THE AGE AND TECTONIC SIGNIFICANCE OF DOLERITE DYKES IN WESTERN NORWAY

Coast-parallel dykes in SW Norway, primarily of Permo-Triassic age, ha ve been linked regionally to the early tectonic evolution of the Norwe gian continental shelf. We demonstrate from palaeomagnetic data (mean declination=206.1 degrees, inclination=-30.1 degrees, alpha(95)=11.8 d egrees) that dolerite dykes in the coastal Sunnfjord region of Western Norway and immediately west of the Devonian Basins are also of Permia n (c. 250-270 Ma) age, and not lower-or pre-Devonian as previously adv ocated. The Sunnfjord dykes appear to be contemporaneous with dykes fr om SW Norway at Sotra (262+/-6 Ma) and the oldest dykes from Sunnhordl and (260-280 Ma), and geochemical data attest to a transition from sub -alkaline to alkaline magmatism at the dawn of the Mesozoic. The Sunnf jord dykes are not simple records of E-W extension and magma intrusion , but instead represent significant mid-late Permian time markers with in a complex zone of fault activation and rejuvenation. Late Mesozoic- Cenozoic magnetic overprinting (mean declination=348.6 degrees, inclin ation=+68.9 degrees, alpha(95)=12 degrees) and metamorphic alteration documented by these dykes are directly dependent upon proximity to maj or E-W brittle faults south of the Hornelen Devonian Basin, hence some motion and related fluid activity do post-date dyke intrusion. The E- W high-angle normal or oblique-slip faults can be regionally traced of fshore to the Oygarden Fault Zone. Onshore, these faults truncate the Hornelen low-angle detachment, which in turn cuts folded Devonian stra ta. These observations, along with evidence for Permian and Late Juras sic-Cretaceous extension from the nearby Dalsfjord region, demonstrate important reactivation of a Late to post-Caledonian detachment and hi gh-angle fault system in Western Norway.