Continental to oceanic transition in the Sardo-Provençal Abyssal Plain - West Mediterranean Sea

The WS_10 project was acquired by OGS in the 2010 to study the West Sardinian margin and the adjacent north-eastern sector of the West Mediterranean abyssal plain (Fig.1). One of the main results of this data is the effect of the Messinian Salinity Crisis (MSC). From a geological point of view, the MSC was a very short event (5.96-5.33Ma) involving the whole Mediterranean Sea. A large drop in the water level of the Mediterranean was followed by significant erosion on the continental margins and deposition of evaporitic sediments. These last were represented by a typical sequence, often defined as the Messinian Trilogy in the West Mediterranean Sea. Thicknesses of the Messinian layers reflect the relative depth of their deposition and coeval subsidence (or uplift), therefore providing some information regarding tectonics which affect the different crustal blocks. An interpretation of the Messinian layers and erosional surface provided an opportunity to analyse the MSC event, but also to explore the different subsidence and sedimentation rates of the various crustal domains after the oceanic opening. A comparison between several seismic profiles (possibly depth migrated or converted) can contribute to knowledge regarding the crustal domains, despite the low resolution of the deeper part of the seismic profiles. In particular, a transitional crust of variable width, between the continental and the oceanic crust, shows peculiar MSC depths and thickness. This can help with the interpretation of the crustal domain even when deep reflectors are not visible. Several seismic profiles cut the Sardinian continental slope orthogonally. They show surprisingly different morphologies and structures, indicating that very complex fracture systems developed during the rifting phase of the West Mediterranean and, sometimes, successively re-activated. Furthermore, the different thicknesses of the Messinian layers suggests important information about the coeval sea bottom depths: in particular, some sectors of the present abyssal plain (about 2800 meter in depth) at the foot of the continental slope, show Messinian layers thinner than in the western sector of the plain. This appears to be due to a shallower depositional domain, representing a transitional crust between the continental and the oceanic crusts: a significant subsidence appears to have affected the oceanic crust during the Plio-Quaternary. Furthermore, the geometric setting of the Pliocene sequence is concordant with the Messinian sequence. At the same time, both the sequences are deepening toward the deep basin, testifying to the recent deepening of the oceanic sea bottom. It is generally very hard to recognize deep reflectors along seismic profiles, due to acquisition parameters, seismic resolution, presence of very high energy absorption, etc. This renders it difficult to interprete deep crustal reflectors. In the West Mediterranean Sea one can note a transitional domain at the foot of the continental slope, currently present below the abyssal plain: it is characterized by thinner MSC layers and by a MU bottom typically at approximately 4.0-4.4 s TWT (about 3500 m), while in the oceanic basin the depth is 5.0-5.2-5.4 s TWT approximately 5000-5500 m). This suggests that information about the Messinian depths and thicknesses could be used to define the different crustal domains.