To explain the lack of magnetization in large portions of the Noachian crust, a lithological origin has been put forward, corresponding to the lack of formation of magnetic minerals in the crustal rocks. An alternative hypothesis is proposed here, with constant amount of magnetic minerals, but variable reduction of total magnetization over the magnetic thickness, due to variable record of successive polarities of the magnetizing field. Large crustal cooling rates would correspond to strongly magnetized zone, and slow cooling rates to poorly magnetized zone. Based on this model we derive an explanation for the proposed confinement of magnetized zones within the inter-tropical band.