extensional faults. K-feldspar ages from the Sjona window yield a pronounced Early Carboniferous signature (335-346 Ma) that we attribute to a second episode of rapid cooling and unroofing, after top-WSW ductile motion on the NSZ had ceased. We relate unroofing to motion on the steeper and younger faults that cut low-angle ductile shear zones like the NSZ. Early Carboniferous unroofing in north-central Norway would have been contemporaneous with widespread, Late Devonian-Early Carboniferous tectonic activity documented around the Caledonian perimeter. These mica and K-feldspar ages are the first published from north-central Norway and promote a picture of widespread, Early-Mid Devonian top-W to -SW ductile extension and la test Devonian-Early Carboniferous unroofing, operating at several tectonostratigraphic levels within the disintegrating Scandinavian Caledonide orogen.
The Nesna Shear Zone, north-central Norway: an 40 Ar /39Ar record of Early Devonian - Early Carboniferous ductile extension and unroofing
elizabeth. eide@ngu. no