A Ligurian (Western Mediterranean Sea) geophysical transect revisited

Abstract

Summary Data from a new deep‐penetration seismic reflection line (LISA cruise 1995), onland seismic 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 Provençal 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 Provençal 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 τ–p domains. The main result is the identification of a transitional zone on the continental margin (Provençal 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.

Structure and evolution of a passive margin in a compressive environment: Example of the south-western Alps–Ligurian basin junction during the Cenozoic

“…We believe that the general structure of the margin does not support such a hypothesis. Indeed, considering that on the other segments of the margin, the observed w15 to 20 tilting of the crustal blocks is demonstrably explained by crustal thinning and normal faulting (Bache et al, 2010;Contrucci et al, 2001;Rollet et al, 2002), the probability of a locally different origin for a very similar observation appears quite unlikely. In particular, the neighbouring Cannes-Antibes segment, despite being very close, was out of the influence of the second Alpine compression phase, as indicated by the Miocene Alpine deformation front located w20 km north of the shoreline in this area (Fig.…”

“…In this hypothesis, SU1eSU2 would belong to a tilted block bounded by major normal faults F1 and F2 (Fig. 4d), as proposed by Contrucci et al (2001) and Rollet et al (2002) for the westernmost part of the study area, offshore of the Castellane subalpine chain. In that case, the uppermost fan-shaped sediment in SU3 would be synrift deposits emplaced during the normal motion along the listric normal fault F1.…”

“…Offshore of the Castellane subalpine chain, multichannel seismic lines indicate that the northern Ligurian margin is structured in a few, w25 km-wide crustal blocks, tilted along large normal faults that accommodate crustal thinning (Contrucci et al, 2001;Rollet et al, 2002). In the MA12 seismic line, south of the Maures-Esterel (Figs.…”

“…Contrucci et al, 2001;Réhault et al, 1984;Rollet et al, 2002). However, the basin offers some peculiar characteristics offshore of the Alps.…”

“…1b), is typical of a rifted margin (Contrucci et al, 2001;Réhault et al, 1984;Rollet et al, 2002) as are the other margins of the western Mediterranean basins (Bache et al, 2010;Chamot-Rooke et al, 1999;Pascal et al, 1993). From multichannel seismic reflection data, north-dipping reflectors on top of the tilted blocks were interpreted either as the pre-rift Mesozoic sedimentary cover (Contrucci et al, 2001) or as pre-and synrift Mesozoic and OligoMiocene sedimentary units (Rollet et al, 2002) tilted during the rifting. On these low-resolution seismic data, the lack of fan-shaped sediment units typical of rifted margins was interpreted as a possible consequence of the Alpine compressive environment, which may have prevented the normal development of the Ligurian rift (Rollet et al, 2002).…”