We derive a three-dimensional shear-wave velocity model of the Ligurian-Provence back-arc basin (Northwestern Mediterranean Sea) using ocean-bottom seismometers (AlpArray OBSs) and land stations from permanent and temporary seismic networks. The quality of OBS continuous records is enhanced by a specific processing that reduces instrumental and seabed-induced noises (transients, tilt, compliance). To further improve the resolution of ambient-noise tomography in the offshore area, we compute the Rayleigh-wave part of the Green functions for OBS-OBS pairs by using onshore stations as virtual sources. 2-D group-velocity maps and their uncertainties are computed in the 4 - 150 s period range by a transdimensional inversion of Rayleigh-wave travel times. The dispersion data and their uncertainties are inverted for a probabilistic 3-D shear-wave velocity model that includes probability densities for Vs and for the depth of layer interfaces. The probabilistic model is refined by a linearized inversion that accounts for the water layer in the Ligurian Sea. Our S-wave velocity and layer boundary probability models correspond well to a recent, high-resolution P-wave velocity cross-section derived from controlled-source seismic profiling along the Ligurian-Provence basin axis. A joint interpretation of the P- and S-wave velocity sections along this profile reveals a thin, anomalous oceanic crust of low P-wave velocities but high S-wave velocities, intruded by a gabbroic body. The illuminated part of the upper mantle appears to be devoid of serpentinization.

Key Points

Efficient processing scheme to remove transients and reduce tilt and compliance from continuous ambient noise recorded by OBSs

Computation of iterative correlations between OBSs based on a virtual reconstruction of the Rayleigh waves

Thin, anomalous oceanic crust with gabbroic intrusions evidenced in the basin axis from a joint interpretation of Vs and Vp models

Plain Language Summary

The Ligurian-Provence basin (Northwestern Mediterranean Sea) is one of the Miocene-Pliocene back-arc basins that resulted from the retreat of the Adria subduction in the plate reorganization due to Africa-Europe convergence. The crustal structure of the basin is still debated, even though it has been probed by active seismic profiling. We compute a high-resolution shear-wave velocity model of the Ligurian-Provence basin and its margins by making optimal use of ambient-noise recordings of seafloor broadband seismometers. In particular, we improve the usually low quality of surface-wave signals in noise correlations between seafloor stations by involving correlations with land stations. The joint interpretation of our S-wave velocity model with a P-wave velocity section obtained in the basin axis by controlled-source seismic profiling provides compelling evidence for the presence of a thick sediment pile above a thin, ∼4.5 km-thick oceanic crust, intruded by gabbroic bodies emplaced at the crust-mantle transition. These results show the potential of a joint interpretation of P- and S-wave velocity models since they provide reliable answers to a number of debated questions on the petrological nature of the crust and uppermost mantle, in particular in the Ligurian-Provence basin.