A new tomographic-petrological model for the Ligurian-Provence back-arc basin (North-Western Mediterranean Sea)

The nature of the crystalline basement of the Ligurian-Provence back-arc (LPB) basin is a matter debate as it remains unexplored by direct drilling methods. Several models have been proposed for the lower crustal structure comprising hyperextended continental crust or serpentinized mantle. In this paper, a new Vp-Vs geophysical dataset and corresponding tomography are used to propose a new petrological model for the LPB basin and for the formation of the crust of this back-arc domain. By crossing values of Vp, Vs and Vp/Vs ratios, the Messinian salt layer can be clearly identified down to 5 km depth, which highlights salt diapir structures into its overlying sedimentary cover. The 7.5 km depth corresponds to the transition with a heterogeneous basaltic oceanic crust about 4.5-5 km thick, intruded by rounded felsic gabbro plutons and underplated by a more mafic/ ultramafic gabbro. This latter results from fractional crystallization of a hydrous magma inherited from the melting of a supra-subduction mantle which interacted with fluids originating from the subducting Adria slab. These magmas can be traced at the surface by magnetic anomalies punctuating the studied profile. Those new data and observations lead to conclude that the crust of the LPB basin resulted from a fast oceanic accretion during the opening of the back-arc. Its nature remains comparable to an immature oceanic crust with an overall basaltic and gabbroic composition and appears devoid of any serpentinized exhumed mantle.