The role of underplated mafic bodies (UPMB) in the localization of deformation is examined using a two-dimensional thermo-mechanical finite element model. Rheological heterogeneity brought about by the UPMB is linked with the following two main physical effects: the UPMB material is assumed to have (1) an anomalous high temperature and (2) a mafic crustal rock composition that is intrinsically weaker than the mantle. The thermal effect will disappear rather quickly, but the rock composition can have an effect at any stage of extension. We show that the UPMB has a strong influence on the style of lithospheric extension, which depends on the thermal condition of the uppermost mantle at the time when the UPMB is emplaced. Since the strength contrast between the weakened and non-weakened regions is the most important factor, the UPMB works more efficiently on the localization of deformation for a colder uppermost mantle. Dependence of the localization of deformation on the temperature and thickness of the UPMB (i.e. more significant localization for the UPMB with higher temperature and greater thickness) also depends on the thermal condition of the uppermost mantle. However, the width of the UPMB has a strong influence on the localization for any thermal condition of the uppermost mantle; a greater amount of thinning is distributed into a narrower weakened region. Such a model behaviour implies that various styles of lithospheric extension, including inward or outward migration of deformation and asymmetric extension, can be simply obtained by considering the emplacement of the UPMB, which also plays an important role in controlling the onset of continental break-up. © 2008 Elsevier B.V. All rights reserved.