At elevated stress levels, the deformation of granular assemblies has the tendency to localize in narrow shear bands. Recent research efforts within the granular material community are focusing on the morphology of deformation patterns within the bands. Understanding of the deformation mechanisms is a prerequisite for the quantification of energy dissipation, hardening and softening in the post-localization range. Despite significant experimental research efforts, a clear understanding of the flow properties and constitutive behaviours within the shear band is still elusive. This could mainly be attributed to the fact that shear bands are thin objects, making a detailed characterization of the particles' behaviour and their interactions within shear bands difficult. As a numerical method, the Discrete Element Method (DEM) has demonstrated its ability to address this problem, owing to its intrinsic characteristic of tracing particle behaviour. This paper presents the results of application of DEM to model two-dimensional, densely packed, cohesionless, polydisperse granular assemblies under simple shear. Meso-scale kinematical information such as displacement field is evaluated. Within the shear band, the deformation field displays a vortex-like structure, and such vortex structures are accompanied by strong pore space production within the shear band. Analysis of the history of force chains inside the localized shear zone reveals that the formation of vortex structures is related to the kinematical evolution of a cycle encompassing buildup, buckling and collapse of the force chains.