A Ligurian (Western Mediterranean Sea) geophysical transect revisited

Abstract

Data from a new deep-penetration seismic reflection line (LISA cruise 1995), onland siesmic recording (LISA) and marine ESPs (expanding spread profiles, CROC II cruise 1981) are integrated to study the crustal structure of the north Ligurian basin. The correlation of these different seismic data, located along an Antibes-Ile Rousse transect, provides accurate information on the nature of the crust. On the Provencal margin, the multichannel seismic (MCS) profile LISA01 shows a major step in the basement. This structure is associated with a crustal change, corresponding to the ocean boundary. On the Corsica margin, we do not observe such a structure and the ocean boundary is constrained by magnetic anomalies. These anomalies are identical to those observed on the Provencal margin above the main basement structure. On the MCS line, the Messinian salt and the bottom water multiple obscure the crustal image below 7 s two-way traveltime (TWTT). Nevertheless, the ESPs provide information about the crust below 7 s TWTT. These ESPs were reprocessed and analysed by matching traveltime and amplitude variations in the X-T and t-p domains. The main result is the identification of a transitional zone on the continental margin (Provencal and Corsica margin) characterized by a layer with velocities of 7.2-7.3 km s-1 in the lower crust. Recordings from land stations on the Corsica margin define the geometry of this layer. In the centre of the basin, the oceanic crust is about 6 km thick. From the Corsica margin to the centre of the basin, the Moho depth decreases from 18 to 13 km. The velocity model deduced from offshore data is consistent with the ESP velocity model and MCS line LISA01. The combined analysis of these data allows us to propose a new boundary of the oceanic crust in the north Ligurian basin.