The Martian surface composition appears mainly mafic but recent observations have revealed the presence of differentiated rocks, only in the Highlands. Here, we demonstrate that differentiated melts can form during the construction of thick crustal regions on Mars by fractional crystallisation of a mafic protolith, without plate tectonics. On a stagnant-lid planet, regions of thicker crusts contain more heat-producing elements and are associated to thinner lithospheres and to higher mantle melt fractions. This induces larger crustal extraction rates where the crust is thicker. This positive feedback mechanism is favoured at large wavelengths and can explain the formation of the Martian dichotomy. We further develop an asymmetric parameterised thermal evolution model accounting for crustal extraction, where the well-mixed convective mantle is topped by two lithospheres (North/South) characterised by specific thermal and crustal structures. We use this model in a Bayesian inversion to investigate the conditions that allow crustal temperatures to be maintained above the basalt solidus during crustal growth, resulting in the formation of evolved melts. Among the thermal evolution models matching constraints on the structure of the Martian crust and mantle provided by the InSight NASA mission, a non-negligible fraction allows partial melting and differentiation of the crust in the south, which can occur very early (<100 Myr) as well as during the Hesperian ; partial melting in the north appears unlikely. Although crustal differentiation may occur on a hemispheric scale on Mars, its vertical extent is limited to less than a third of the crustal thickness.