Late-orogenic progressive shearing in eastern Bergslagen, central Sweden

Abstract

Structural studies of deformation zones in northeastern Bergslagen with emphasis on the NNE-SSW Österbybruk-Skyttorp Zone (ÖSZ) and the N-S Gimo Zone (GZ) indicate that both the ÖSZ and the GZ are splays from the Singö shear zone (SSZ) system where they originally enveloped tectonic lenses. Strain in the SSZ was possibly enhanced and later pinned by the emplacement and solidification of a c. 1.8 Ga granite within the SSZ. This distributed strain southwards leading to rotation and folding of the lenses. Fold axes are steep and rotate both the regional foliation as well as the stretching lineation demonstrating modification of pre-existing structures. The ÖSZ shows east-side-up kinematics related to the east-vergent folding. An earlier (?) west-side-up pattern along the western margin of the ÖSZ is here interpreted to be related to a dextral northeast-side-down oblique shear along the SSZ, and east-side-up shear along the GZ, prior to the folding. The GZ, which is located closer to the SSZ, generally shows a higher metamorphic grade than the ÖSZ. Microstructures and chlorite-bearing shearbands indicate upper greenschist facies conditions during deformation of the ÖSZ, whereas striped gneisses and a C-S fabric with stable hornblende and biotite along both planes indicate amphibolite facies conditions along the GZ. Overlapping shear zones in the hinge of one east-vergent fold were probably formed during flexural slip related to the folding. The structural pattern and indicated timing fit into the previously suggested S-N or SSW-NNE convergence of the accretionary, Svecokarelian orogen.