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

•Shear zones are difficult to form without a dynamic weakening mechanism.•Dynamic weakening can quickly localize strain into an interconnected weak zone.•Competing weakening and strengthening processes impact shear zone pattern.•Shear zones are dynamic in time and space within a single deformation event.•Finite strain patterns do not fully represent the evolution of a shear zone network.