The age and tectonic significance of dolerite dykes in western Norway

Abstract

Coast-parallel dykes in SW Norway, primarily of Permo-Triassic age, have been linked regionally to the early tectonic evolution of the Norwegian continental shelf. We demonstrate from palaeomagnetic data (mean declination = 206.1°, inclination = — 30.1°, a 95 = 11.8°) that dolerite dykes in the coastal Sunnfjord region of Western Norway and immediately west of the Devonian Basins are also of Permian (c. 250-270 Ma) age, and not lower- or pre-Devonian as previously advocated. The Sunnfjord dykes appear to be contemporaneous with dykes from SW Norway at Sotra (262 ± 6 Ma) and the oldest dykes from Sunnhordland (260–280 Ma), and geochemical data attest to a transition from sub-alkaline to alkaline magmatism at the dawn of the Mesozoic. The Sunnfjord dykes are not simple records of E-W extension and magma intrusion, but instead represent significant mid-late Permian time markers within a complex zone of fault activation and rejuvenation. Late Mesozoic-Cenozoic magnetic overprinting (mean declination = 348.6°, inclination = + 68.9°, a 95 =12°) and metamorphic alteration documented by these dykes are directly dependent upon proximity to major 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 offshore to the Øygarden Fault Zone. Onshore, these faults truncate the Hornelen low-angle detachment, which in turn cuts folded D evonian strata. These observations, along with evidence for Permian and Late Jurassic-Cretaceous extension from the nearby Dalsfjord region, demonstrate important reactivation of a Late to post-Caledonian detachment and high-angle fault system in Western Norway.