The tectonic framework of the upper crust along the FIRE 4 profile is less well constrained than in most of the other parts of Finland but broadly follows a similar transition from Paleoproterozoic rifting of the Archean craton (ca. 2.5–2.05 Ga) to convergent amalgamation and accretion of reworked Archean and newly generated Proterozoic crustal components (ca. 2.0–1.85 Ga; Patison et al., 2006).
Archean crust along the surface of the FIRE 4 profile is limited to the rocks of the Ranua complex (Hölttä et al., 2012) in the southern part and to the presumably Archean metasedimentary rocks of the Vuojärvi group (Lahtinen et al., 2015) along the northern flank of the Central Lapland granitoid complex (CLGC). Metasedimentary and -volcanic rocks deposited during the Paleoproterozoic extensional stage are represented by the rocks of the Peräpohja belt in the south and the Central Lapland greenstone belt (CLGB) in the north (Patison et al., 2006; Kyläkoski et al., 2012; Lahtinen et al., 2015). These crustal blocks were further deformed and metamorphosed during accretion onto the margins of the Archean craton and followed by the intrusion of the granitic rocks of the CLGC between ca. 2.13–1.76 Ga (Patison et al., 2006; Ahtonen et al., 2007; Lauri, 2013). The significance and detailed origin of the CLGC is still unclear but most likely it records a rather diverse magmatic and tectonic history that lasted over 300 m.y. (e.g., Lauri, 2013 and references therein).
From isotopic evidence (Huhma, 1986; Ahtonen et al., 2007; Lauri et al., 2012) Archean middle and lower crust of the Karelian craton is inferred to underlie the entire FIRE 4 profile and the Proterozoic rocks to occupy only the topmost 5–20 km of the section (Patison et al., 2006). The seismic fabric of the Archean middle crust underneath the Archean Ranua complex and the adjacent Peräpohja schist belt has been inferred to record the Paleoproterozoic extension in the form of partially melted domains and abundant mafic dykes (Patison et al., 2006). Otherwise the tectonic deep features of the entire profile have been interpreted to reflect the later compressional stages (Patison et al., 2006).