Mechanisms of shear zone deformation

The accommodation of displacement within discrete zones of ductile deformation is a feature of many metamorphic rocks. The mechanisms by which the deformation becomes concentrated within these shear zones, as opposed to being equally distributed throughout the rock, are the subject of this study. A model is presented based on the analogy of a brittle crack propagating in an elastic medium. This is then used to define the stress field around the tip of a propagating ductile shear zone. The patterns of stress, strain and the resulting stress release at the tip are examined. The consistency of the model is investigated by determining the ratio of driving stress to stress release at the tip. The model predicts that shear zones will only propagate in materials where the power (n) to which the stress is raised is < 3. Using this conclusion it is possible to determine the interrelationships between the temperature, propagation velocity, length and applied stress for the shear zone. The model is then developed to include displacement on the zone and the orientation of foliation around the tip. It is found that variations in propagation velocity are required to accommodate changes in the shear zone length and changes in rheology. The model is then applied to examples of shear zones from the Lewisian and estimates of propagation velocity and applied stress are obtained for specific shear zones. Conclusions from the model regarding Increased displacement associated with higher temperatures are confirmed from the fieldwork, and it is concluded that the displacement/width ratio is a function of the local P/T conditions. It is proposed that the majority of shear zones develop by propagation but an element of localization is necessary in the initial stages. The nature of this nucleation event is of importance in determining the final dimensions of the shear zone