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).
FIRE 3 Tectonic overview
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).
The upper crust along the FIRE 3 profile has been divided into three domains: the Outokumpu (1), Höytiäinen (2), and Pielinen (3) domains following the demarcation by Sorjonen-Ward (2006).
The middle crust has been divided into two domains: 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).
Geographic domain constraints
The Outokumpu domain corresponds to the part of the FIRE 3 profile that transects the western part of the North Karelian schist belt from the eastern contact of the Archean Sotkuma basement inlier (CMP 2500) to the Svecofennian granodiorites of the Heinävesi intrusive suite in the WNW (CMP 0000). The profile is roughly E-W along CMP 2500–1200 and follows a NW-SE strike along CMP 1200–0000.
In the eastern end of the Outokumpu domain (CMP ~2500–1800) the FIRE 3 profile follows the southern contact of the Archean Sotkuma inlier with the metasedimentary rocks that have been interpreted to belong to the autochtonic lower Kaleva assemblages. Towards W the profile crosses over allochtnonous upper (western) Kaleva metasediments of the Savo Province (Viinijärvi suite) that are dominated by monotonous turbiditic biotite paraschists (CMP 1800–0450) intercalated by the serpentinites and black schists of the Outokumpu assemblage (CMP 0880–0840; Peltonen, 2005; Peltonen et al., 2008). In the WNW the upper Kaleva metasediments are intruded by the granites and granodiorites of the Paleoproterozoic (1870–1850 Ma) Heinävesi intrusive suite (CMP 0470–0000).
Archean Sotkuma dome
The southern part of the Sotkuma inlier alongside the FIRE 3 transect is composed of migmatitic TTG gneiss but there are also some local occurences of (potentially basal) conglomerates in the southern contact towards the Kaleva metasediments (Huhma, 1975).
Kaleva sediments and Outokumpu assemblage
The upper (western) Kaleva allochtonous metasediments of the Outokumpu domain comprise a relatively homogeneous unit of metaturbiditic greywackes interbedded with local black schists (Laajoki, 2005; Peltonen et al., 2008; Lahtinen, 2010). The black schists are associated with a local sequence of highly strained ultramafic and mafic intrusions comprising the Outokumpu ophiolite complex that was over-thrusted alongside the allochtonous upper Kaleva metasediments onto the Archean basement and autochtonous lower Kaleva sequences (Peltonen, 2005; Peltonen et al., 2008).
Heinävesi suite granitic rocks
The Maarianvaara granites and granodiorites in the western end of the FIRE 3 transect belong to the Heinävesi intrusive suite (Huhma, 1986), which intruded into the upper Kaleva metasediments along the entire length of the North Karelian schist belt (Lahtinen, 2000).
The U-Pb zircon age of the Outokumpu ophiolite sequence is 1972 ± 18 Ma (Huhma, 1986). The maximum depositional age of the allochtonous upper Kaleva rocks within the Outokumpu domain is constrained by age of the ophiolite sequences in Outokumpu and Jormua and detrital zircon grains at around 1.95–1.90 Ga (Huhma, 1986; Kontinen, 1987; Claesson et al., 1993; Peltonen et al., 2008; Lahtinen et al., 2010). Their detrital zircon studies have recorded an Archean component (2.63–3.02 Ga) but are dominated by Paleoproterozoic ages (1.91–2.1 Ga; Lahtinen et al., 2010). Minimum age of deposition for the allochtonous western Kaleva sequence is defined by the plutonic rocks (Heinävesi suite) that intruded the formation at 1.87 Ga (Huhma, 1986; Lahtinen et al., 2010).
Structural and metamorphic geology
The suggested tectonic boundary between the Kalevian sequences coincides with the eastern contact of the Archean Sotkuma inlier and has been interpreted from the west-dipping reflectors that can be followed down to the depth of ~10 km (CMP 2500–1500). As a dominantly allochtnonous unit, tectonic stacking and multiple folding are common features of the upper Kaleva rocks (Lahtinen et al., 2010). The Outokumpu ophiolite assemblage with associated black schists is tectonically enclosed in the upper Kaleva metasediments but represents at least locally also their deposition surface (Lahtinen et al., 2010). The dominant structural feature in the upper crust west of the Sotkuma inlier is the Viinijärvi synform (hinge plane NE-NNE, moderate dip SW; surface expression at CMP ~1800–800; Sorjonen-Ward, 2006). The surface intersection of the interpreted western (east-dipping) limb of the synform coincides with the FIRE 3 intersection of the Outokumpu sequence (CMP ~800). Non-reflective zones underneath the Viinijärvi syncline have been correlated with the granitic rocks of the Heinävesi suite (Maarianvaara/Kermajärvi) in the west and south. Zones of high reflectivity between CMP 900–1200 have been correlated to Outokumpu-type sequences reached through a deep drill core (Kukkonen et al., 2006; Heinonen et al., 2011). The metamorphic grade increases from east to west across the domain and is seen in recrystallization, segregational banding, and migmatization, which become stronger in the western part (Lahtinen, 2000; Sorjonen-Ward, 2006).
Geochemistry and isotope geology
The allochthonous western Kaleva metasedimentary rocks are compositionally and isotopically very homogeneous but based on geochemical criteria they have been divided into two groups (Lahtinen, 2000; Lahtinen et al., 2010): 1) metapsammites and 2) SiO2-poor metapelitic rocks with higher levels of elements incorporated in clay minerals (Al2O3, MgO, FeO, K2O). Based on trace element evidence (high Sc/Th and Cr/Th ratios) the material of the western Kaleva rocks is interpreted to have a substantial mafic component (Lahtinen, 2000). The Sm-Nd model ages (TDMNd) of the Upper Kaleva metasediments ranges from 2.23 to 2.41 Ga and εNd values from −0.6 to −2.8 at 1.9 Ga (Lahtinen et al., 2010). A Heinävesi suite granodiorite from Viitalahti has a Sm-Nd mantle model age (TDMNd) of 2.20 Ga and εNd value of -2.1 at 1.87 Ga (Huhma, 1986). The geochemical and geochronological evidence suggests that the source of the western Kaleva sediments was dominantly juvenile but with some Neoarchean material (Lahtinen et al., 2010).
Geographic domain constraints
The Höytiäinen domain (Sorjonen-Ward, 2006) is constrained in the east by the surface expression of the boundary (CMP ~3700) between the Neoarchean Ilomantsi subterrane (Pielinen domain) and the Proterozoic autochtonous Höytiäinen province (Ward, 1987; 1988; Kohonen, 1995). In the west it is limited by the Suhmura thrust contact to the Savo province (Outokumpu domain; CMP 2500–0000) and the Archean Sotkuma basement inlier (Kohonen, 1995; CMP ~2500, coincident also with an acquisition break in the FIRE 3 transect between 2700–2500).
Höytiäinen domain belongs to the southern part of the Paleoproterozoic North Karelian Schist Belt and includes (par)autochtonous eastern (lower) Kaleva (in the east) with autochtonous upper Kaleva (in the west) and their underlying Jatuli platform rocks (easternmost part of the domain; Ward, 1987; Kohonen, 1995; Lahtinen, 2000; Lahtinen et al., 2010). The rocks have been interpreted as syn- and post-rift sequences deposited within a marginal basin of the Archean basement during early Proterozoic multistage extension and deformed during the convergent stage of a subsequent basin inversion (Kohonen, 1995; Lahtinen et al., 2010).
The sequence consists of autochtonous pelitic and psammitic eastern Kaleva (or Kalevian) metasediments (greywackes and mica schists; CMP 3660–2700) underlain by a thin occurrence of Jatuli platform quartzites (CMP 3680–3660) that delineate the eastern contact towards the Archean rocks of the Pielinen domain. The Jatuli quartzites and conglomerates do not necessarily represent a basal unit for the sequence but most likely the location of a W–SW dipping fault zone that was reactivated multiple times during both extensional and convergent stages (Kohonen, 1995; Laajoki, 2005).
Lithologically the eastern Kaleva is heterogeneous in different scales with interbedded metagreywacke and metapelite dominated units and metaquartzitic intercalations (Lahtinen et al., 2010). The metasediments in the northern part of the Höytiäinen province have been separated in five distinct lithologic assemblages (LA 1–5; Kohonen, 1995). LA 1, 2, and 3 are dominated by laminated pelitic schists with quartzite interbeds, minor carbonate rocks (LA 2; Kohonen, 1995), and graphite-bearing interlayers (LA3; Kohonen, 1995). LA 4 and 5 include a larger amount of psammitic material but are interbedded with pelitic layers (Kohonen, 1995).
Based on the U-Pb geochronology of volcanic rocks underlying the lower Kaleva (Tohmajärvi volcanic complex, 2105 ± 15 Ma; Huhma, 1986), maximum deposition age of the autochtonous Kalevian sequences is traditionally assumed to be ~2.1 Ga (Pekkarinen, 1979; Huhma, 1986; Pekkarinen and Lukkarinen, 1991; Lahtinen et al., 2010). Detrital zircon studies have, however, only yielded Neoarchean U-Pb ages (Claesson et al., 1993; Lahtinen et al., 2010). The maximum depositional age of the autochtonous upper Kaleva rocks within the domain is constrained by ages of the ophiolite sequences and detrital zircon grains from the western Kaleva (Outokumpu domain) at around 1.95–1.90 Ga (Huhma, 1986; Kontinen, 1987; Claesson et al., 1993; Peltonen et al., 2008; Lahtinen et al., 2010). Their detrital zircon studies record dominantly Paleoproterozoic ages (1.91–2.1 Ga; Lahtinen et al., 2010). Minimum age of deposition for the whole Kalevian (Kaleva) sequence is defined by intrusive plutonic rocks (Heinävesi suite) 1.87 Ga in age (Huhma, 1986; Lahtinen et al., 2010).
Structural and metamorphic geology
The general structural framework of the Kaleva sequences suggests extension and subsidence were followed by an eastward convergence with over-thrusting of the western allochtnonous Kaleva sequences and basement nappes on top of the (par)autochtonous eastern Kaleva sequences. Two-stage tectonic evolution is evident in the structure of the rocks of the Höytiäinen domain: early extensional features during the rifting of the Archean basement (~2.1–2.05 Ga) and convergent structures related to the inversion of extension into a collisional tectonic setting (~1.95–1.9 Ga). The convergent stage reactivated many of the earlier extensional features as the originally west-dipping eastern contact developed into an eastward over-thrust and the western east-dipping marginal faults into rotational basement thrusts (Kohonen, 1995). Poorly-defined near-vertical reflectors in the Höytiäinen domain (CMP 3000–3100) are interpreted as correlatives of mafic volcanic sequences (Tohmajärvi) exposed on the surface southeast of the profile transect (Sorjonen-Ward, 2006).The Höytiäinen province is characterized by lower to medium amphibolite facies metamorphic conditions that increase towards the west (Kohonen, 1995; Lahtinen et al., 2010).
Geochemistry and isotope geology
The major element and Sm-Nd isotope geochemistry of the lower Kaleva rocks in the Höytiäinen domain suggests a source dominated by Neoarchean weathered paleosols and derived sedimentary rocks with local input from Neoarchean unweathered and Proterozoic volcanic and subvolcanic rocks (Huhma, 1987; Kohonen, 1995; Lahtinen, 2000; Lahtinen et al., 2010). The upper Kaleva rocks in the western part of the domain have somewhat lower TDMNd values and differ also geochemically from the lower Kaleva assemblages, which suggests a dominantly Paleoproterozoic source for them (Lahtinen et al., 2010).
Geographic domain constraints
The FIRE 3 line transects the south-central Finnish part of the Archean Ilomantsi subterrane of the Archean Central Karelian subprovince in a roughly E-W direction. The Pielinen domain covers the upper crustal CMP range 3700–6300 of the FIRE 3 profile. It commences in the Archean sanukitoid suite Sysmäjärvi quartz diorites (CMP 6300) in the east and terminates in the west at the Archean–Proterozoic boundary (~CMP 3700) towards the Proterozoic Höytiäinen domain (CMP 3700–2500).
The upper crust of the Pielinen domain belongs to the Neoarchean Ilomantsi subterrane of the Archean Central Karelia subprovince (Sorjonen-Ward, 1993; 2006). Most of the surface rocks along the FIRE 3 profile belong to the Ilomantsi complex, which consists of several Neoarchean granitic intrusions (tonalites, granodiorites, and quartz diorites) with sanukitoid affinities (Lobach-Zhuchenko et al., 2005; Heilimo et al., 2011; 2012), migmatitic TTG (tonalite-trondhjemite-granodiorite) gneisses (Hölttä et al., 2012), and GGM (granodiorite-granite-monzogranite) series granites (Käpyaho et al., 2006; Hölttä et al., 2012) intercalated by greenstone belts (Sorjonen-Ward, 1993; 2006; Hölttä et al., 2016).
The granitic rocks of the domain include the Sysmäjärvi (Möhkö) quartz diorite (CMP 6300–6100) and Pogosta (Ilomantsinjärvi) granodiorite (CMP 5900–4700) that belong to the Kuittila sanukitoid suite, undiscriminated tonalitic TTG series migmatites (CMP 4900–3800), and the dominantly monzogranitic rocks of the Kutsu (CMP 4800–3700) and Naarva (CMP ~6100) granite suites that belong to the Archean GGM series (Käpyaho et al., 2006; Hölttä et al., 2012).
The Ilomantsi greenstone belt has two major branches. The Hattu schist belt (CMP 6100–5900) lies in the eastern part of the domain, between the Kuittila Suite sanukitoids; the Sysmäjärvi quartz diorite (Möhkö tonalite) in the east and the Pogosta (Ilomantsinjärvi) granodiorite in the west. The Kovero branch of the greenstone belt in the western part of the domain is separated into three discrete zones: Sonkaja (CMP ~4600), Otravaara/Kuusijärvi (CMP 4400–4200), and Ylinen/Keskijärvi (CMP ~3800) zones, which are intercalated within TTG series tonalitic migmatites and Kutsu suite granites.
The rocks of the domain are dominantly Archean but are intruded by minor Paleoproterozoic gabbro and diabase bodies and dikes designated to three distinct arrays that intersect the FIRE 3 profile (Sorjonen-Ward, 2006): the Marjovaara array (CMP 5100–4500), Havukkakallio array (CMP ~5500), and Kuuksenvaara array (CMP 6100–5600).
The eastern branch of the Ilomantsi greenstone belt, the Hattu schist belt, consists principally of feldspathic volcaniclastic deposits with andesitic, basaltic, and locally also ultramafic volcanic interlayers (Sorjonen-Ward, 1993, 2006). In its southern part the belt is dominated by turbiditic and pelitic metasedimentary rocks, mainly greywackes. The Kovero greenstone belt is dominated by mafic and ultramafic volcanic and subvolcanic units with minor felsic and sedimentary interlayers (Sorjonen-Ward, 1993, 2006).
Kuittila suite sanukitoids
The sanukitoid rocks of the Kuittila suite (Lobach-Zhuchenko et al., 2005; Heilimo et al., 2011, 2012) make up the majority of the eastern part of the Pielinen domain. They belong to the older (2.75–2.73 Ga) Eastern Sanukitoid Zone (Heilimo et al., 2011, 2012) of the Central Karelian Province. Major Kuittila suite sanukitoids along the FIRE 3 profile include the Sysmäjärvi quartz diorite (Möhkö tonalite, Sorjonen-Ward, 2006; CMP 6300–6000), Kuittila tonalite (CMP ~5900), and the Pogosta granodiorite (Ilomantsinjärvi granodiorite, Heilimo et al., 2011; CMP 5800–4900).
TTG gneisses and GGM granites
Tonalite-trondhjemite-granodiorite (TTG) gneisses and migmatites make up most of the Archean Karelian Province and are prominent also in the western part of the Pielinen domain along the FIRE 3 profile. Granodiorite-granite-monzogranite (GGM) suite rocks (Käpyaho et al., 2006; Hölttä et al., 2012) are also more prominent in the western part of the domain towards the contact of the Proterozoic Höytiäinen domain (CMP 3800–3700). The largest GGM suite intrusion is the Kutsu granite that lies about 5 km south of the FIRE 3 profile at CMP ~4650. In the eastern part of the domain, the Sysmäjärvi quartz diorite is intruded by the Lukanvaara leucogranite (CMP ~6100) that belongs to the Naarva GGM granite suite.
The bulk of the Ilomantsi greenstone belt including the Hattu schist belt was formed at about 2750 Ma and most of the granitic rocks in the Kuittila suite intruded either contemporaneously or shortly after (Vaasjoki et al., 1993; Heilimo et al., 2011; Huhma et al., 2012). The Sysmäjärvi quartz diorite and Kuittila tonalite in the eastern part of the domain have coinciding U-Pb TIMS and SIMS ages at around 2745 Ma [Sysmäjärvi quartz diorite TIMS age (Vaasjoki et al., 1993): 2744 ± 3 Ma and a SIMS age (Heilimo et al., 2011): 2744 ± 5 Ma, Kuittila tonalite TIMS age (Vaasjoki et al., 1993): 2745 ± 10 Ma and SIMS age (Heilimo et al., 2011): 2741 ± 6 Ma] and the Pogosta granodiorite seems to be slightly younger at about 2730 Ma [TIMS ages (Vaasjoki et al., 1993): 2733 ± 6 and 2724 ± 5 Ma and a SIMS age (Heilimo et al., 2011): 2728 ± 7 Ma]. Youngest Ilomantsi area GGM age for the Lukanvaara suite based on unpublished data quoted by Hölttä et al. (2016) is 2.63 Ga. The Kovero greenstone belt has two distinct age groups. In addition to felsic dykes and gabbroic rocks contemporaneous with the main part of the Ilomantsi greenstone belt at ~2.75–2.76 Ga, felsic volcanic rocks in the Kovero belt have been dated at 2878 ± 2 Ma (Huhma et al., 2012).
Structural and metamorphic geology
Due to the lack of major Paleoproterozoic deformation, the seismic fabric of the Archean rocks in the eastern part of FIRE 3 profile has been interpreted to principally record Neoarchaean orogenic processes (Sorjonen-Ward, 2006). The relatively featureless upper crust in the Archean Pielinen domain has been interpreted to reflect its granitoid-dominated geology. The greenstone belts along the profile do not correlate with any apparent reflectivities. Contacts between the granitoids and the greenstone belts in the Pielinen domain are not observed to be depositional but are either intrusional or tectonic (Hölttä et al., 2012). Greenstone belts have generally steeply dipping structures with N–NNE strike trends and their deformation follows the intrusional features of the granitic rocks (Sorjonen-Ward, 2006). The Archean rocks of the Pielinen domain experienced two metamorphic stages: a Neoarchean event at 2.66–2.62 Ga coeval with the late stage GGM magmatism and a Paleoproterozoic thermal event at ~1.84 Ga evidenced by resetting of K-Ar isotopic systems and monazite U-Pb ages (Hölttä et al., 2016). The Proterozoic overprinting is, however, less pronounced in the Ilomantsi subterrane than in the Western Karelia Subprovince (Hölttä et al., 2012). Deformation related to the Proterozoic metamorphosis in the Pielinen domain was in general weak and restricted mostly to fracture zones (Sorjonen-Ward, 2006). The southern part of the Ilomantsi subterrane lacks systematic P/T determinations but the central parts were metamorphosed in lower to mid-amphibolite facies conditions (<5.5 kbar/<600°C; Hölttä et al., 2012). Metamorphic grade is higher in the northern parts of the subterrane varying from upper amphibolite to granulite facies (>6 kbar/>600°C; Hölttä et al., 2012). In general, greenstone belts display consistently lower metamorphic grade (especially in the inner parts) than the granitic rocks surrounding them (Tuisku, 1988; Hölttä et al., 2012; 2016).
Geochemistry and isotope geology
Most of the Archean bedrock in Finland belongs to the Western Archean subprovince but the Ilomantsi subterrane belongs to the Central Karelian subprovince. This is also reflected in the geochemistry of the Ilomantsi complex rocks that differs somewhat from the corresponding rock types elsewhere in the Finnish Archean.
The volcanic rocks of the Ilomantsi greenstone belt have arc-type (BADR, basalt–andesite–dacite–rhyolite) geochemistry (Hölttä et al., 2012; 2016). The Ilomantsi komatiites have high LREE/HREE ratios and negative Nb, Ta, and Ti-anomalies. Compositional similarity between the komatiites and associated felsic volcanic rocks suggests a petrogenetic connection (Hölttä et al., 2012). The Ilomantsi komatiitic basalts, however, have less fractionated incompatible trace element patterns than the komatiites, which suggests simple crystal fractionation may not be adequate to explain their geochemical relationship (Hölttä et al., 2012). Kovero komatiites and basaltic komatiites are geochemically distinct from the main Ilomantsi belt komatiites as they have less fractionated REE patterns and higher Al2O3/TiO2 and Cr/MgO ratios.
Archean TTG:s that make up most of the western part of the Pielinen domain have dominantly adakitic geochemistry (Richards and Kerrich, 2007; Hölttä et al., 2012). In contrast to most Finnish TTG series rocks the Ilomantsi TTG:s also display the diagnostic higher Cr and Ni values (Ruotoistenmäki, 2012). Archean granitic rocks of the Ilomantsi subterrane are on average relatively high in SiO2 and alkalis. The adakitic rocks of the Ilomantsi area are defined more clearly by post-collisional–late-orogenic geochemical signatures in contrast to broader pre- to post-orogenic ranges observed elsewhere in the Finnish Archean (Ruotoistenmäki, 2012). The granitic rocks of the eastern part of the Ilomantsi complex are dominantly sanukitoids, which have a distinct geochemical signature in elevated contents of both compatible (e.g., Mg, Ni, Cr) and incompatible (e.g., K, Ba, Sr, LREE) elements and Mg# at a given SiO2 when compared to the TTG series rocks (Heilimo et al., 2010; 2012).
Sm-Nd isotope systematics of the Ilomantsi complex rocks are strongly affected by later metamorphic remobilization but meticulous examination of a large sample volume (O’Brien et al., 1993; Huhma et al., 2012) has produced a consistent petrogenetic picture. Neodymium isotope mantle model ages (TDMNd ~2.75–2.9 Ga) of the Ilomantsi complex rocks suggest a predominantly juvenile origin for most of the granitic and volcanic rocks. Last stage GGM rocks have not been directly studied in the Ilomantsi area but are elsewhere suggested to be generated by remelting of recently generated Neoarchean crust.
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).
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