We use a thermomechanical modeling approach to study the development of extensional gneiss domes in a thickened and thermally relaxed lithosphere. Our models consider a compositional and thermally dependent rheological lithosphere layering, with a 60-km-thick crust and Moho temperatures in the range 840–1040 °C. No discontinuity or detachment fault is assumed to preexist within the upper crust. However, to initiate localized deformation, a density anomaly is placed at the base of middle crust. Extension is applied to one model boundary at constant rates of 2.0 and 0.66 cm/yr. Models illustrate the progressive development of domes and associated strain patterns at crustal scale. Extension first localizes in the upper crust as a nearly symmetrical graben, allowing the underlying middle and lower ductile crust to rise up, initiating a dome. Dome amplification is further accommodated by convergent channel flow in the lower crust. Strain localization displays a complex pattern of shear zones at the crustal scale, first nearly symmetrical, and progressively becoming asymmetrical, giving, in particular, an upward convex detachment on one side of the dome. During extension, Moho geometry and depth vary as a function of boundary displacement rate. At the lower boundary displacement rate used in the calculations, the Moho remains rather flat and rises up at a constant rate. Results are discussed in the light of field examples and compared to previous models.