Paleostress field reconstruction in the Oslo region

Abstract

The geodynamic history of a region is archived in its geologic record which, in turn, may reflect deformation patterns that causally can be related to certain configurations of paleostresses. In the Oslo Region, the exposed geological record ranges from Precambrian high-grade metamorphic rocks through Cambro-Silurian sedimentary rocks to Permo-Carboniferous sedimentary and magmatic rocks, the latter being related to the development of the Oslo rift system. We investigate the kinematics of outcrop-scale faults to derive the diversity of paleostress states responsible for the observed strain. For this purpose, we combine different graphical and numerical approaches to separate heterogeneous fault-slip data sets and estimate the associated reduced stress tensors. A reduced stress tensor consists of the directions of the three principal stress axes with σ 1 ≥ σ 2 ≥ σ 3 and the ratio of principal stress differences, R = ( σ 2 − σ 3)/( σ 1 − σ 3). The present study gives evidence for three major regional paleostress fields that affected the Oslo Region: A compressional stress field with a NW–SE-directed maximum compression ( σ 1) has been identified as a Caledonian imprint. The most prominent regional stress field, however, is tensional, characterised by a WNW–ESE directed minimum compression ( σ 3), and related to the Permo-Carboniferous rifting. Later, the area was affected by a wrench regime with a roughly N–S directed σ 1 and a maximum age of Permian, whereas the absolute timing of this stress field is unclear due to the lack of exposed rocks younger than Permian. For a large number of estimated stress states, none of the principal axes are sub-vertical. These “oblique” paleostresses cannot be integrated into a common regional stress field but rather correspond to local effects of intense magmatic activity during the Permo-Carboniferous phase of rifting. The present study tends to show that the Oslo Region remained unaffected by major tectonic activity for much of the Mesozoic and Cenozoic – which is when the Central European Basin System to the south experienced several phases of intense deformation.