Cracking of viscoplastic material, which cools down in the conditions of plane strain, and formation of channels through which melt is transported in the heated mantle are studied by numerical simulation. The pattern of deformations in the cooling mantle suggests that bifurcation of the deformation process leads to the formation of zones characterized by rather strong decompaction, in which plastic deformations are localized. The orientation of the zones is controlled by the preceding strain state, which binds them to the direction of the maximum initial compression. Heating and partial melting of the mantle masses perturb the stress state and also lead to the formation of the localization zones. These weakened zones may act as the channels for melt filtration in the mantle.