⁴⁰ Ar/ ³⁹ Ar geochronological constraints on the tectonothermal evolution of the Eastern Segment of the Sveconorwegian Orogen, south-central Sweden

Abstract

Abstract A 40 Ar/ 39 Ar study to constrain the tectonothermal evolution across the Eastern Segment of the Sveconorwegian Orogen has been initiated in the area north and east of lake Vänern, south-central Sweden. This segment of the orogen is confined by two major deformation zones, the Sveconorwegian Frontal Deformation Zone (SFDZ) in the east and the Mylonite Zone in the west. Previous structural work and the prograde character of the metamorphism within the study area suggest that an older (< c. 1.57 Ga), regional foliation was formed by ductile shear deformation in a compressional tectonic regime. The orientation of this foliation was subsequently modified by later rotation along younger ductile shear zones in the easternmost, frontal part of the orogen (SFDZ). The 40 Ar/ 39 Ar ages for hornblende suggest that the regional foliation is Sveconorwegian. Furthermore, white mica ages demonstrate that the Sveconorwegian tectonothermal overprint continues at least 40 km east of the traditionally accepted limit situated along the ‘Protogine Zone’. These results also provide age constraints for different phases of Sveconorwegian tectonothermal evolution with an older group of ages from 1009–965 Ma and a younger set from 930–905 Ma. The older ages are inferred to constrain a minimum age for crustal thickening during which the regional foliation and metamorphism developed, while the younger are associated with later compressional movement along the SFDZ.