Early proterozoic deposition and deformation at the karelian craton margin in southeastern Finland

Abstract

The margin of the Karelian craton underwent complex, though not chaotic, deformation during the Svecokarelian orogeny. The resultant structural sequence can be subdivided into an early phase that involved translation of thrust nappes and recumbent folds verging towards the craton, and a younger phase having more zonal and upright structures. These latter include NW-trending ductile shear zones associated with widespread pluton emplacement and later N-trending faults and shear zones having substantially vertical displacements. These features are interpreted in terms of a relative increase in vertical crustal stresses resulting from gravitational and thermal disequilibrium in the overthrust Archaean basement. Despite this structural complexity, two distinct depositional provinces are recognized in North Karelia, separated by a major thrust zone. The more easterly Höytiäinen province is a narrow, essentially intracratonic feature containing locally derived Archaean and Proterozoic detritus. An early phase of chemogenic and pelitic deposition with sporadic tholeiitic volcanism was followed by an influx of coarse clastic detritus, largely deposited from turbidity currents within a prograding submarine fan. The province apparently records deposition in a transtensional en echelon system of basins initiated near the craton margin at about 2.1 Ga. In contrast, the Savo province is largely allochthonous and includes the distinctive Outokumpu assemblage commonly interpreted as a remnant of lower Proterozoic oceanic crust. Provenance for the bulk of the overlying metasediments remains problematical although local evidence for deposition on Archaean basement exists. If an accretionary margin context is considered, several potential niches for Savo province deposition can be found that appear to satisfy constraints imposed by both isotopic data relating to provenance and the sequence and geometry of Svecokarelian deformation.