AN INTERLINKED SYSTEM OF FOLDS AND DUCTILE SHEAR ZONES - LATE-STAGE SVECOKARELIAN DEFORMATION IN THE CENTRAL FENNOSCANDIAN SHIELD, FINLAND

The Svecokarelian deformation of the Karelian and Svecofennian Provinc es in Finland involved four main phases. Isoclinal folding and associa ted thrusting were the main processes of D-1, which was followed by mo re 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 de formation 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 i n the rocks than has commonly been supposed. The shear zones formed du ring D-4 are referred to collectively as the Finlandia Shear System. T he Karelian province is composed of Archaean basement and its autochth onous-allochthonous Palaeoproterozoic sedimentary cover. Svecokarelian tectonics split the basement into separate complexes, the rheological properties of which differed markedly from those of the adjacent Prot erozoic supracrustal units. On the northern and southern margins of th e Archaean complexes, folding with steep E-W-trending axial surfaces w as the main response to D-4 deformation, whereas on the eastern and we stern margins of the complexes the response to D-4 was the development of sinistral and dextral shear zones trending NE-SW and NW-SE, respec tively. On a large scale, these areas of folding and of shearing form an interlinked system, the changes from one style of deformation to th e other being gradual. The principal D-4 elements in the area of juven ile Svecofennian crust are folds with E-W-trending axial surfaces.