An interlinked system of folds and ductile shear zones—late stage Svecokarelian deformation in the central Fennoscandian Shield, Finland

Abstract

The Svecokarelian deformation of the Karelian and Svecofennian Provinces in Finland involved four main phases. Isoclinal folding and associated thrusting were the main processes of D 1, which was followed by more open folding in D 2. Subsequent deformation was localized during D 3 and D 4. This paper focuses on the late stage ( D 4) Svecokarelian deformation that created most of the ductile shear zones in the central Fennoscandian Shield and was the last penetrative deformational event to affect the entire Palaeoproterozoic domain. Deformation during D 4 occurred approximately from 1.85–1.80 Ga, and left a greater imprint in the rocks than has commonly been supposed. The shear zones formed during D 4 are referred to collectively as the Finlandia Shear System. The Karelian province is composed of Archaean basement and its autochthonous-allochthonous Palaeoproterozoic sedimentary cover. Svecokarelian tectonics split the basement into separate complexes, the rheological properties of which differed markedly from those of the adjacent Proterozoic supracrustal units. On the northern and southern margins of the Archaean complexes, folding with steep E-W-trending axial surfaces was the main response to D 4 deformation, whereas on the eastern and western margins of the complexes the response to D 4 was the development of sinistral and dextral shear zones trending NE-SW and NW-SE, respectively. On a large scale, these areas of folding and of shearing form an interlinked system, the changes from one style of deformation to the other being gradual. The principal D 4 elements in the area of juvenile Svecofennian crust are folds with E-W-trending axial surfaces.