The Gulf of Lion margin has been created by an Oligo-Aquitanian rifting followed by oceanic accretion in the Provencal basin during the Burdigalian. Two main periods have been detailed in this study. The Messinian event represents a clear marker (between 7 and 5.3 Ma) within the history of the basin. The study of sedimentary geometries on the margin and in the deep basin gave new elements for the understanding of the messinian crisis: • The first element is the identification of a thick detritic series (up to 1000 m) at the transition between the sub aerially eroded shelf and the slope. The base of this series corresponds to the very first event of the Messinian crisis. • The second important element is the identification of a thick stratum of lower evaporites (over 1500 m), between the first detritic episode and the deposition of halite in the deep basin. • We also identified a submarine surface of erosion related to the transgressive movement of the coast (the ravinement surface). This surface occurs on the outer-shelf-upper-slope between the top of halite deposits and the sub aerial surface of erosion. The transition between the sub aerial erosional surface and this ravinement surface due to waves occurs at a constant depth of 1.6 twtt. This limit represents the position of the shoreline just before a very rapid transgression that fossilised the subaerial surface without further erosion (around 5.3 Ma). We therefore identified about 3500 m of sediments deposited during the messinian crisis (s. l.) (from the first erosional event to the re-flooding of the basin). The shelf registered a loss of up to 1000 m thick of sediments. This implies enormous amount of sediment transfer from the shelf to the slope and the deep sea. This, in turn, also has strong implications on margin's evolution. The initial period of margin formation, from the beginning of continental extension (35-30 Ma) to oceanisation (around 20 Ma). The study of crustal structures and early sedimentation in the Gulf of Lion margin showed several major erosional surfaces within the Miocene series that enabled us to conclude that: • The substratum is largely eroded. This erosion concerns the entire margin as far as the «oceanic zone». This demonstrates a high position of the area, until to the end of the rifting period. • From Upper Aquitanian time (end of the rifting) the break-up occurs but seams to be restricted to a narrow zone of around 50 km. From that time on, the Gulf of Lion subsides. Those results led us to propose a model for the formation of the Gulf of Lion in three steps. First of all, a deep thermic event keeps the shelf subaerially exposed during early phase of rifting. In a second step, rupture occurs (during Upper Aquitanian, around 20 Ma ago and the first «unusual oceanic crust» (transitional zone) is built. The third step corresponds to the formation of typical oceanic crust at the centre of the basin. [NOT CONTROLLED OCR]