Many of the most important and commonly occurring, emergent geological structures such as shear bands, fault zones and folds may be understood as a consequence of changes in the type of the governing model equations. Such changes, or bifurcations, depend strongly on the details of the constitutive relationships including the presence of strain softening, thermal or chemical effects, and whether the flow rule is associated or non-associated, coaxial or non-coaxial. Here we focus on the influence of non-coaxiality of the flow rule on the development and evolution of shear bands. The term non-coaxial refers to the non-coincidence of the principal axes of the stress and of the plastic strain rate tensor. Non coaxial plasticity models were originally proposed by Josselin de Jong and A. J. M. Spencer.