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).

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.

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.

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.

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).

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