Finite-element modelling of shear zone development in viscoelastic materials and its implications for localisation of partial melting

Abstract

The development of shear zones initiating on random weaker initial perturbations is modelled numerically for low Deborah number viscoelastic materials, considering the influence of effective viscosity contrast, power law rheology, strain softening, and different imposed bulk deformation geometries, ranging from pure to simple shear. Conjugate shear zones initiate at ∼90° to one another, and rotate with increasing bulk deformation, the basic pattern not being markedly influenced by the vorticity of imposed deformation. The rate of propagation of individual conjugate shear zones is little affected by increased effective viscosity contrast between matrix and inclusion but is promoted by power-law rheology. However, the most marked effect is observed for strain softening behaviour, where rapid propagation produces straighter and narrower shear zones. The localisation of strain is reflected in a correspondingly heterogeneous stress distribution. In particular, mean stress or pressure is higher in the extending, near planar, weaker zones of localised shear. Melting of gneissic or pelitic compositions is pressure dependent. With free water present, increased pressure promotes melting, whereas the opposite is true for water-absent melting. For water-present conditions, a positive feedback could develop between localised shearing, increased pressure and partial melting. This is potentially more effective in concentrating melt in shear zones than shear heating, where melt-related softening has a negative feedback effect.