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
.