Accretion and lateral extension in an orogenic wedge: evidence from a segment of the Seve-Ko¨li terrane boundary, central Scandinavian Caledonides

The Seve-Ko¨li boundary in the central Scandinavian Caledonides represents the transition from Baltica continental margin-related rocks (Seve) to outboard, westerly derived terranes (Ko¨li). In this study we present and discuss structural and kinematic data recording the syn- and post-accretionary evolution of this boundary in west central Sweden. olite-facies thrusting, mylonitization, imbrication and folding occurred before and during the establishment of the Seve-Ko¨li boundary. Subsequent asymmetric lateral extension considerably modified earlier structures and the boundary at lower grade. Contractional ramps and extensional synforms developed and translation continued during progressive deformation. tic analysis indicates dominant movement towards the east or southeast. Local movement in the opposite direction may be due to stretching and rotation in the hanging wall above extensional shear zones. A pattern of minor shear zones and shear bands demonstrate stretching in an approximately E-W direction. esence of extensional structures developed during top-to-the-east movement suggests alternative mechanisms for the formation of some characteristic Caledonian structures e.g. late synforms and thin lenticular tectonic units. These features may be due to asymmetric extension during Scandian top-to-the-east motion of the nappe pile. Contemporaneously, basement antiforms may have been stacked at lower tectonic levels.