Patterns of strain localization in heterogeneous, polycrystalline rocks – a numerical perspective

The spatial and temporal patterns of strain localization in materials with pre-existing heterogeneities are investigated via a series of two-dimensional numerical models. Models include (i) a dynamic feedback process, to simulate rheological weakening in response to the transition from non-linear flow (dislocation creep) to linear flow (diffusion creep/grain boundary sliding), and (ii) a time dependent strengthening process, counteracting the weakening process. Different load bearing framework geometries with 20% weak component are used to evaluate the impact of geometry on the strength of the material and its ability to localize strain into an interconnected weak layer (IWL). Our results highlight that during simple shear, if dynamic weakening with or without strengthening feedbacks is present, strain is quickly localized into an IWL, where an increasing proportion of weak material increases the interconnections between the IWLs, thereby increasing the anastomosing character of the shear zones. We establish that not only bulk strain localization patterns but also their temporal patterns are sensitive to the dominance of the weakening or strengthening process. Consequently, shear zones are dynamic in time and space within a single deformation event. Hence, the pattern of finite strain can be an incomplete representation of the evolution of a shear zone network.