Basement/Cover Relationships of the Bergsdalin Area, Central South West Norway

Abstract

The Bergsdalen area of central southwest Norway comprises late Archaean and early Proterozoic gneisses that are intruded by amphibolites and Sveconorwegian granites and gabbros; the gneisses were tectonically stacked, forming four structural units: 1. Autochthonous and parautochthonous acid and intermediate late Archaeanorthogneisses. 2. Parautochthonous Mixed Gneisses that contain lithologies of both the basement and the succeeding allochthon. 3. Allochthonous Egg jane Nappe, derived in the main from a Proterozoic quartz diorite pluton. 4. Allochthonous Lower Bergsdalen Nappe comprising Proterozoic calc-alkaline and tholeiitic metavolcanics and metasediments.

The Basal orthogneisses were deformed at amphibolite facies during both the Svecofennian and Sveconorwegian orogenic cycles; Caledonian effects are minimal.

The allochthonous gneisses have a pervasive amphibolite facies mylcnitic foliation, thought to be of Sveconorwegian origin, instead of Caledonian, as previously suggested.

The allochthonous gneisses were uplifted, and reworked during a phase of nappe emplacement along mylornitic and phyllonitic thrust zones, during late Sveconorwegian times. This contrasts with the traditional view of the Lower Bergsdalen Nappe being Caledonian in age.

Superimposed Green schist facies Caledonian folding and thrusting was restricted to higher structural levels, culminating in a cross fold to form the Bergen Arcs.