Laboratory experiments on detachment systems in an extending brittle-ductile crust showed that the rise of a domal core complex in the ductile layer required that shear be localized above a soft inclusion in a laterally inhomogeneous ductile layer. Gravity spreading of models consisting of laterally homogeneous layers of sand over ductile silicone resulted in homogeneous patterns of tilted blocks. Systematic and reproducible patterns of our models simulate features classically observed in natural detachment systems. The extensional system is composed principally of a main detachment fault with a convex-upward shape and at least one listric accommodation fault, which facilitates progressive steepening of the footwall during the rise of the core complex. All faults begin as steeply dipping normal faults that rotate to low dips during extension.