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Abstract

Caledonian rocks form a north-south-trending orogenic belt along the western edge of Arctic Scandinavia and an east-west-trending belt in northernmost Norway. The change in direction of the belt separates two distinct regions of Caledonian geology.

Structurally, the two regions are similar. Both consist of a geosynclinal sequence broken into three distinct tectonic units, thrust easterly in the southern region and southeasterly in the northern region, over the autochthonous cover of the Archean Fennoscandian shield. The lower two units contain sequences similar to that of the autochthon; the sediments of the middle unit are mildly metamorphosed and overlie imbricated slices of basement. The upper unit contains highly metamorphosed geosynclinal sequences. In the south, this unit has been imbricated into a series of nappes. The eastern leading edges (in Sweden) are disjunctive on a brittle basement, whereas in the west (Norway) the trailing edges were drawn out when the rocks were hot and semiplastic; thus they thin progressively in a conjunctive, pseudo-autochthonous sequence deformed together with the “caledonized” basement.

Polyphase deformation and metamorphism is evident throughout the belt. The nappe folds and the initiation of thrusting developed during the first phase. Large quantities of differentiated mafic and ultramafic material, related to a geophysically detected zone of upthrust mantle rocks, were intruded along the western seaboard during the interval of maximum metamorphism between the first and second phases of deformation. The main transport of the nappes occurred during the second phase and was accompanied by intense flattening. The third and following phases deformed the thrust surfaces.

Historically, the two areas are distinct. Upper Precambrian to Lower Ordovician rocks form the autochthonous cover and the two lower tectonic units of both areas, although the northern rocks are much thicker (4,000–5,000 m and possibly 14,500 m). The geosynclinal sequences of the upper unit are composed of Ordovician to Silurian rocks containing thick volcanic sections in the south, but are composed of upper Precambrian to Upper Cambrian rocks in the north. The main period of metamorphism and the development of nappe folds occurred during the latest Silurian (396 ± 48 m.y.) in the south but during the latest Cambrian (530 ± 35 m.y.) in the north. The dia-chronous nature of the orogenic phases is interpreted as a migration along the continental margin of the point of collision of the lithospheric plate with the subduction zone.

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