The Sveconorwegian orogenic belt, a case of late-proterozoic plate-collision

Abstract

A geotetctonic model is proposed for the Sveconorwegian orogeny (1200-850 Ma), based on compilation of lithological and structural evidence. This analysis suggests a N—S trending, asymmetrical orogenic belt with longitudinal zonation. In the western-most core zone, synkinematic intrusions dominate, while the central zone consists primarily of reworked basement with subordinate intrusions and metasediments. In the easternmost marginal zone, series of pre-Sveconorwegian mega-units are separated by thrust zones and finally bordered by the Sveconorwegian frontal thrust towards the east. Graben formation in the Telemark area preserves the deposits of previous epicratonic stages and marks the initiation of Sveconorwegian activities at about 1190 Ma. The subsequent orogenic development, associated with lithospheric subduction, shows tight folding about N—S axes with steep axial planes in the west, and more open NW—SE folding with gentle westward dipping axial planes towards the east. Regional metamorphism of low-P, high-T conditions imply steep geothermal gradients and suggests hercynotype crustal evolution, in agreement with a Cordilleran-type plate collision. In the Bamble district, important structural alignment forms a zone of crustal anisotropy which was repeatedly reactivated and ultimately became the site for Permian rifting forming the Oslo Graben. The lithological and structural characters as well as the crustal evolution for the Sveconorwegian orogeny show strong similarities with the Grenville development in southeastern Canada. Structural vergence and orogenic zonation suggest that the Sveconorwegian front and the Grenville front are equivalent and form different parts of a single orogenic belt, permitting a tentative reconstruction of the Late-Proterozoic, North-Atlantic Shield.