Abstract Ductile shear zones with dextral transpressive deformation separate the Ljusdal lithotectonic unit from the neighbouring units (Bothnia–Skellefteå and Bergslagen) in the 2.0–1.8 Ga Svecokarelian orogen. Sedimentation steered by regional crustal extension at c. 1.86–1.83 Ga was sandwiched between two separate phases of ductile strain with crustal shortening and predominantly high-grade metamorphism with plutonic activity. Metamorphism occurred under low-pressure, medium- to high-temperature conditions that locally reached granulite facies. The earlier shortening event resulted in the accretion of outboard sedimentary and c. 1.89 Ga volcanic rocks (formed in back- or inter-arc basin and volcanic arc settings, respectively) to a continental margin. Fabric development (D 1 ), the earlier phase of low-pressure and variable temperature metamorphism (M 1 ) and the intrusion of a predominantly granitic to granodioritic batholith with rather high ε Nd values (the Ljusdal batholith) occurred along this active margin at 1.87–1.84 Ga. Thrusting with westerly vergence, regional folding and ductile shearing (D 2–3 ), the later phase of low-pressure and variable temperature metamorphism (M 2 ), and the subsequent minor shear-related intrusion of granite, again with relatively high ε Nd values, prevailed at 1.83–1.80 Ga. Mineral deposits include epithermal Au–Cu deposits hosted by supracrustal rocks, V–Fe–Ti mineralization in subordinate gabbro and norite bodies inside the Ljusdal batholith, and graphite in metasedimentary rocks.

Abstract Six separate lithotectonic units, referred to from north to south as the Överkalix, Norrbotten, Bothnia–Skellefteå, Ljusdal, Bergslagen and Småland units, are identified inside the western part of the 2.0–1.8 Ga Svecokarelian orogen, Fennoscandian Shield, Sweden. Apart from the boundary between the Norrbotten and Bothnia–Skellefteå lithotectonic units in northern Sweden, which is defined on the basis of a change in crustal basement from Neoarchean (and possibly older) in the NE (Norrbotten) to juvenile Paleoproterozoic crust further south (Bothnia–Skellefteå), all the boundaries are defined by shear zones or combinations of zones that, in places, form broader shear belts up to several tens of kilometres thick. The identification of lithotectonic units provides a necessary foundation for a more detailed synthesis of the tectonic evolution of the 2.0–1.8 Ga orogeny in northern Europe, emphasizing in particular the allochthoneity between most of these units inside this part of the orogen.

Abstract The Småland lithotectonic unit in the 2.0−1.8 Ga Svecokarelian orogen, southeastern Sweden, is dominated by a c. 1.81−1.77 Ga alkali–calcic magmatic suite (the Transscandinavian Igneous Belt or TIB-1). At least in its central part, the TIB-1 suite was deposited on, or emplaced into, c. 1.83–1.82 Ga calc-alkaline magmatic rocks with base metal sulphide mineralization and siliciclastic sedimentary rocks (the Oskarshamn–Jönköping Belt). Ductile deformation and metamorphism under low- to medium-grade conditions affected the Oskarshamn–Jönköping Belt prior to c. 1.81 Ga. Both suites were subsequently affected by low-grade ductile deformation, mainly along steeply dipping, east–west to NW–SE shear zones with dip-slip and dextral strike-slip displacement. Sinistral strike-slip NE–SW zones are also present. In the northern part of the lithotectonic unit, 1.9 Ga magmatic rocks, c. 1.87–1.81 Ga siliciclastic sedimentary rocks and basalt, and c. 1.86–1.85 Ga granite show fabric development, folding along steep NW–SE axial surfaces and medium- or high-grade metamorphism prior to c. 1.81 Ga and, at least partly, at c. 1.86–1.85 Ga; base metal sulphide, Fe oxide and U or U–REE mineralizations also occur. Magmatism and siliciclastic sedimentation along an active continental margin associated with subduction-related, accretionary tectonic processes is inferred over about 100 million years.

Beginning ca. 2.76 Ga, evolution of the Kola-Karelia crust was related to the intracontinental high-temperature metamorphic (up to granulite facies) and magmatic events in combination with formation of the basins related to rifting and infilling with intracontinental volcanic and sedimentary sequences initiated by plume-type processes in the mantle. The geological events corresponding to intracontinental evolution were expressed not only in the formation of new rock associations, juvenile to a significant extent, but also in reworking of previously formed rocks. The age, content, and mode of geological activity are somewhat different in the Kola and the Karelian-Belomorian regions. The Karelian-Belomorian region is oval in plan view. The long axis of this oval extends for 600–700 km in the meridional direction; its maximum width is 400–450 km. The southern part of this oval structure is cut off along the NW-trending boundary with the Paleoproterozoic Svecofennian accretionary orogen. The main constituents of the Karelian-Belomorian region are: epicontinental sequences of greenstone belts (Kostomuksha, Khedozero-Bolsheozero, Gimoly-Sukkozero, Jalonvaara) and paragneiss belts (Hattu, Nurmes); granulite-gneiss complexes and intrusive enderbite-charnockite series; sanukitoid-type granitoid intrusions and lamprophyre dikes, along with migmatization and emplacement of within-plate young granites; and local manifestations of granulite-facies metamorphism superposed on older rocks. Concentric spatial distribution of related geological units is characteristic of the Karelian-Belomorian region. The geometric pattern of the region can be satisfactorily explained assuming initial activity of a mantle plume ca. 2.76 Ga in the central part of the region. A peak of activity was related to the events that occurred ca. 2.74–2.70 Ga. The geochronological data show that a region of high-temperature processes expanded from its center (2.76–2.73 Ga) to the periphery (2.74–2.70 Ga). The concentric character of the tectonic structure was eventually formed as a result of these processes. Widespread high-temperature magmatism and metamorphism in combination with formation of synformal and linear sedimentary basins indicate the setting of anorogenic extension and vigorous influx of extracrustal heat, i.e., a large event related to a mantle plume. In contrast to the Karelian-Belomorian hot region, the coeval Kola region of intracontinental manifestations of high-temperature metamorphism and magmatism is characterized by oval-block geometry. This area, confined to the central part of the Kola Peninsula, extends for 600 km in the northwestern direction, having a width of ~200 km. It is possible that this area extends further to the southeast beneath the platform cover. The main tectonic units are the intracontinental greenstone belts (Sør-Varanger, Titovka, Uraguba, Olenegorsk, Voche-Lambina, Kachalovka, Runijoki–Khikhnajarvi, and Strelna system) in the Inari-Kola microcontinent, the granulite-gneiss Central Kola complex, and the Keivy volcanotectonic paleodepression. Sanukitoid intrusions play a modest role. The Keivy volcanotectonic paleodepression is situated in the eastern Kola Peninsula. Rocks of this tectonic unit are peculiar, and many of them have no obvious analogs in the Fennoscandian Shield or elsewhere. The major Neoarchean amphibolite-gneiss association consists of calc-alkaline to subalkaline garnet-biotite and subalkaline-peralkaline aegirine-arfvedsonite gneisses, as well as biotite-amphibole and amphibole gneisses, amphibolites, and rheomorphic alkali granites. In the western part of the paleodepression, gneisses (metavolcanic rocks) are cut through by small Sakharjok and Kuljok nepheline syenite intrusions. Geochronological estimates characterize two outbursts of magmatic activity separated by a long gap. The early outburst corresponds to magmatic crystallization of calc-alkaline metavolcanic rocks at 2.90–2.87 Ga. The second vigorous outburst documented at 2.68–2.63 Ga corresponds to eruption of subalkaline and subalkaline-peralkaline volcanic rocks, emplacement of alkali and nepheline syenites, and crystallization of gabbro-anorthosite of the Tsaga-Acherjok complex. The duration of the main magmatic phase is ~50 m.y., whereas the preceding gap lasted for ~200 m.y. A model of a volcanotectonic depression largely filled with pyroclastic flows seems plausible to explain pre-metamorphic events. Such manifestations of volcanic activity are inherent to intracontinental domains and related to activity of mantle plumes; similar processes can also develop in the back extensional zone of active continental margins. The synchronism of felsic volcanism and emplacement of the typically intracontinental gabbro-anorthosites form a sound argument in favor of an intracontinental setting for the Keivy paleodepression. The geometry of the Kola region can be satisfactorily explained in terms of mantle-plume activity noted ca. 2.76 Ga in the marginal part of this region; a peak of activity in its central part is related to the events that happened ca. 2.68–2.63 Ga.

In this chapter, the available seismic and geological data are integrated and applied to the East European craton as a whole. The deep structure and seismic characteristics of the granite-greenstone crust in the Archean microcontinents vary significantly. The unevenly distributed and vaguely oriented short reflections are occasionally gathered into packets. The intensely reflecting bodies with high acoustic impedance correspond to greenstone belts reaching many tens of kilometers in extent. The oval acoustically transparent domains in the middle-crustal level are composed of moderately dense rocks that correspond to granitoid plutons. The total thickness of the crust in granite-greenstone domains can vary from 40 km in the Kola-Karelia continent to 50 km in the Kursk microcontinent. The Archean granulite-gneiss complexes form the delineated belts localized in the upper crust. These are synformal tectonic nappes, the vertical thickness of which reaches 15 km. A special case is the Volgo-Uralia continent, the crust of which is completely composed of granulite-gneiss rocks partly replaced by retrograde metamorphic assemblages. The granulite-gneiss crust is distinguished by significant thickness (~60 km; maximum 65–70 km). The lower-crustal “layer,” ~35 km thick, consists of inclined tectonic sheets plunging in a northwestern direction and penetrating the upper mantle. The interface between upper and lower crust is replaced by acoustically transparent granitoid crust 10–20 km in thickness. The Middle Paleoproterozoic East Voronezh intracontinental collisional orogen between the Archean Kursk and the Khopior microcontinents is represented by a “crocodile-jaw” structure. The structural pattern in the seismic image of the crust clearly indicates a Paleoproterozoic age of lower-crustal layer and shows the absence of the Paleoproterozoic lower-crustal complex at the base of the Kursk craton proper. The Late Paleoproterozoic intracontinental Lapland–Mid-Russia–South Baltia orogen surrounds in a wide arc the Karelian craton in the northeast, east, south, and southwest. The upper crust in the inner zone of the orogen in the Mid-Russia sector is composed of alternating granulite-gneiss and gneiss-migmatite-amphibolite tectonic sheets 5–10 km thick, deformed in gentle synformal folds. The marginal zones consist of south-plunging tectonic sheets of the Totma belt in the north and the Aprelevka belt in the south. The Totma sheet, reaching 10 km in thickness and dipping at a mean angle of 5°–10°, is traced by reflectors from the basement surface (interval 1700–1800 km) to the crust-mantle interface (interval 2000–2200 km). These parameters, along with composition of the rocks, allow interpretation of the Totma belt as a suture zone, separating the synformal structural assemblage from the lower crust. In the Paleoproterozoic Svecofennian accretionary orogen, interpretation of the FIRE-1 profile shows that the Central Finland granitoid massif is a nearly horizontal, sheetlike intrusive body that conceals an accretionary complex—a succession of tectonic sheets, 10–20 km thick, which plunges northeast at angles of 10°–12° down to the crust-mantle interface at a depth of 65 km and can be traced beneath the margin of the Karelian craton for more than 150 km. The lower-crustal layer, often called a reflectivity zone, is always present at the base of the Lapland–Mid-Russia–South Baltia orogen and the Archean cratons surrounded by this arcuate orogen. This layer was formed in the Early Paleoproterozoic as a result of under- and interplating of mantle melts accompanied by granulite-facies metamorphism. The increase in thickness of the lower-crustal layer is related to hummocking (mutual over- and underthrusting and wedging) of tectonic sheets at the base of the crust. The lower-crustal layer of the Kola-Karelia continent was formed before the main collisional events, which took place in the Late Paleoproterozoic. The structure of the crust depicted by seismic reflectors indicates in some cases that the crust-mantle interface has remained unchanged since the time of crust formation, whereas in other situations, this boundary is younger than the bulk crust. The crust-mantle interface beneath the East European craton reveals manifold deviations from its persistent near-horizontal outline due to bending, plunging, and apparent dissolution of lower-crustal sheets in the mantle. The underlying upper mantle reveals a number of indistinct reflectors imaged as dashed lines, which trace lower-crustal structural elements incorporated into the mantle. These domains are regarded as crust-mantle mixtures. The crust images along the seismic lines exhibit widely varying structural features and degrees of contrast (sharpness) of the crust-mantle interface. The following structural and morphological types of the crust-mantle interface are distinguished beneath the East European craton: (1) a smooth, generally flat or horizontal or slightly sloping boundary with an abrupt decrease in number of reflectors at the lower edge of intensely reflecting layered lower crust; (2) a boundary similar to the previous type but periodically interrupted at sites where the sheetlike fragments of the lower crust sharply bend and sink into the mantle and acoustically as if they dissolve therein; (3) a serrated boundary in the regions of consecutive plunging lower-crustal tectonic sheets into the mantle; these domains are commonly conjugated: reverse-thrust assemblages in the upper crust rise in the same direction as the lower-crustal sheets plunge; (4) a serrated boundary confining from below the ensemble of inclined tectonic sheets that form the crust completely or partly; (5) a diffuse crust-mantle interface that is observed where a distinct lower-crustal reflectivity zone is absent; and (6) a phantom (disappearing) crust-mantle interface that separates acoustically transparent crustal and mantle domains that can be detected by seismic-refraction exploration. Integration of the entire body of information allowed us to simulate the 3D deep structure of the Early Precambrian crust of the East European craton as a whole using sections along the 1-EU geotraverse, cross-traverse 4B, TATSEIS regional profile, and FIRE-1 and FIRE-4 profiles. The 3D deep structure of the East European craton is represented by the tectonically delaminated Early Precambrian crust with a predominance of gently dipping boundaries between the main tectonic subdivisions and a complexly built crust-mantle interface. The integral model includes Archean granite-greenstone domains and granulite-gneiss areas and Paleoproterozoic accretionary and intracontinental collisional orogens.

The earliest events in the geological history of the Early Precambrian crust of the East European craton (3.5–2.93 Ga) resulted in the emergence of spatially separate and genetically independent areas of continental crust (continental embryos), the dimensions of which rarely exceeded a few hundred kilometers across. The period between 3.05 Ga and ca. 2.75 Ga was a time of mainly plate-tectonic development: origin, evolution, and accretion of ancient island-arc systems, and collision of microcontinents. The Vedlozero-Segozero and Sumozero-Kenozero systems of greenstone belts, Tipasjärvi-Kuhmo-Suomussalmi and Central Belomorian greenstone belts in Karelia, and the Kolmozero-Voronya greenstone belt in the Kola Peninsula are interpreted as accretionary systems transformed in collisional orogens. The Belomorian eclogite province is structurally linked with the Central Belomorian greenstone (suture) belt. The Kovdozero granite-greenstone terrane is formed by granitoids and gneisses hosting metasediments and metavolcanics of several greenstone belts, which belonged to the Parandovo-Tiksheozero island arc. The amalgamation of the continental domains that made up the bulk of the Archean crust in the growing East European composite craton took from 2.82 to 2.66 Ga, but the main events had terminated by 2.75 Ga. During the period from 2.79–2.55 Ga, specific areas of intracontinental thermal and tectonic activity (hot regions) developed in the inner portions of the recently formed continent: Karelian-Belomorian and Kola areas in the eastern Fennoscandian Shield and extensive Volgo-Uralia granulite-gneiss area in the eastern part of the East European craton. These processes marked a principally new evolutionary episode in the Early Precambrian history of the East European craton. Widespread high-temperature magmatic and metamorphic processes and the development of synformal structures and linear sedimentary basins testify to an anorogenic extensional environment and a significant influx of heat to the crust, i.e., a significant event of mantle-plume type. During the Paleoproterozoic (2.53–1.87 Ga), a number of intracontinental collisional orogens were produced within the East European craton. The largest of them are the Lapland–Mid-Russia–South Baltia intracontinental orogen and the Svecofennian accretionary orogen. The Lapland–Mid-Russia–South Baltia orogen surrounds the Karelian craton as a wide arc, separating it from Volgo-Uralia and Sarmatia. The orogen extends for more than 3000 km; its width in the northern and central segments is 400–700 km and increases to 1000 km in the southwest. The Lapland sector of the orogen is characterized by spatial distribution of tectonic belts composed of low-grade metavolcanic-metasedimentary rocks and belts of high-grade metamorphic rocks, including granulite-gneiss complexes. The former are localized along the orogen boundaries; in turn, the axial zone of the orogen is mainly formed by alternation of low-angle tectonic sheets varying in thickness from a few to 20–25 km: sheets composed of Paleoproterozoic granulite-gneiss complexes with a predominance of metamorphosed juvenile intrusive and volcanic bodies and sedimentary rocks alternate with the sheets of Archean granite-greenstone and amphibolite-gneiss complexes. The Paleoproterozoic evolution of the Kola-Karelia continent and, accordingly, the Lapland–Mid-Russia–South Baltia orogen, is subdivided into four episodes: (1) ca. 2.53–2.3 Ga: failed rifting of the Archean continent; (2) 2.3–2.1 Ga: quiescent within-plate activity and diffuse rifting that can be regarded as “failed attempts” to break the supercontinent; (3) 2.1–1.95 Ga: rifting of the Kola-Karelia continent; and (4) 1.95–1.87 Ga: origin of the intercontinental collisional orogens. The Paleoproterozoic pulse of tectonic activity, which transformed the Neoarchean Kola-Karelia continent, continued for more than 600 m.y. Globally speaking, Early Paleoproterozoic magmatic and thermal activities were largely constrained within the ancient continent that then included North America and most of the eastern European continent, including the Fennoscandian Shield (Lauroscandia). Analysis of the extensive data leads us to distinguish a new type of tectonic structure: the intracontinental oval orogen formed in the inner portions of continents under the effect of large mantle plumes. It is an oval-shaped tectonic ensemble of regional rank with diameters from 600–1000 to 2500–3000 km, of which at least a part is characterized by concentric structure and metamorphic zonation or which contains bowl-shaped crustal structures. Intracontinental orogens contain granulite-gneiss complexes, derivatives of juvenile (though crust-contaminated) mafic magmas (gabbro-anorthosites and layered mafic-ultramafics), intrusions of “dry” high- temperature, within-plate–type granites, enderbites, and charnockites, and low-grade sedimentary-volcanic belts. The oval or oval-concentric structure excludes the possibility that intracontinental orogens originated as a result of processes at convergent plate boundaries. Their size and morphology and the evidence of a vast influx of mantle heat make intracontinental orogens comparable to oceanic plateaus and large igneous provinces on the continents. The fundamental changes in Earth's geological evolution that occurred at the Mesoarchean-Neoarchean boundary (ca. 2.75 Ga) can be related to the transition from Archean “microplate tectonics” to Paleoproterozoic “supercontinent tectonics” (or “microocean tectonics,” with regard to the limited size of the Red Sea–type oceans that were formed within the partly fractured supercontinent). The origin of Earth's first supercontinent, a landmass covering much of Earth's surface, by 2.80–2.76 Ga, should have played an extremely important role in restyling the system of convection cells in the underlying mantle. The style of tectonic processes and the geodynamic environment of plate tectonics in the Neoarchean–Paleoproterozoic differ from those in both the Archean and the Phanerozoic: the Archean tectonics of multiple “miniplates” was much more similar to Phanerozoic plate tectonics than to the Neoarchean–Paleoproterozoic “tectonics of supercontinents.”