The accretionary Svecofennian orogen—insight from the BABEL profiles

Abstract

The BABEL profiles B, C, 1–7 form a 1200 km long nearly continuous cross-section through the Svecofennides. The near-vertical marine reflection profiles display a wide range of crustal structures that can be associated with both the accretionary Svecofennian orogeny (1.96–1.75 Ga) and the following Subjotnian and Jotnian rift-stages (1.65–1.11 Ga). The Svecofennian accretionary orogeny took place when a number of micro-plates with island arc affinities and surface expression of a large archipelago accreted to the continental Karelian plate. Some of the accreting terranes seem to have had older cores that have acted as crustal indentors during the collision. BABEL profiles 3 and 4 image a series of collisional terrane boundaries between Karelian continental margin, Savo arc (SA) and Central-Finland arc (CFA). In the north, the Karelian margin has been both over- and underthrust by the Savo arc. CFA comprises a folded thrust package on top of a continental nucleus and the Savo arc. The associated subduction zone and accretionary prism are interpreted to lie to the south, underneath the Bothnian basin area, where prominent NE-dipping, lower to middle crustal reflections are found along BABEL profiles 1 and 4. An oblique collision of the Central Finland arc and the continent resulted in the development of the strike-slip fault on the young, hot Savo arc. BABEL profiles 1, 6, 7 and C image the internal architecture of the Southern and Central Finland arcs. The unusually thick crust (55–60 km) hosts unreflective, high density, mafic intrusions acting as a crustal indentor. A highly reflective antiform structure developed on the southern side of it. Southern Finland arc complex (SFA) is an imbrication structure comprising stacked slices of arc-related crust on an older continental nucleus, Bergslagen nucleus. Prior to the collision, the SFA suffered from gravitational collapse during which the crust was thinned. Profile B images the architecture of the Central and Southern Swedish Svecofennides. The Sörmland terrane (SöB) is interpreted as the accretionary prism of the Southern Finland arc squeezed between the Svecofennian collage in the north and the advancing continent to the south. The southern continent/island arc is characterised by NE dipping crustal reflections and Moho offsets as well as step-wise increasing thickness of the crust. After the final collision, large volumes of mantle-derived material intruded the crust as large mushroom-shaped plutons. They are interpreted as the heat source for the TIB magmatism in southern and western Scandinavia.