A correlation imaging velocimetry technique is applied to recover displacement fields in a granular material subjected to extended shear. A thick (10 cm) annular sand sample (grain size: 1 mm) is confined at constant pressure (σ=0.5MPa) against a rough moving wall displacing at very low speed (˙δ=83μms−1). Localization of the strain rapidly forms a shear band (seven particles wide) in which comminution develops. We focused on the strain field outside this shear band and observed a rich dynamics of large and intermittent mechanical clusters (up to 50 particles wide). Quantitative description of the radial velocity profile outside the shear band reveals an exponential decrease. However, a significant slip evolution of the associated characteristic length is observed, indicative of a slow decoupling between the shear band and the rest of the sample. This slow evolution is shown to be well described by power laws with the imposed slip, and has important implications for friction laws and earthquake physics.