The Bergslagen ore province, Sweden: Review and update of an accreted orocline, 1.9–1.8 Ga BP

Abstract

We have revisited and re-interpreted deformation structures in the classical ore province of west Bergslagen, part of the ∼1.9–1.8Ga old Svecofennian orogen in south-central Sweden. Four successive phases and styles of deformation reflect (1) continental rifting associated with bimodal magmatism in continental back-arc basins (D1), (2) closure of the rift(s) by upright tight to isoclinal folds on any scale (D2), (3) renewed rifting (D3) with deposition of argillites, conglomerates and submarine basalts in restricted rift basins, and (4) intense buckling and folding of west Bergslagen along variably but generally steeply plunging folds (D4). Strain localization during D4 initiated a network of conjugate retrogressive shear zones and closely related large scale vertical drag folds, with the southwest–northeast striking West Bergslagen Boundary Zone system acting as the main tectonic boundary along which west Bergslagen slipped relative to the central part of the province. D4 transpression and variably oblique D4 shear may have caused the preservation of a low grade domain in west Bergslagen as well as the exhumation of a granulite facies domain near the hinge of the orocline. D4 structures include a regional scale oroclinal bend of 135° in southwest Bergslagen, connecting common southwest–northeast strikes in west Bergslagen to east–west strikes to the south and east. In central Bergslagen, the oroclinal bend tightens to 180°. To the south, it lines up with another orocline, the two forming an S-shaped pair of large-scale buckle folds. The orocline finds expression in the regional patterns of form lines and magnetic anomalies, and in the spatial distribution of specific intrusive rock suites. The northern structural boundary of the province is known as an east–west striking zone of dip slip movement and local wrenching, which we also interpret as a D4 effect. The D4 structures imply the evolution of a separate ribbon-like Bergslagen micro-continent, at least 700km long, rifted from the former active continental margin during the D3 event, and finally accreted, slightly obliquely, to pre-existing Fennoscandia to the present north. By correlating D4 deformation structures with the areal distribution and previously determined radiometric ages of ‘late orogenic’ anatectic granites, we derive a provisional estimate, 1.83–1.82Ga, as the lower age limit to the initiation of terrane accretion. D4 accretion is Svecofennian orogeny proper in this part of the belt, younger than the currently preferred estimate of ∼1.86Ga. West Bergslagen's synsedimentary hydrothermal ore deposits, originating from 1.91 to 1.89Ga old volcanism and rifting, have been remobilized to various extent during the ‘late-orogenic’ D4 phase, and show pronounced concentration along D4 shear zones.