Shallow Structure of the Nile Deep-sea Fan: Implications on Sedimentary Dispersal and Fluid Ascents

By

 Lies Loncke¹, Jean Mascle², Virginie Gaullier³, Bruno Vendeville⁴

(1) Géosciences Azur, 06235 Villefranche-sur-mer, France (2) Geosciences Azur, Villefranche-sur-Mer, France (3) LSM, Perpignan, France (4) University of Texas at Austin, 78713-8924 Austin, TX

 In many areas of the Mediterranean Sea, deformations result from interplays between thick-skinned, crustal tectonics and thin-skinned, gravity-driven deformation of Messinian evaporites and overlying sediments. The Nile deep-sea fan (NDSF), recently surveyed by swath bathymetry, backscatter images and seismic data, is a good example of downslope progression along salt-bearing passive margins: There gravity spreading of the salt-sediment package induces proximal thin-skinned extension and distal contraction. The NDSF however displays a strong lateral dissymmetry. Its Western and Central provinces are poorly deformed, while its Eastern domain displays a more than 200 km long, NW-SE oriented tectonic corridor, bounded to the NE by a 400 meters high salt-bearing scarp, facing the Eratosthenes seamount (ESM). The origin of this corridor being controversial (deep-seated or salt-related?), structural analysis and physical experiments were carried out. They emphasize the importance of Messinian paleo-topographies on gravity spreading. Messinian paleo-reliefs, such as ESM, constitute passive butresses whereas depressions, infilled by thick piles of evaporites, form preferential channels for further gravity spreading. Features, such as paleo-Messinian shelf-breaks, localize linear fault zones within the Plio-pleistocene sedimentary cover. These reliefs, possibly generated by previous crustal tectonics, and re-shaped by erosional processes, seem to have induced most of the structural features observed in surface. The Messinian heritage has thus strong implications on the location of deep-sea channels, that appear restricted into the tectonized Eastern province, and highly ramified elsewhere. Fluid ascents and releases seem also to be influenced by salt distribution, itself depending on interplays between gravity spreading and Messinian paleo-reliefs.