Johannes Wiest

Title: Constraints from the Lyderhorn Gneiss on the tectonomagmatic evolution of the Øygarden Complex, SW Norway

The Øygarden Complex and the Bergen Arc System are enigmatic large-scale structures in the SW Norwegian Caledonides. So far, the lack of published U-Pb zircon ages from the Øygarden Complex limits clear interpretations. Located in the poorly studied eastern part of the Øygarden Complex, the Lyderhorn Gneiss forms the core of the Bergen Arc System. Detailed field mapping was conducted and seven samples were dated by SIMS U-Pb zircon geochronology.

Igneous protoliths formed during the Gothian-Telemarkian (1504 Ma) and Sveconorwegian (1041 Ma; 1026 – 1022 Ma) orogenic periods. Metamict zircons were reset by an Early Ordovician (483 Ma) thermal event and sustained elevated temperatures annealed previous radiation damage. Caledonian regional metamorphism left no measurable effect on the U–Pb system.

The structure of the Lyderhorn Gneiss is characterized by lineation-parallel, recumbent folds that record constrictional strain. Consistently oriented, top-to-ENE shearing prevailed during deformation from amphibolite facies conditions to the brittle-plastic transition. Fluid-assisted strain weakening formed E-dipping, phyllonitic detachments.

Mesoproterozoic protolith ages correlate the Lyderhorn Gneiss (and Øygarden Complex) with the Telemarkia domain of the Baltican basement and show a relation with the Sirdal Magmatic Belt. Our results suggest a continuous NNW-trend of Sveconorwegian domain boundaries and contradict widespread Sveconorwegian high-T metamorphism.
The structure of the Lyderhorn Gneiss is inconsistent with the current tectonic model of the Bergen area, however, it is well explained by an alternative model that considers core complex exhumation of the Øygarden Complex. The discussed model reconciles the interpretation of basement culminations along the entire Norwegian margin and enhances onshore-offshore correlations.