It has been largely demonstrated that basement thrust-slices were emplaced over the southeastern margin of the Ebro basin during Eocene times (Fontboté, 1954) due to sinistral, convergent wrenching in a north-south regional shortening context (Julia y Santanach ,1984; Guimerà, 1984), and that the deformation migrated from the northern to the southern areas through the active margin (Anadôn et al, 1985). Three groups of compressional structures deform the basin margin in the area between Vacarisses Slice and Montserrat. Subsequently to emplacement, continental syntectonic, sediments were deposited in the adjacent areas o f the basin, which together with the unconformities bounding their depositional sequences, recorded the evolution of the deformation along the time. Thus, in a first step, apile of north-verging thrust slices, involving the basement and the Triassic-Paleocene cover, were emplaced in a piggyback sequence; in a second step, the northwards progression of the system was partially blocked, and the previous structures were deformed by backthrusts involving Triassic and the deposited Eocene cover; and in a third step, as the system was completely blocked, formed a basement-involving fold, which southern limb evolved to a basement-involving thrust, and emplaced, in an overstep sequence, Paleozoic slices over the previously deformed basin margin. The emplacement of such basement slices at different heights in the sedimentary record, together with the trace in map view of the segments forming the Vallès fault, suggest a positive flower geometry for the main original wrench fault. We relate these three steps to the migration of the deformation proposed by the aforementioned authors. During the Neogene, the most internal structures with suitable dips were inverted. As a consequence, the segments forming the old strike-slip fault were transformed in overstepping normal-fault segments, linked by relay ramps.
Seismic and field data show that one of these ramps corresponds to the "Llobregat fault
