FIRE 3A Tectonic overview

The FIRE 3A profile runs for 448 km from east in the Outokumpu domain towards west over the eastern part of the Central Finland granitoid complex (CFGC) to the Äänekoski region, where it takes a turn towards northwest and crosses over the rest of the CFGC and terminates in the northwest nearby the town of Kokkola in the Bothnian Belt.

The upper crust along the FIRE 3A profile is divided into four domains:

• Outokumpu (CMP 0000–3800) (1),
• Savo (CMP 3800–6000) (2),
• Keitele (CMP 6000–14800) (3), and
• Bothnian (CMP 14800–17800) (4) domains.

The FIRE 3 and 3A lines run semi-parallel across the Outokumpu domain in the eastern end of the FIRE 3A profile (ca. CMP 0000–0500). Towards the west along FIRE 3A the domain is defined by the Paleoproterozoic upper Kaleva metasedimentary sequences (Viinijärvi suite; Sorjonen-Ward, 2006) underlain by Archean mid- to lower crust and intruded by the 1.88–1.86 Ga Heinävesi suite granitic rocks with Archean source characteristics and emplaced during local shearing and deformation (Sorjonen-Ward, 2006).

The areal extent of the upper Kaleva sediments is considered to delineate the western margin (CMP 3800) of the Outokumpu domain along the FIRE 3A profile.

The Savo domain (CMP 3800–6000) corresponds to the early Paleoproterozoic Savo belt (Kähkönen, 2005), which along FIRE 3A profile consists mostly of gneisses and migmatites with supracrustal origins and juxtaposed between the slightly older upper Kaleva metasediments in the east (Viinijärvi suite; Sorjonen-Ward, 2006) and the Proterozoic Central Finland Granitoid complex in the west (Mikkola et al., 2016). The belt also coincides with the lithosphere-scale Raahe-Ladoga shear zone (RLSZ), which has been interpreted to represent the deep suture between the Archean Karelian province and the Proterozoic Svecofennian domain (e.g., Sorjonen-Ward, 2006).

Along the FIRE 3A the Savo migmatites belong to the Suonenjoki suite paragneisses with some local enrichment in more volcanic source materials and they are intercalated with the rocks of the Sorsakoski granitoid suite (Mikkola et al., 2016). The western contact of the Savo domain towards the Central Finland granitoid complex (Keitele domain) is delineated by the Iisvesi mylonite zone (Mikkola et al., 2016), which also marks the western extent of the RLSZ. Archean crust of the Kuopio complex (Hölttä et al., 2012) is inferred to underlie the entire Savo domain but it is exposed only on the northern flank of the FIRE 3A profile in the central part of the domain.

The Keitele domain (CMP 6000–14800) corresponds roughly to the area of the Central Finland granitoid complex (CFGC) that comprises a variety of orogenic, dominantly I-type granitic rocks and minor associated supracrustal rocks with a relatively evolved geochemical character (Nironen, 2005).

Majority of the CFGC consists of 1.89 to 1.87 Ga calc-alkaline granodiorites and granites synkinematic with the major orogenic stage corresponding to the collision of the Southern Finland arc complex (Bergslagen microcontinent and the Häme belt of the southern Svecofennia sub-province; Lahtinen et al., 2016) with the Arc complex of Western Finland (Keitele and parts of the presumed Bothnian microcontinents or the central Svecofennia sub-province; Lahtinen et al., 2016) at ca. 1.9 Ga. The 1.88 to 1.86 Ga postkinematic granitic rocks with more alkaline geochemical character (Nironen, 2005) form smaller, often discordant, intrusions with transitional I- to A-type characteristics and can be divided into three geochemical types (types 1, 2, and 3; e.g., Elliott, 2003 and references therein).

Based on the more juvenile isotopic character of the granitic rocks and the presence of larger amount of mafic rocks Lahtinen et al. (2016) also defined a separate boundary zone on the northwestern flank of the CFGC between the Bothnian domain and bulk of the CFGC.

The Bothnian domain (CMP 14800–17800) comprises two Paleoproterozoic metasedimentary suites of the Bothnian belt that is a part of the Arc complex of Western Finland, the Lappfors suite (Williams et al., 2008) in the west and the Pirttikylä suite, in contact with the Central Finland granite complex (CFGC), in the east (Suikkanen et al., 2014; Kotilainen et al., 2016a, 2016b).

The supracrustal rocks rim the migmatitic granitoid rocks of the Vaasa migmatite complex, the bulk of which is located west of the FIRE 3A profile (Chopin et al., 2012; Suikkanen et al., 2014; Kotilainen et al., 2016a, 2016b). The Vaasa migmatites are interpreted to represent the center of an exhumed mid-crustal metamorphic core complex generated by partial melting of the metasedimentary rocks of the Bothnian belt at ca. 1.88–1.87 Ga (Suikkanen et al., 2014; Kotilainen et al., 2016a).

Upper to mid-crustal reflectors (ca. CMP 12000–17000) that dip E–SE under the CFGC have been correlated with the contacts of the supracrustal units on the surface (Sorjonen-Ward, 2006). These reflectors extend through the present-day middle crust and are interpreted as marking the contacts of the Bothnian belt blocks with each other and “the Pirttikylä block” towards the CFGC (Sorjonen-Ward, 2006). Tectonic interpretations of these reflectors suggest them to be either thrust-related (Sorjonen-Ward, 2006) or reactivated extensional (Nikkilä et al., 2015) features.

Chopin, F., Korja, A., Hölttä, P., 2012. Tectonometamorphic evolution of the Bothnian belt within the Svecofennian orogen – a case study for the building of a large dome in the Paleoproterozoic. LITHOSPHERE 2012 Symposium, November 6-8, 2012, Espoo, Finland, Abstracts, pp. 5–8.

Elliott, B.A. 2003. Petrogenesis of the Post-kinematic Magmatism of the Central Finland Granitoid Complex II; Sources and Magmatic Evolution. Journal of Petrology 44, 1681–1701.

Kähkönen, Y., 2005. Svecofennian supracrustal rocks. In: Lehtinen, M., Nurmi, P.A., and Rämö, O.T. eds. Precambrian Geology of Finland – Key to the Evolution of the Fennoscandian Shield. Developments in Precambrian Geology 14, 343–406

Kohonen, J., 1995. From continental rifling to collisional crustal shortening – Paleoproterozoic Kaleva metasediments of the Höytiäinen area in North Karelia, Finland. Bulletin of the Geological Survey of Finland 380, 79 p.

Korja, A. and Heikkinen, P.J., 2008. Seismic images of Paleoproterozoic microplateboundaries in the Fennoscandian shield. Geological Society of America, Special Paper 440, 229–248.

Korsman, K., Koistinen, T., Kohonen, J., Wennerström, M., Ekdahl, E., Honkamo, M., Idman, H., Pekkala, Y., (comp.) 1997. Bedrock Map of Finland, 1: 1 000 000. Geological Survey of Finland.

Kotilainen, A.K., Mänttäri, I., Kurhila, M., Hölttä, P., and Rämö, O.T., 2016a. Evolution of a Palaeoproterozoic giant magmatic dome in the Finnish Svecofennian; New insights from U-Pb geochronology. Precambrian Research 272, 39–56.

Kotilainen, A.K., Mänttäri, I., Kurhila, M., Hölttä, P., and Rämö, O.T. 2016b. New monazite U-Pb age constraints on the evolution of the Paleoproterozoic Vaasa granitoid batholith, western Finland. Bulletin of the Geological Society of Finland 88, 5-20.

Lahtinen, R., Korja, A., and Nironen, M., 2005. Paleoproterozoic tectonic evolution. In: Lehtinen, M., Nurmi, P.A., and Rämö, O.T. eds. Precambrian Geology of Finland – Key to the Evolution of the Fennoscandian Shield. Developments in Precambrian Geology 14, 481–531.

Lahtinen, R., Huhma, H., Lahaye, Y., Lode, S., Heinonen, S., Sayab, M., and Whitehouse, M.J., 2016. Paleoproterozoic magmatism across the Archean-Proterozoic boundary in central Fennoscandia: Geochronology, geochemistry and isotopic data (Sm-Nd, Lu-Hf, O). Lithos 262, 507–525.

Mikkola, P., Lukkarinen, H., Luukas, J. 2016. Suonenjoen kartta-alueen 3241 kallioperä ja kivilajiyksiköt. Geological Survey of Finland, Archive Report, 29/2016, 30 p.

Nikkilä, K., Korja, A., Koyi, H., and Eklund, O., 2015. Analog modeling of one-way gravitational spreading of hot orogens – A case study from the Svecofennian orogeny, Fennoscandian Shield. Precambrian Research 268, 135–152.

Nikkilä, K., Mänttäri, I., Nironen, M., Eklund, O., and Korja, A., 2016. Three stages to form a large batholith after terrane accretion – An example from the Svecofennian orogen. Precambrian Research 281, 618–638.

Nironen, M., 2005. Proterozoic orogenic granitic rocks. In: Lehtinen, M., Nurmi, P.A., and Rämö, O.T. eds. Precambrian Geology of Finland – Key to the Evolution of the Fennoscandian Shield. Developments in Precambrian Geology 14, 443–479.

Sorjonen-Ward, P., 2006. Geological and structural framework and preliminary interpretation of the FIRE 3 and FIRE 3A reflection seismic profiles. In: Kukkonen, I. and Lahtinen, R., eds. Finnish Reflection Experiment 2001–2005, Geological Survey of Finland Special Paper 43, 105–159.

Suikkanen, E., Huhma, H., Kurhila, M., and Lahaye, Y., 2014. The age and origin of the Vaasa migmatite complex revisited. Bulletin of the Geological Society of Finland 86, 41–55.

Williams, I.S., Rutland, R.W.R., and Kousa, J., 2008. A regional 1. 92 Ga tectonothermal episode in Ostrobothnia, Finland: implications for models of Svecofennian accretion. Precambrian Research, 165, 15–36.