40Ar/ 39Ar muscovite dates from the nappe region of southwestern Norway: dating extensional deformation in the Scandinavian Caledonides

Abstract

The Caledonian thrust nappes and basal décollement zone above the Baltic shield record penetrative Caledonian deformation related to ESE-directed nappe transport, and subsequent WNW-directed ductile shearing during extensional collapse of the orogen. Muscovite within Proterozoic quartzite conglomerates in the Lower Bergsdalen Nappe that record only ESE-vergent Caledonian deformation display internally concordant 40Ar/ 39Ar age spectra which yield plateau ages of 403 Ma and 398 Ma. Similar plateau ages (402 and 399 Ma) were obtained from muscovite from mica schists which record additional penetrative WNW-directed shearing, suggesting that muscovite intracrystalline argon systems did not cool below appropriate closure temperatures until during or after extensional WNW-vergent movements. Quartz microfabrics indicate that temperatures were higher than those required for argon retention in muscovite until the end of the WNW-directed nappe translation (extension), and that lower temperatures were maintained during subsequent movements along W- and NW-dipping extensional ductile shear zones. The 40Ar/ 39Ar muscovite ages are therefore interpreted to post-date the extensional WNW-directed shearing along the décollement zone, and closely date the initiation of W- and NW-dipping extensional shear zones. When compared to the age of contractional deformation, the results confirm that extension followed orogenic contraction very shortly after the main, Scandian orogenic phase.