The properties of shear zones forming in slow three-dimensional granular flow are investigated. We simulate a straight version of the split-bottom shear cell. It is shown that the same type of wide shear zones is obtained in the presence as well as in the absence of gravity. We investigate the relaxation of the material toward stationary flow and analyze the stress and the velocity fields. A functional form of the widening of the shear zone inside the bulk is given. We discuss the growth of the region where the material is in the critical state. The growth of this critical zone turns out to be responsible for the initial transient of the shear zone.