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Workreport 2020-13



Ductile Deformation and Deformation Structures of Olkiluoto


Kärki, A., Paananen, M., Aaltonen, I., Engström, J., Mattila, J., Paulamäki, S.



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The available report contains the structural synthesis dealing with the ductile deformation of the Olkiluoto study site. The report is based on the results of geological and geophysical studies carried out during the last decades. Deformation has taken place in four main stages in the course of Paleoproterozoic tectonic evolution. The first stage is associated with the Svecofennian Accretionary Orogeny and certain tectonic events at the continental margin prior to that. The most typical deformation structures generated by this deformation process are various foliations ranging from gneissosity to a variety of foliated migmatite structures and other elements resulted from the most intensive phase of regional migmatization.

The second stage of ductile deformation has overprinted the first stage elements still in high grade metamorphic environment and tight to isoclinal folds with certain migmatite structures were again generated. The third phase of ductile deformation has affected wide subareas of the bedrock but regional variation of the deformation mechanisms and intensity has been wide. In the subunits nowadays located in the central and southern parts of Olkiluoto, the complexity of the interference structures has increased by this deformation due to refolding and generation of new dextral F3 components to the interference structures. Various shear structures dominate the northern part of the island and certain sub-areas in the South, where rather wide ductile shear zones have been generated by the stage D3.

The fourth phase of regional deformation has still taken place in a relatively high-grade metamorphic environment due to a tectonically remarkable phase of convergence associated with the E-W directed compression. As a whole, fold structures with N-S striking axial surfaces with subparallel shear structures are the most important D4 elements but regional variation of the magnitude has been strong and impact of this deformation is forcefully localized and remarkable D4 structures are mostly in the eastern part of Olkiluoto detected.


Olkiluoto, structural evolution, structural synthesis, ductile deformation, tectonic structure


WR 2020-13_web (pdf) (5 MB)
Appendix 1_web (pdf) (1.8 MB)
Appendix 1b_web (pdf) (91.8 KB)


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