SVECONORWEGIAN EXTENSION-PARALLEL DEFORMATION STRUCTURES - AN EXAMPLEFROM THE HAMMARO SHEAR ZONE, SW SWEDEN

On the northern shore of Lake Vanern, southern Varmland, southwestern Sweden, various orthogneisses are affected by a major, steeply dipping shear zone, the Hammaro 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 Sout hwest Scandinavian Domain in the Fennoscandian Shield, from the major Mylonite Zone in the west to the northeastern shore of Lake Vanern. Th ree fold phases, extensive dynamic and static recrystallisation, and a conspicuous, gently plunging lineation are the main features of the H ammaro Shear Zone. Early deformation occurred under amphibolite-facies conditions, but narrow zones with a greenschist-facies assemblage dem onstrate a lower grade of metamorphism during the late stages. Orthogn eisses are variably overprinted by the shear deformation. No indisputa ble supracrustal rocks are found adjacent to the zone, and within the zone there is an extensive tectonic overprinting on primary and pre-sh ear zone structures. The shear zone is intimately related to large-sca le regional folding (F-4) which also occurs south of Lake Vanern. The steep mylonitic foliation and the horizontal stretching lineation are interpreted to be the result of the final stages of horizontal E-W ext ension of the crust during the Mesoproterozoic D-4 event.