A Flexotir seismic reflection profile made in 1970 from the Gulf of Valencia to the Ligurian Sea defines the sedimentary formations and the main structural provinces in the northwestern Mediterranean basin.

This basin has a sedimentary section 6,000-7,000 m thick and contains a Pliocene-Quaternary group 1,000-1,500 m thick with a velocity of 1.7-2.8 km/sec; an evaporitic group consisting of a bedded upper unit of Messinian (late Miocene) age 600 m thick with a velocity of 3.5 km/sec and a homogeneous lower unit of salt, 0-1,200 m thick with a velocity of 4.3 km/sec; an infrasalt group 1,000-4,000 m thick; and an acoustic basement visible only near the basin margins.

In the southern Gulf of Lion the salt group, although thick, is not disturbed by diapirs, whereas farther east (Ligurian Sea) diapirs are highly developed, probably in connection with Pliocene faults. The infrasalt group is horizontal; its composition is unknown; its age could be as old as Oligocene in some areas.

Two types of basin margins were revealed. In the first type (Provence), all the formations in the basin pinch out against the abrupt continental slope. In the second type (north Balearic zone, Corsica), between the continental shelf and the abyssal plain, there is a broad intermediate marginal zone. This marginal zone is characterized by a shallow continental basement, a thick infrasalt group localized in restricted basins, the absence of salt, and the presence of a Pliocene-Quaternary group similar to the one in the abyssal plain. There are two different types of structural boundaries; the first, now at great depth, limits the continuous infrasalt group and the salt layer, and the second corresponds to the present margin related to Pliocene-Quaternary faulting.

The structural history of the basin apparently is not governed solely by Pliocene-Quaternary tectonics; foundering began earlier, probably as early as the Oligocene or early Miocene.