Caledonian Deformation in the Oslo Region, updating the regional tectonic model.

Abstract

Tectonically the Oslo Region is part of the Osen-Røa nappe which is the most external part of the Scandinavian Caledonides. The region extends roughly 100 km north and south of the city of Oslo, Norway, and contains a roughly 2000 m thick Lower Paleozoic sedimentary of shales and limestones. The proportion of competent units increases up stratigraphy culminating in the very competent Ringerike Group. Contrasting structural styles due present in the stratigraphy lead to discrepancies in shortening experienced by the Cambrian to Lower-Ordovician strata and the Middle-Ordovician to Silurian sediments. A layer parallel detachment to account for this discrepancy was postulated by Morley (1987a). The entire region is underlain by the Osen-Røa basal thrust which, is constricted to the Alum Shale Formation. This study focusses on structures of this Osen-Røa thrust in the Hønefoss area. A detailed description of structures and southward directed transport direction of the Osen-Røa basal thrust in the Hønefoss area are presented. Combining and analysing the structural data gathered in the Hønefoss area with previously published structural data, a very homogeneous regional transport direction towards the SE is found. In order to gain insight into the mechanical parameters controlling the localization of deformation, stress transfer up stratigraphy and their influence on structural style analogue models where utilized. Results are mainly applied to large thrust splaying from the Osen-Røa thrust which cut the entire stratigraphic succession. It is concluded, based on modelling results, that the structural style of such large faults is controlled by effective lateral competence contrasts. By using data from various sources, the regional tectonic model initially presented by Bruton et al. (2010) is updated to include a layer parallel detachment in the Ordovician strata and involvement of basement in the Osen-Røa basal thrust.