The tectonic division and structural evolution of the eastern Fennoscandian Shield

Y.J. Systra (1, 2)

1) Department of Mining, Tallinn University of Technology, Ehitajate tee 5, EE-19086 Tallinn, Estonia (systra@staff.ttu.ee),
(2) Institute of Geology, Karelian Research Centre RAS, Pushkinskaya St.11, Petrozavodsk, RU-185910 Karelia, Russia

The eastern part of the Fennoscandian Shield (Kare¬lian Region) comprises three major NW-SE trending geostructures: the Karelian Craton (KC) in the centre is flanked to NE by the Belomorian Fold Belt (BFB) and to SW by the Svecofennian Domain (SD). Each of them has different structural evolution.
The Archean basement of the KC was formed 3.5-2.65 Ga and its Early Proterozoic volcano-sedimentary cover, preserved in relict synclinal cores, 2.5–1.75 Ga ago. Four regional fold generations occur in cover rocks: the oldest with axial planes (AP) striking 10-20° NE were likely formed during 1.95 Ga rifting, when the Jormua ocean on the western margin of craton opened. Three other generations with AP 310–330° NW, 10–50° NE and 275–295° NW belong to the Svecofennian orogeny (1.9–1.8 Ga). In the SW part of KC, where the litosphere has maximum thickness (Lehtonen et al., 2004), no Svecofennian fold was found. The Archean basement has common folds with near N-S axial planes, and NW striking folds. Near the contact with BFB, E-W folds are also developed. The SW and NE margins of KC are complicated by 1.8 Ga thrusting, the NW margin by shear duplex and drag folds.
The strongly folded and highly metamorphosed 3.0–2.8 Ga BFB represents the oldest orogenic belt of the Fennoscandian Shield, and is composed by mig¬matized gneisses and amphibolites. A system 1.9–1.8 Ga conical folds with AP 10–50° NE is superposed on E-W Archean folds with curved axial planes.
The SD has a complex folded structure: 5 systems of linear and conical folds with AP 290–305°, 315–340°, near N-S, NE and near E-W are found here (Koistinen, 1981), some of which can be correlated with folds in KC. In addition, domal structures occur.
Magmatism in the KC and BFB is similar in age and composition, while in the SD is significantly another.

Koistinen, T.J. (1981). T. R. Soc. Edin. Earth Sci. 72 115–158.
Lehtonen, M.L. et al. (2004). Lithos 77, 593–608.