Superimposed fabrics due to reversal of shear sense: an example from the Bergen Arc shear zone, western Norway

Abstract

A profile through the upper margin of a large scale ductile shear zone (the Bergen Arc Shear Zone, western Norway) has been studied in detail. Within the shear zone, asymmetric structures indicate dextral or top-to-NW movement, which is interpreted as representing late or post-Caledonian extensional tectonics. The hanging-wall of the shear zone is characterised by kinematic indicators, such as shear bands associated with curved or inclined foliation and asymmetric boudins, indicating sinistral or top-to-SE movements. These are interpreted as related to thrusting during the Caledonian Orogeny. The studied profile shows the progressive overprinting of the sinistral structures by the later dextral shear. This first led to a decrease in the obtuse angle between shear bands and the curved/inclined foliation. Subsequently, backfolding of asymmetric boudins in quartz veins occurred, and a composite cleavage developed from shear bands and associated curved/inclined foliation. Dextral asymmetric extensional shear bands were developed at a later stage when the folded quartz veins had attained tight to isoclinal geometries.