Using geophysical observations and some basic geological ideas on the present structure of the Ventura basin, we model its possible mechanical evolution during the last 4 million years. We use a two-dimensional finite element model with frictional and elastoviscoplastic rheologies that allow strain localization. This allows us to study the mechanical origin of the San Cayetano thrust fault (SCF) and its interaction with the Oak Ridge fault (ORF). The results of our modeling indicate that the geometry and activity of a shear zone which mimics the San Cayetano fault are conditioned both by the preexistence of the Oak Ridge fault and by the shape of the brittle-ductile transition. Furthermore, the presence of a d¿collement level is shown to favor the development of the San Cayetano fault shear zone as a low-angle structure. The value of the effective friction on the Oak Ridge fault is evaluated from this model. Possible values of the viscosity of the sediments are also estimated; it is shown that low sediment viscosity may completely hinder the activity of such a fault at the surface, while slip at depth continues. The stratigraphic profile of the basin is also shown to depend on the relative strengths of the ORF and SCF, which also provide an explanation for the shortening and rapid subsidence of the basin. ¿ 1998 American Geophysical Union