We develop a thermal-mechanical model for describing the formation of shear zones. We consider shear deformation of a two-dimensional rectangular region composed of a viscous fluid under a constant velocity at the boundary. Viscosity of the material is assumed to depend only on temperature for simplicity. In order to enhance the shear deformation, we included a small inclusion whose viscosity differs from that of the surrounding material. We carried out time-marching simulations and monitored the evolution of temperature and strain around the inclusion. Our results show that the deformation localizes in a narrow region when sufficient heat is generated by viscous dissipation. In the zone of localized deformation, temperature increases by several hundred degrees owing to strong dissipative heating. We found that the zone of localized deformation develops in the region where the greatest heat is generated at the initial stage of evolution. The zone of localized deformation pierces the inclusion when the inclusion is weaker than the surrounding material, while it develops away from the inclusion when the inclusion is stiffer than the surrounding material. These findings, though qualitatively, suggest that heating by viscous dissipation may play an important role in the formation of decollements around subducted seamounts.