A. Karki

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.

Zircon and monazite geochronology of deformation in the Pielavesi Shear Zone, Finland: multistage evolution of the Archaean–Proterozoic boundary in the Fennoscandian Shield

The Raahe–Ladoga Shear Complex is a major crustal structure representing the Archaean–Palaeoproterozoic boundary in the Fennoscandian Shield. The complex developed during the Svecofennian Orogeny (c. 1.9 – 1.8 Ga) beginning with regional thrust tectonic phases D1 and D2, followed by large-scale shearing events D3 and D4. The Pielavesi Shear Zone is a vertical north–south-trending shear zone within the Raahe–Ladoga Shear Complex formed during regional D3 shearing and later reactivated during the regional D4 phase. Three north–south-trending elongate granitoid intrusions were selected as representative of silicic melts that intruded the transtensional Pielavesi Shear Zone during the regional D3 phase. The oriented magmatic fabric of the granitoids indicates that they intruded coeval to the deformation event. The zircon U–(Th)–Pb secondary ionization mass spectrometry (SIMS) ages of these intrusions (1888 ± 4, 1884 ± 6 and 1883 ± 5 Ma) overlap within error and provide a direct age for the regional D3 deformation. εHf(T) (−1.1 to +3.4) and εNd(T) (−1.2 to +0.4) values from these granitoids are both consistent with a predominantly juvenile source affected by a minor Archaean component. U–(Th)–Pb SIMS analyses of metamorphic monazite formed within a crosscutting blastomylonite provide an age for the regional D4 phase and associated fluid activity of 1793 ± 3 Ma. Supplementary material: Analytical methods and tabulated analytical data are available at https://doi.org/10.6084/m9.figshare.c.3498501