Abstract We have studied the crustal structures of the Palaeoproterozoic Svecofennian ( c. 1.9 Ga) Orogeny with the help of large scale seismic reflection surveys (FIRE 1–3), preliminary structural field work and geological and geophysical databases. The central part of the orogen is occupied by the Central Finland Granitoid Complex, which comprises two suites of granitoid rocks and associated mafic and volcanic rocks. The complex and the surrounding supracrustal belts are cut and deformed by numerous shear zones and faults; here divided into six groups. The most prominent reflections are usually shear zones or faults on outcrops. The granitoid complex is interpreted as a deep, lower-level section of an old core complex, where the younger granitoid intrusions form the basins and older granitoid intrusions and associated volcanic rocks form the horsts. The upper–middle crust detachment zone is exposed at the northeastern edge of the complex and middle crust is exposed in the migmatitic domes at northern and western margins. The seismic reflection sections display a frozen image of orogenic thickening and lateral spreading. The decoupling of the upper, middle and lower crust during spreading resulted in the formation of layered superstructure–infrastructure of the crust.

Abstract Accretionary processes contributed to major continental growth in Fennoscandia during the Palaeoproterozoic, mainly from 2.1 to 1.8 Ga. The composite Svecofennian orogen covers c . 1×10 6 km 2 and comprises the Lapland–Savo, Fennia, Svecobaltic and Nordic orogens. It is a collage of 2.1–2.0 Ga microcontinents and 2.02–1.82 Ga island arcs attached to the Archaean Karelian craton between 1.92 and 1.79 Ga. Andean-type vertical magmatic additions, especially at c . 1.89 and c . 1.8 Ga, were also important in the continental growth. The Palaeoproterozoic crust is the end product of accretionary growth, continental collision and orogenic collapse. Preserved accretional sections are found in areas where docking of rigid blocks has prevented further shortening. The Pirkanmaa belt represents a composite accretionary prism, and other preserved palaeosubduction zones are identified in the Gulf of Bothnia and the Baltic Sea areas. In the southern segment of the Lapland–Savo orogen collision between the Archaean continent (lower plate) and the Palaeoproterozoic arc–microcontinent assembly (upper plate) produced a special type of lateral crustal growth: the Archaean continental edge decoupled from its mantle during initial collision and overrode the arc and its mantle during continued collision.

Deep seismic-reflection data from the BABEL and FIRE profiles across the Fennoscandian Shield image Svecofennian crust that is made up of a collage of terranes. The data suggest sequential accretion of island arcs and microcontinents to the Karelian craton (1.9–1.8 Ga). These accretionary events may have caused temporary changes to arc geometries, accretionary episodes or collisional phases, and westward growth of the continent. The accreted terranes experienced gravitational collapse that stabilized the crust and exhumed the medium- to high-grade rocks to their present positions. The structures froze after isostatic balance was achieved and thus have been protected from later tectonic deformation. The accretionary growth period was terminated by continent-continent collision, after which it was possible for the Wilson cycle to operate at the margin of the newly formed continent. As in modern accretionary systems, several tectonic environments are found. These include paleosubduction, obduction, continental transforms, collision of hot and cold terranes, and collapse of hot and cold collisions, which are all supportive of plate tectonics operating in the Paleoproterozoic. From the different collision zones, the following tectonic units can be recognized: hinterland-foreland fold-and-thrust belts, metamorphic cores, accreted arcs and basins, and foreland fold-and-thrust belts. The metamorphic cores are associated with granitoid complexes and/or core complexes. The platetectonic theory together with gravitational balancing is a viable model to explain the evolution of the Svecofennian orogen.

Abstract Based on an integrated study of geological and geophysical data, a tectonic model for the Palaeoproterozoic evolution of the Svecofennian orogen within the Fennoscandian Shield at the northwestern corner of the East European Craton is proposed. The Svecofennian orogen is suggested to have formed during five, partly overlapping, orogenies: Lapland-Savo, Lapland-Kola, Fennian, Nordic and Svecobaltic. The Svecofennian orogen evolved in four major stages, involving microcontinent accretion (1.92–1.88 Ga), large-scale extension of the accreted crust (1.87–1.84 Ga), continent-continent collision (1.87–1.79 Ga) and finally gravitational collapse (1.79 and 1.77 Ga). The stages partly overlapped in time and space, as different processes operated simultaneously in different parts of the plates. In the Lapland-Savo and Fennian orogenies, microcontinents (suspect terranes) and island arcs were accreted to the Karelian microcontinent, which itself was accreting to Laurentia in the Lapland-Kola orogeny. The formation of the Svecofennian orogen was finalized in two continental collisions producing the Nordic orogen in the west (Fennoscandia-Amazonia) and Svecobaltic orogen in the SSW (Fennoscandia-Sarmatia). The collisions were immediately followed by gravitational collapse.