The Grenvillian–Sveconorwegian orogeny in Fennoscandia: Back-thrusting and extensional shearing along the “Mylonite Zone”

Abstract

New structural and geochronological investigations of the “Mylonite Zone” (MZ), an arcuate terrane boundary in southwest Scandinavia, contribute to a refined conceptual model for the Grenvillian–Sveconorwegian tectonic evolution of the Mesoproterozoic Sveconorwegian orogenic belt. During late convergence, around 970 Ma, the MZ acted as a top-to-the-SE thrust that accommodated crustal shortening in the eastern part of the orogen by juxtaposing the Idefjorden Terrane in the hanging wall in the west against the “Eastern Segment” footwall in the east. The eastward vergence of the MZ and of similarly oriented second-order nearby shear zones is interpreted as reflecting late back-thrusting within the overall W-vergent orogeny. Back-thrusting was possibly promoted by the backstop role played by the rigid block formed by the 1810–1650 Ma Transscandinavian Igneous Belt. During subsequent E–W crustal extension, the MZ thrust-related fabrics were reactivated in an extensional fashion with bulk top-to-the-W kinematics. This was triggered by gravitational instabilities resulting from crustal overthickening during the shortening phase. 40Ar- 39Ar biotite and white mica ages from a greenschist-facies and extension-related mylonite range between 922 and 860 Ma. This long-lived episode is expressed by extensional structures that evolved continuously from purely ductile to brittle during progressive exhumation of the footwall. The “Eastern Segment” is interpreted as an immature asymmetric core complex, exhumed in the footwall of the extensional MZ, through the antithetic normal displacement of the MZ itself and of the top-to-the-E Sveconorwegian Frontal Deformation Zone farther to the east. The core complex is bound to the north by the transtensional Hammarö Shear Zone, characterized by penetrative constrictional fabrics, interpreted as indicative of an overall transtensional regime.