Oroclines, or curvatures of previously linear arcs (or belts), are tectonic structures that bridge the manufacture of arcs and formation of stable, equant continental blocks. Oroclines are well documented and spatially distributed on a global scale, and they form curved mountain belts of varying degrees in both young and ancient orogens. Johnston et al. (2013) recognized two distinct types of oroclines: progressive and secondary. Progressive oroclines are thin-skinned (thrust sheet/thrust belt) and develop during thrusting in response to the same orogen-perpendicular stress responsible for thrust sheet emplacement. Secondary oroclines occur at the scale of an orogen, are plate-scale features that affect the crust and lithospheric mantle, and form in response to an orogen-parallel principal shortening direction. Fennoscandia comprises two major Paleoproterozoic oroclines: the Inari and the coupled Bothnian oroclines (Lahtinen et al., 2014, 2016).
The Inari orocline is a collage of the Lapland granulite belt, marginal zone, and Inari arc forming an arcuate shape (Fig.), which has typically been considered to have formed coeval with thrusting. This would suggest that the Inari orocline constitutes a progressive orocline. New field data demonstrate that both D1 and D2 structures are bent along the arcuate shape and D3 folding has axial traces orthogonal to the main foliation, forming a fan-like structure. Abundant fractures follow a similar radial pattern. According to the proposed model, layer-specific short fractures and conical folds in the Inari orocline are radial features that formed during large-scale buckling about a vertical axis of rotation in response to orogen-parallel principal compressive stress. This would favor the Inari orocline being a secondary orocline.
Lahtinen et al. (2014) modeled the non-linear but curved part of central Fennoscandia, including 1.90–1.88 Ga Skellefte, Pohjanmaa, and Tampere arc rocks, to have formed by buckling of a linear orogen about vertical axes of rotation into coupled Bothnian oroclines (Fig.). This interpretation is strongly supported by a crustal-scale conductance anomaly (Korja et al., 2002) favoring a lithosphere-scale phenomenon. Later studies (Lahtinen et al., 2017) have indicated similar pre-1.91 Ga evolution in the Western Finland supersuite (PoB, PB), TB, and HB, and contemporaneous arc magmatism at 1.90–1.88 Ga in the Skellefte district (SD), Central Finland Granitoid Complex (CFGC), TB, and HB. These data open new possibilities to interpret the formation of the Bothnian coupled oroclines and their continuation. Two models have been presented (Fig.): a) all the 1.90–1.88 Ga volcanic rocks formed in one linear arc system from the SD to the HB; b) there have been two separate arcs, SD-CFGC-TB and HB, of a similar age with double-plunging subduction zones.
The Fennoscandian shield forms the ancient geological core of the supercontinent Nuna/Columbia. Igneous and high-grade metamorphic rocks are the main constituents and the shield is a collisional root zone affected by multiple tectonic, metamorphic and structural events. The bending of mountain belts seen in younger orogens is lacking, which makes the occurrence of oroclines self-evident in such cases. Anyhow, both the Inari and Bothnian oroclines are large and crustal-scale features and fulfill the classification criteria for oroclines. In both orocline systems, orogen-parallel shortening and buckling are the proposed mechanisms. In the Inari orocline, the occurrence of radial features favors buckling as the main mechanism, but in the case of the Bothnian oroclines, the formation mechanism could be more complex, and more studies are needed to resolve the evolutionary history of the Bothnian oroclines and their continuations.
Johnston, S.T., Weil, A.B., Gutiérrez-Alonso, G., 2013. Oroclines: Thick and thin. Geological Society America, Bulletin 125, 643–663.
Korja, T., Engels, M., Zhamaletdinov, A.A., Kovtun, A.A., Palshin, N.A., Smirnov, M.Yu., Tokarev, D.A., Asming,V.E., Vanyan, L.L., Vardaniants, I.L. and the BEAR Working Group, 2002. Crustal conductivity in Fennoscandia—a compilation of a database on crustal conductance in the Fennoscandian Shield. Earth, Planets, Space 54, 535–558.
Lahtinen R., Johnston S.T. and Nironen M., 2014. The Bothnian coupled oroclines of the Svecofennian Orogen: a Palaeoproterozoic terrane wreck. Terra Nova 26, 330-335.
Lahtinen, R., Sayab, M., Johnston, S.T., 2016. Inari orocline – progressive or secondary orocline. LITHOSPHERE 2016 Symposium, November 9-11, 2016, Espoo, Finland, 69-70.
Lahtinen R, Huhma H., Sipilä, P. and Vaarma, M., 2017. Geochemistry, U-Pb geochronology and Sm-Nd data from the Paleoproterozoic Western Finland supersuite – a key component in the coupled Bothnian oroclines. Precambrian Research 299, 264–281.
Teksti: Raimo Lahtinen
Raimo Lahtinen is Research Professor of geological modeling and mineral economy. He has worked for over 40 years at GTK on various assignments, from rock crushing to management and research. His main scientific focus for the last 25 years has been the tectonic evolution and metallogeny of Fennoscandia.