FIRE 3 Geographic constraints

The eastern part the FIRE 3 line transects the south-central Finnish part of the Archean Ilomantsi subterrane of the Archean Central Karelian subprovince in a roughly east-west direction. Towards the west the profile crosses the Archean-Proterozoic boundary into the North Karelian schist belt that consists of autochtonous and allochtonous Paleoproterozoic platform sedimentary sequences of the Karelian formations that are intercalated with Archean basement inliers and black schist interlayers associated with ophiolitic rocks. The profile terminates in the west-northwest in the Paleoproterozoic granitic rocks of the Heinävesi suite (Sorjonen-Ward, 2006).

FIRE 3 Geological and tectonic division

The upper crust along the FIRE 3 profile has been divided into three parts: the Outokumpu (1), Höytiäinen (2), and Pielinen (3) domains following the demarcation by Sorjonen-Ward (2006).

The middle crust has been divided into two parts: the Allochtonous Archean middle crust (4) and Autochtonous Archean middle crust (5) blocks.

The lower crust (6) is treated as a single block due to its lack of seismic fabric and observed structural features. Moho depth is approximated from Grad et al. (2009).

Geological interpretations of the deep structures along the FIRE 3 profile (CMP 0000–6300)

The mid and lower crust layers of the autochtonous (eastern) Archean block are relatively featureless with the exception of the easternmost part (CMP 4700–6300) of the mid crust section in which more reflective zones can be observed at the depth of 10–20 km. These zones have been interpreted either as high strain zones at the base of Archean granite plutons, tectonic imbrication of greenstone belt remnants, or refractory material in pluton source regions (Sorjonen-Ward, 2006). Alongside with the west-dipping general seismic fabric in the otherwise  featureless Archean upper crust, the overall structure is attributed to orogenic convergence, thickening, and generation and emplacement of granitic rocks (Sorjonen-Ward and Luukkonen, 2005; Sorjonen-Ward, 2006).

A moderately west-dipping discordance in reflectivity that projects to the surface at the Archaean–Proterozoic unconformity (CMP~3700) is interpreted to result from Paleoproterozoic rifting, basin subsidence, and deposition of the marginal Kaleva sequences (Sorjonen-Ward, 2006). Some eastward dipping reflectors at the Archean-Proterozoic boundary have also been identified but are interpreted as late orogenic deformational features. The overall seismic fabric is attributed to complex interaction between thrusting and strike-slip shear systems with maximum compressive direction subparallel to the buried craton margin.

The mid crust block west of the surface expression of the Archean–Proterozoic unconformity has distinct seismic fabric that is interpreted to reflect its potential allochtonous nature and separation from the more homogeneous Archean mid crust in the east beneath the Pielinen domain (Sorjonen-Ward, 2006).

At lower crustal levels the seismic fabric is more homogeneous and no definite structural interpretations have been made. Interpretations on the thickness of the crust vary from ca. 40 to 60 km along the FIRE 3 profile (Sorjonen-Ward, 2006; Korja and Heikkinen, 2008).

FIRE 3 Tectonic overview

The structural geology along the FIRE 3 profile has been interpreted in the context of extension and break-up of the Neoarchean Karelian basement at around 2.2–2.0 Ga and the following rift-related deep water sedimentation that generated the bulk of the Karelian platform cover sequences (Kohonen, 1995; Sorjonen-Ward, 2006). The extensional stage changed into a convergent tectonic regime after 2.0 Ga, which resulted in inversion of the extensional structural features and partly syn-sedimentary over-thrusting of the sedimentary cover, ophiolitic sequences, and basement nappes onto the Archean basement (Kohonen, 1995). The granitic rocks exposed in the northwestern part of the profile intruded into the sedimentary sequence at 1.87–1.85 Ga (Huhma, 1986). The preferred interpretation of the deep crustal structure along the profile has been a west-dipping unconformity between the Archean basement (in the east) and Proterozoic cover sequences (in the west). General regional structural evidence and isotope compositions of the Heinävesi suite granitic rocks suggests that Archean crust underlies the entire profile and that the Paleoproterozoic rocks form only the topmost <10 kilometers of the crust in the western part (Sorjonen-Ward, 2006).

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