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Evolution Oligo Miocène des marges du micro océan Liguro-Provençal

Abstract : 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 thesis:
· The Messinian event that needs to be differentiated from earlier strata deposition. The event also represents a clear marker (between 5,96 and 5,33 Ma) within the history of the basin.
· The initial period of margin formation, from the beginning of continental extension (35-30 Ma) to oceanisation (around 20 Ma)
The database for this study has been built using all available data (seismic and wells from industry) on the shelf, the slope and the deep basin of the Gulf of Lion. The gathering of this huge database occurred in the framework of the French GDRMarges program.

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 erodes directly the Miocene shelf and corresponds to the first trace of the Messinian crisis in the central basin. This first detritic event corresponds to what we called the « messinian erosional crisis » that brings the shelf to sub aerial erosion, but this occurs far ahead from the deposition of evaporites in the deep basin. A sea level drop of a few hundreds meters can explain the erosion of the shelf.
· 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. This event represents the beginning of the «messinian salinity crisis ».
· A third result concerns the flooding of the basin, just after halite deposition associated to the lowest sea level during the crisis. 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. First attempts of 1D backstrippings, taking into account our new observations, showed that the basin might have been 3500m deep just before the messinian crisis. A full study of isostatic readjustments related to the messinian event remains to be done; this would bring a better understanding of the reaction of the crust to varying loadings.

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 (Mesozoic and Palaeozoic) is largely eroded. This erosion concerns the entire margin as far (seaward) as the «oceanic zone». This demonstrates a high position of the area, until to the end of the rifting period. This high position is very different from what happens in the Camargue zone where an important Oligocene synrift basin has been described.
· From 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.

The questions that arise now are: why has the Gulf of Lion been uplifted and therefore submitted to erosion during the rifting before subsiding? Is this due to tectonic heritage or to the opening of the Provencal basin?
At basin scale, the Sardinian homologous margin is largely depressed between the Iglesiente and the Nurra zones that staid in high position. As in the Gulf of Lion, synrift deposits are poorly developed. This gives us arguments to say that it is really the opening of the Provencal Basin that guides the vertical evolution of the Gulf of Lion.

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.
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https://tel.archives-ouvertes.fr/tel-00326616
Contributor : François Bache Connect in order to contact the contributor
Submitted on : Friday, October 3, 2008 - 4:56:54 PM
Last modification on : Friday, April 1, 2022 - 2:56:04 PM
Long-term archiving on: : Thursday, June 3, 2010 - 10:18:25 PM

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  • HAL Id : tel-00326616, version 1

Citation

François Bache. Evolution Oligo Miocène des marges du micro océan Liguro-Provençal. Géologie appliquée. Université de Bretagne occidentale - Brest, 2008. Français. ⟨tel-00326616v1⟩

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