Timing and kinematics of Caledonian thrusting and extensional collapse, southern Norway: evidence from 40Ar/ 39Ar thermochronology

Abstract

40Ar/ 39Ar dating methods have been applied to rocks across the Caledonian orogen in southwestern Norway. In the eastern nappe area, K-feldspar thermochronological modeling, microfabric characteristics and conodont color alteration together indicate that temperatures were not above the closure temperature of muscovite for any significant period of time. Two groups of muscovite and biotite ages from this area (415–408 for most samples with top-to-the-SE fabrics and 402–394 for samples with top-to-the-NW fabrics) are therefore interpreted as ages of contractional (thrusting) and extensional (hinterland-directed nappe translation) deformation, respectively. In the west (hinterland), peak Caledonian temperatures were higher, and 40Ar/ 39Ar plateau ages are generally interpreted as cooling ages. The western basement and lower nappes cooled rapidly through ∼500°C (basement) at ∼404 Ma and 350°C (basement and lower nappes) shortly after, i.e. during extensional top-to-the-NW transport of the orogenic wedge. In addition, tectonostratigraphically higher nappes in the hinterland show evidence of earlier cooling, probably following Ordovician orogenic activity prior to the main collisional stage. The new 40Ar/ 39Ar data conform to kinematic observations that contraction and extension in the Caledonian nappe region were sequential, and that the change from contraction (convergence) to extension (divergence) was quick (between 408 and 402 Ma).