SEDIMENTARY MECHANISMS AND UNDERLYING TECTONIC STRUCTURES OF THE NORTHWESTERN MEDITERRANEAN MARGIN, AS REVEALED BY COMPREHENSIVE BATHMETRIC AND SEISMIC SURVEYS

The tectonic structures, lithological contrasts and sedimentary processes of the northwestern Mediterranean margin are revealed by a comprehensive survey, including recently acquired Seabeam and seismic profiling data. Mass-wasting by gravity flow processes occurs over this entire area. Gravity slides are concentrated on the thick Plio-Quaternary formations located in the interchannel areas. They can lead to surprisingly steep walls, and contribute to the widening of the canyons. The contrasts between these superficial formations and the underlying Neogene formations are morphologically well expressed. The slide products, sometimes directly observed as voluminous erratic blocks, can seal the deep-sea channels, and cause their migrations. Areal extent of these slides can be very large especially with respect to the small size of the basin. However, the turbidity currents are the main gravity flow processes leading to the construction, during the Quaternary glacial epochs, of the Rhone deep-sea fan and other sedimentary ridges. They are responsible for the formation of particular bedforms such as circular depressions into the valley floors or sediment waves on the interchannel areas. The wide extent of the gravity flow processes can be explained by the particular geological setting of this margin, which is a mountainous borderland with steep slopes and rapid deposition of undercompacted sediments. The general instability is favored by recent tectonic movements, linked with the Alpine setting, and especially in the deepest areas, by the lubricating effects of the underlying Messinian salt layer. Under the loading effect of the sedimentary cover, this salt layer crops out around the Rhone deep-sea fan as a peripheral belt of diapirs having shapes controlled by deep tectonic structures.