The structural and metamorphic development of the Bergen Arc/Bergsdalen Foreland Gneisses of Osteroy, S.W. Norway

Abstract

North Osteroy comprises autochthonous and para-autochthonous basement which has been reworked during the orogenic development of the adjacent Bergsdalen and Bergen Arc nappes.

The divisions of the basement, the Southern and Northern Gneiss Units, are composed of acid gneisses, paragneisses, migmatites, orthoamphibolites and granites. Three sedimentary complexes are identified, the first (circa 1500 Ma.) resembles the Fjordane complex. The second is correlated with sediments in the Bergsdalen Nappes (older than 1000 Ma.), and with the Samnanger complex in the Bergen Arcs. The third sedimentary sequence is the Ashgillian, Holdhus sequence of the Bergen Arcs.

The rocks of the Southern Gneiss Unit are derived from, and emplaced over, the Northern Gneisses during four deformations, each of which is separated by a sedimentary interval. The deformation involves 'mylonitic' processes acting at high temperatures and high strain rates.

The first deformation forms part of the Svecofennian orogenic cycle. The second and third form part of the Sveconorwegian cycle and produce folds with north-easterly trending axial traces. The third is responsible for the emplacement of the primary Bergsdalen Nappes and contrary to current opinion these Nappes are thought to root in the south east.

The final deformation is part of the Caledonian orogenic cycle and involves the development of the Bergen Arcs and their emplacement over the Bergsdalen Nappes. In the early stages of this cycle, structures formed on an east-north-east Bergsdalen trend, but later, non-cyclindrical folds with north-westerly trending axial traces developed, giving rise to the Bergen Arc trend. Of the two deformations which produce the Bergen Arc and Bergsdalen Nappes, the Sveconorwegian event is the more important.

The interpretation of the geology of N. Osteroy and consideration of its relationships to adjacent regions allows a new plate tectonic interpretation of Western Norway.