Two of the most important segments of the west European Rift, the Rhinegraben and the Massif Central grabens, show in plan and in cross section a very different crustal structure. The Rhinegraben and the Massif Central grabens are roughly parallel and formed in the same time interval (i.e., Priabonian/Oligocene). The Rhinegraben in the north is a single half graben of ~35 km wide that resulted from the activity of a major detachment fault running from the surface to a MOHO discontinuity at depth. The Massif Central in the south is composed of two lateral half garbens, similar to the Rhinegraben, and a near-symmetric central graben. The opposing detachment faults of the two lateral half grabens reveal a striking mirror symmetry on either side of the symmetric central graben. Experiments have been conducted to explain mechanically this contrasted structural evolution in the same rift system. It is shown that this difference may be attributed to (1) the number of ruptures in the brittle part of the mantle lithosphere and (2) variations in the extension rate. A single rupture in the brittle mantle lithosphere results in either a single half graben or a pair of half and symmetric grabens in low and high extension rate experiments, respectively. Two ruptures in the brittle mantle lithosphere result in two half grabens in low rate experiments. The number of grabens is dependent upon the distance between the two ruptures in the brittle mantle lithosphere in high extension rate experiments and may vary from 2 to 4. Estimation of the extension rate in the Rhinegraben and the Massif Central matches experimental results and shows that the strength ratio between lower and brittle crust is the key parameter in determining the graben geometry. ¿ 2000 American Geophysical Union