CRUSTAL EXHUMATION OF THE WESTERN GNEISS REGION UHP TERRANE, NORWAY: 40AR/39AR THERMOCHRONOLOGY AND FAULT-SLIP ANALYSIS

New ⁴⁰Ar/³⁹Ar muscovite and K-feldspar thermochronology combined with existing data reveal the timing and patterns of late-stage exhumation across the Western Gneiss Region (U)HP terrane. Muscovite age contours show that exhumation into the mid-upper crust progressed westward over a ~20 Myr period (~400–380 Ma). This exhumation was caused by i) E–W stretching and eastward tilting north of Nordfjord, where muscovite ages decrease from the foreland allochthons westward into the UHP domains, and ii) differential exhumation south of Nordfjord, where muscovite ages depict a NE–SW dome-like pattern and the Western Gneiss Region is bounded by overlying units little affected by the Scandian metamorphism. Exhumation of the UHP domains into the mid-upper crust by late folding continued through ~374 Ma. The smooth gradient of fairly flat muscovite age spectra demonstrates minimal influence of excess Ar, which is relatively unusual for a (U)HP terrane. ⁴⁰Ar/³⁹Ar spectra and modeled cooling histories from K-feldspar combined with brittle–ductile and brittle fault data indicate continued exhumation on local structures into the Permian.