We present experiments and numerical simulations dealing with the growth of faults in thin brittle/ductile systems to understand deformation modes in the continental lithosphere. Experiments were uniaxial shortening of layers of dry sand and silicone putties of various viscous resistances. For large strength ratios between the brittle and ductile layers (R>5--10), the deformation localizes into two shear bands; the fault pattern is created before reaching 10% shortening, and has fractal dimensions varying between 1.6 and 1.8. For small strength ratios (R<5--10), deformation never localizes; the fault pattern is homogeneous with a trivial dimension of 2, and grows continuously during deformation. The transition between localized and homogeneous deformation occurs when the mechanical resistance of brittle layers is 5--10 times larger than the resistance of ductile layers. This transition was also investigated by means of electrical analog simulations. A fuse network, which represents an elasto-brittle layer, is coupled with a capacitor layer which models strain-rate dependent fluids. An AC potential is applied and the fuses progressively burned out until they form a connected network. The AC-potential frequency, f, is a tuning parameter similar to the applied strain rate in experiments. A critical frequency is obtained marking a transition between a localization mode where the density of burned fuses decreases as the system size increases, and a delocalization mode where the density of burned fuses remains constant with increasing system size. The scaling dependency of the fracture process, as well as the critical frequency, are consistent with experimental results. Available information on the rheology of the continental lithosphere shows that this mechanical transition is bracketed by the possible range of brittle-to-ductile strength ratios. ¿ American Geophysical Union 1995