SHEAR ZONE STRUCTURES IN THE OYGARDEN AREA, WEST NORWAY

Investigation of shear-related sheath-like folds from blastomylonitic gneisses in the Øygarden Complex, Norway, shows that the folds have not initiated with fold axes systematically normal to or with another fixed angle to the lineation. Their short limbs do, however, pass from a period of thickening to final thinning as the folds close and the short limbs become inverted. The formation of the folds appears to be controlled by local geometries within the shear zone, and we suggest three fold-generating mechanisms. In one, the mylonitic foliation curves ahead of a tectonic lens and starts to fold and in another, mesoscopic shear bands with normal fault geometries cut subplanar mylonites and folds are generated by the accretion of material in the evolving gap between the "hangingwall" and the "footwall". The third mechanism involves perturbation of the main foliation by slip along an inclined S-foliation so that shear folds may develop. All the mechanisms involve a rotation of the mylonitic foliation into the compressional field of the instantaneous strain ellipse. © 1990.