Microstructures associated with the Sottunga-Jurmo shear zone and their implications for the 1.83–1.79 Ga tectonic development of SW Finland

This petrographic study of rock samples from the area of a large-scale shear zone, the Sottung-Jurmo shear zone, in SW Finland, illuminates the thermal development of and strain distribution within the rocks during the last development stages of the shear zone. The results indicate relatively high temperatures during the deformation phases that created the gneisses and the mylonites, strain partitioning through time and continued transpression from S-SW until at least c. 1.79 Ga. This has implications to the latest tectonic models since the results together with previous studies suggest acompartmentalisation of regional stresses between the area SW of the shear zone and central Fennoscandia. The results also suggest that the uplift rate increased during the late stage of the transpression.