Sveconorwegian extension‐parallel deformation structures; an example from the Hammarö Shear Zone, SW Sweden

Abstract

On the northern shore of Lake Vänern, southern Värmland, southwestern Sweden, various orthogneisses are affected by a major, steeply dipping shear zone, the Hammarö Shear Zone. It is several kilometres wide and comprises a network of individual shear zones (less than 100 m wide), which strike approximately E‐W across the Eastern Segment of the Southwest Scandinavian Domain in the Fennoscandian Shield, from the major Mylonite Zone in the west to the northeastern shore of Lake Vänern. Three fold phases, extensive dynamic and static recrystallisation, and a conspicuous, gently plunging lineation are the main features of the Hammarö Shear Zone. Early deformation occurred under amphibolite‐facies conditions, but narrow zones with a green‐schist‐facies assemblage demonstrate a lower grade of metamorphism during the late stages. Orthogneisses are variably overprinted by the shear deformation. No indisputable supracrustal rocks are found adjacent to the zone, and within the zone there is an extensive tectonic overprinting on primary and pre‐shear zone structures. The shear zone is intimately related to large‐scale regional folding (F4) which also occurs south of Lake Vänern. The steep mylonitic foliation and the horizontal stretching lineation are interpreted to be the result of the final stages of horizontal E‐W extension of the crust during the Mesoproterozoic D4 event. Berglund, J., Larson, S.Å. & Vinnefors, A., 1997: Sveconorwegian extension‐parallel deformation structures; an example from the Hammarö Shear Zone, SW Sweden. GFF, Vol. 119 (Pt. 2, June), pp. 169–180. Stockholm. ISSN 1103–5897.