Influence of initial preferred orientations on strain localisation and fold patterns in non-linear viscous anisotropic materials

Deformation localisation can lead to a variety of structures, such as shear zones and bands that range from grain to crustal scale, from discrete zones to anastomosing networks, and shear zone related folds. We present numerical simulations of the deformation of an intrinsically anisotropic material with a single maximum crystal preferred orientation (CPO) in simple shear. We use the Viscoplastic Full-Field Transform (VPFFT) crystal plasticity code coupled with the modelling platform ELLE to achieve very high strains. The VPFFT-approach simulates deformation by dislocation glide, taking into account the different available slip systems and their critical resolved shear stresses. We vary the anisotropy of the material from isotropic to highly anisotropic, as well as the orientation of the initial CPO. To visualize deformation structures, we use passive markers, for which we also systematically vary the initial orientation. At low strains the amount of strain rate localisation and resulting deformation structures depend on the initial CPO in all anisotropic models. Three regimes can be recognised: distributed shear localisation, synthetic shear bands and antithetic shear bands. However, at very high strains localisation behaviour always tends to converge to a similar state, independent of the initial CPO. Shear localisation is often detected by folded layers, which may be parallel to the anisotropy (e.g. cleavage formed by aligned mica), or the deformation of passive layering, such as original sedimentary layers. The resulting fold patterns vary strongly, depending on the original layer orientation. This can result in misleading structures that seem to indicate the opposite sense of shear.