Early carboniferous unroofing in western Norway: A tale of alkali feldsparthermochronology

Ar-40/Ar-39 thermochronologic data from an extensional detachment zone in w estern Norway document punctuated late Paleozoic through Mesozoic tectonoth ermal activity. Diffusion and thermal modeling of alkali feldspars sampled in a profile through the detachment replicate laboratory argon release patt erns when feldspars comprise three or four diffusion domains with differing activation energies that represent high-, intermediate-, and low-closure t emperatures. These domains correspond to distinct changes in late Paleozoic cooling rates: (1) slow cooling at 0.4 degrees-2.2 degrees C/m.yr. from ca . 380-360 Mai (2) rapid cooling at >15 degrees C/m.yr. in Late Devonian-Ear ly Carboniferous time (360-340 Ma); (3) slow cooling at 0.4 degrees-1.7 deg rees C/m.yr. after ca. 340 Ma, with partial argon loss in Permian and Late Jurassic-Early Cretaceous times. The Early Carboniferous rapid cooling even t recorded by the feldspars links the western Norway margin to contemporane ous unconformities, regional extension, basin inversion, and igneous activi ty previously identified around the proto-North Atlantic perimeter. We attr ibute this Early Carboniferous rapid cooling event in western Norway to an episode of unroofing. Unroofing was a consequence of increased topography a nd erosion; movement of the rocks toward the surface (higher topography) ma y have been triggered by thermal underplating combined with regional, trans current tectonics. The results support the judicious application of multidi ffusion domain analysis of Ar-Ar alkali feldspar data from areas with well- constrained tectonostratigraphy and other, independent means of age control .