Strain partitioning in ductile shear zones: an example from a Lower Pennine nappe of Switzerland

Abstract The rocks of the Maggia nappe, a Lower Pennine nappe of the Swiss Alps, were deformed in at least three phases during Alpine orogeny. Hercynian granites in the core of the nappe developed ductile shear zones during the early phases of deformation, but between shear zones they remained almost undeformed. Both heterogeneous simple shear and heterogeneous volume change played an important role in the formation of the ductile shear zones. Partitioning of different strain components indicates a volume loss of nearly 80–90% towards the centres of some of the shear zones. Such a large amount of volume loss is also corroborated by strain analysis of deformed xenoliths in other shear zones of the same region. A comparative study of theoretical strain paths and the natural results indicates constant shear strain rate but changing dilation rate during deformation.