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François Bache, Ifremer - University of Bretagne Occidentale - Intechmer (France)
Jean-Louis Olivet, Ifremer (France)
Christian Gorini, University of Lille - University of Paris VI (France)
Marina Rabineau, University of Bretagne Occidentale (France)
Juan Baztan, University of Bretagne Occidentale (France)
Daniel Aslanian, Ifremer (France)
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The Gulf of Lions margin has been created by an Oligo-Aquitanian rifting followed by oceanic accretion in the Provence Basin during the Burdigalian. Two main periods have been detailed in this study. The Messinian Event which represents a clear marker (between 5.96 and 5.3 Ma) within the history of the basin and the initial period of margin formation, from the beginning of continental extension (35-30 Ma) to oceanisation (around 20 Ma). 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 1,000 m) at the transition between the subaerially eroded shelf and slope. The base of this series corresponds to the very first episode of the Messinian Crisis. • The second important element is the identification of a thick stratum of lower evaporites (over 1,500 m), between the first detritic episode and the deposition of halite in the deep basin. • We also identified a submarine abrasion surface related to the transgressive movement of the coast. On the outer-shelf-upper-slope, this surface is sandwiched between the top of evaporite deposits in the basin and the subaerial erosional surface that impacted the Miocene shelf. Transition between the subaerial erosional and abrasion surfaces caused by wave action occurs at a constant depth of 1.6 twtt. This limit represents the position of the shoreline just before a very rapid transgression that fossilized the subaerial surface without further erosion (at around 5.3 Ma). We therefore identified about 3,500 m of sediments deposited during the Messinian Crisis (s. l.) (from the first erosional event to the re-flooding of the basin). The shelf recorded a loss of up to 1,000 m thick of sediments. This implies huge 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 study of crustal structures and early sedimentation in the Gulf of Lions 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 Late Aquitanian (end of the rifting) the break-up occurs but seems to be restricted to a narrow zone of around 50 km. From that time, the Gulf of Lions subsides. These results led us to propose a model for the formation of the Gulf of Lions in three steps. First, a deep thermic event keeps the shelf subaerially exposed during the early phase of rifting. In a second step, rupture occurs (during Late 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 in the centre of the basin.
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