FIRE 4B Tectonic overview

The FIRE 4B profile transects the northern part of the major tectonic boundary between the Archean Norrbotten and Karelia crustal domains (Lahtinen et al.

, 2005, 2015; Patison et al., 2006) called variably the Pajala shear zone (PSZ; Kärki et al., 1993; Lahtinen et al., 2015), Baltic-Bothnian megashear (Berthelsen and Marker, 1986), Tornio deformation zone (Sorjonen-Ward, 1993), or Kolari shear zone (Väisänen et al., 2000).

The PSZ runs through in north–south direction for over 400 km and along the FIRE 4B separates the ca. 2.0 Ga Kittilä suite metavolcanics (Lehtonen et al., 1998) in the east from the >2.2 Ga metasedimentary rocks of the Virttiövaara formation in the west (Patison et al., 2006). Formations on both sides of the PSZ were later intruded by granitic and monzonitic magmas in several stages from ca. 1.9 Ga to 1.75 Ga (Väänänen and Lehtonen, 2001; Patison et al., 2006). The eastern part of the FIRE 4B profile also crosscuts the Sirkka/Kittilä shear zone interpreted as the overthrusting surface of the allochtonous Kittilä suite (Patison et al., 2006).

The PSZ is suggested to have formed either as a collisional suture between two entirely separate Archean crustal blocks (Norrbotten and Karelia; Lahtinen et al., 2015 and references therein) or as a dextral strike-slip system after a failed rifting event (Lahtinen et al., 2015).

FIRE 4B Geographic constraints

The FIRE 4B runs in an east–west/southeast–northwest direction for 62.5 km from the township of Sirkka in the east to the village of Muonio in the west.

FIRE 4B Geological and tectonic division

The upper crust along the FIRE 4B profile has been divided into two domains:

  • Kittilä (CMP 0000–1650) (1) and
  • Muonio (CMP 1650–2500) (2) domains.
Geological description of the Kittilä domain (FIRE 4B CMP 0000–1650)

The Kittilä domain (CMP 0000–1650) corresponds to the Paleoproterozoic metasedimentary and -volcanic sequences of the Central Lapland greenstone belt (CLGB), which are at least partly correlated with the rocks of the Peräpohja schist belt (Lehtonen et al., 1998; Lahtinen et al., 2015 and references therein) and share a similar geological history.

The rocks of the ca. 2.0 Ga Kittilä suite (a.k.a. the Kittilä allochton) form a group of dominantly metavolcanic rocks with diverse geochemical signatures that are inferred to originate in a plume environment and, in contrast to the generally extensional tectonic regime, to have been emplaced in a convergent tectonic event on top of the rift-related supracrustal sequences (Lehtonen et al., 1998; Hanski and Huhma, 2005; Patison et al., 2006; Lahtinen et al., 2015).

The eastern part of the FIRE 4B profile crosses the westernmost part of the Kittilä suite metavolcanic rocks that are also intruded by the Ruskisrova intrusive complex (CMP 1000–1300). The easternmost occurences of the Virttiövaara formations mark the beginning of the Pajala shear zone (Lahtinen et al., 2015) and the western edge of the Kittilä domain.

Geological description of the Muonio domain (FIRE 4B CMP 1650–2500)

The Muonio domain (CMP 1650–2500) corresponds to the rocks of the Archean Norrbotten block on the western side of the Pajala shear zone (PSZ) (Lahtinen et al., 2015).

The rocks of the domain belong to the dominantly metasedimentary >2.2 Ga Virttiövaara formation that was intruded by the granitic and monzonitic rocks of the (1.9–1.86 Ga) Haaparanta (Haparanda) and (1.81–1.77 Ga) Kittilä suites (Väänänen and Lehtonen, 2001; Patison et al., 2006). The upper crust of the domain along FIRE 4B profile is divided into two blocks that are separated by a west-dipping unconformity that runs parallel to the inferred eastern flank of the PSZ (Patison et al., 2006).

FIRE 4B References

Berthelsen, A. and Marker, M., 1986. 1.9–1.8 Ga old strike-slip megashears in the Baltic Shield, and their plate tectonic implications. Tectonophysics 128, 163–181.

Daly, S.J., Balagansky, V.V., Timmerman, M.J., and Whitehouse, M.J., 2006. The Lapland-Kola orogeny: Palaeoproterozoic collision and accretion of the northern Fennoscandian lithosphere. Geological Society of London, Memoirs 32, 579–598.

Hanski, E. and Huhma, H., 2005. Central Lapland Greenstone belt. In: Lehtinen, M., Nurmi, P.A. & Rämö, O.T. (eds.) The Precambrian Geology of Finland − Key to the Evolution of the Fennoscandian Shield. Developments in Precambrian Geology 14, Amsterdam: Elsevier, 139–194.

Hölttä, P., Väisänen, M., Väänänen, J., and Manninen, T., 2007. Paleoproterozoic metamorphism and deformation in Central Lapland, Finland. Geological Survey of Finland, Special Paper 44, 7–56.

Hörmann, P.K., Raith, M., Raase, P-. Ackermand, D., and Seifert, F., 1980. The granulite complex of Finnish Lapland: petrology and metamorphie conditions in the Ivalojoki - Inarijärvi area. Bulletin of the Geological Survey of Finland 308, 95 p.

Kärki, A., Laajoki, K., and Luukas, J., 1993. Major Paleoproterozoic shear zones of the central Fennoscandian Shield. Precambrian Research 64, 207–223.

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

Lahtinen, R., Huhma, H., Lahaye, Y., Jonsson, E., Manninen, T., Lauri, L.S., Bergma, S. , Hellström, F., Niiranen, T., and Nironen, M., 2015. New geochronological and Sm–Nd constraints across the Pajala shearzone of northern Fennoscandia: Reactivation of a Paleoproterozoic suture. Precambrian Research 256, 102–119.

Lehtonen, M., Airo, M-L., Eilu, P., Hanski, E., Kortelainen, V., Lanne, E., Manninen, T., Rastas, P., Räsänen, J., and Virransalo, P., 1998. Kittilän vihreäkivialueen geologia, Lapin vulkaniittiprojektin loppuraportti. Summary: The stratigraphy, petrology and geochemistry of the Kittilä greenstone area, northern Finland. A report of the Lapland Volcanite Project. Geological Survey of Finland, Report of Investigation 140, 144 p.

Merilänen, K., 1976. The granulite complex and adjacent rocks in Lapland, northern Finland. Bulletin of the Geological Survey of Finland 281, 129 p.

Nironen, M., Lahtinen, R., and Koistinen, T., 2002. Suomen geologiset aluenimet – yhtenäisempään nimikäytäntöön, Summary in english: Subdivision of Finnish bedrock – an attempt to harmonize terminology. Geologi 54, 8–14.

Patison, N. L., Korja, A., Lahtinen, R., Ojala, V. J., and the FIRE Working Group 2006. FIRE seismic reflection profiles 4, 4A and 4B: Insights into the Crustal Structure of Northern Finland from Ranua to Näätämö. Geological Survey of Finland, Special Paper 43, 161–222.

Sorjonen-Ward, P., 1993. Structural history, alteration and gold mineralization in the Lapland Greenstone Belt, Finland. In: Mid- to lower-crustal metamorphism and fluids conference, Mount Isa, July 26–August 1, 1993, Geological Society of Australia, Abstracts 35, 88–90.

Sorjonen-Ward, P., Nironen, M., and Luukkonen, E. 1997. Greenstone Associations in Finland. In: de Wit, M. & Ashwal, L. (eds.) Greenstone Belts. Clarendon Press, London, 677–698.

Väisänen, M., 2002. Structural features in the central Lapland greenstone belt, northern Finland. Geological Survey of Finland, Report K21.42/2002/3.

Väänänen, J. and Lehtonen, M.I. 2001. U-Pb isotopic age determinations from the Kolari-Muonio area, western Finnish Lapland. Geological Survey of Finland, Special Paper 33, 85–93.

Ward, P., Härkönen, I., Nurmi, P., and Pankka, H. S. 1989. Structural studies in the Lapland greenstone belt, northern Finland and their applications to gold mineralizations. Geological Survey of Finland, Special Paper 10, 71–78.