Pinch-and-swell structure and shear zones in viscoplastic layers

Two-dimensional finite element simulations are presented for the extension of stiff viscoplastic layers embedded in a weaker viscous matrix. Layers and matrix exhibit power-law flow laws and the layers exhibit additionally a von Mises yield stress. The power-law flow law applies to rock deformation in the diffusion and dislocation creep regime and the von Mises plasticity to the low-temperature plasticity regime (e.g. dislocation glide). Simulations show that pinch-and-swell structure forms for small viscosity ratio (i.e. 10–20) and typical power-law stress exponents (i.e. 1–5). The pinches in layers with initial random geometrical perturbation form consecutively (i.e. not simultaneous). In multilayers, pinches on both the single-layer- and the multilayer-scale develop. Furthermore, shear zones develop due to the oblique linkage of pinches across the multilayer. These shear zones have a stable position, cause a normal drag geometry and exhibit significant displacement. The numerical results and the importance of low-temperature plasticity are supported by field observations, microstructural observations and EBSD orientation maps for pinch-and-swell structure in calcite veins. The presented models can explain strain localization by shear zone formation during bulk pure shear extension of viscoplastic multilayers without any material softening or feed-back mechanism (e.g. shear heating).