Résumé
The offshore-onshore project METYSS-METYSAR aims at getting a better understanding of the post-Messinian relationships between crustal tectonics, salt tectonics, and sedimentation along the East-Sardinian margin. In this key-area, the Tyrrhenian back-arc basin underwent recent rifting (9-5 Ma), pro parte coeval with the Messinian Salinity Crisis (MSC, 5.96-5.33 Ma), sea-floor spreading starting during Pliocene times, and post-rift reactivation. Thereby, the Tyrrhenian basin and the East-Sardinian margin are excellent examples for studying the mechanisms of extreme lithospheric stretching and thinning, the role of pre-existing structural fabric during and after rifting, and the reactivation of a passive margin and the associated deformation and sedimentation patterns during the MSC. We looked up at the respective contributions of crustal and salt tectonics in quantifying vertical and horizontal movements, using especially the seismic markers of the MSC that were mapped from METYSS seismic-reflection data. Overall, data investigation has allowed us to delineate the history of rifting and tectonic reactivation in the area. METYSAR field work onshore was conducted in the Orosei region and showed that the main present-day river, the Cedrino river, follows the trend of a paleo-valley that cuts through the underlying granitic basement and alterites. These deposits, along with the basement, were likely eroded during Messinian times, then reworked during a marine transgression. Micro-fauna in these fine-grained marine sediments are of Upper Pliocene age. The strata dip by 20 degrees to 30 degrees and trend NNE-SSW, a direction which is subparallel to the main tectonic structures implicated in rifting of the margin. The tilted Pliocene strata were overlain by volcanic flows, some of which dating from Upper Pliocene time. Field mapping has evidenced that there was a paleo-topographic relief, trending NNE-SSW, that controlled the deposition of sedimentary series. These results clearly indicate that the post-Messinian tectonic activity, which is also visible offshore, controls the sedimentary architecture and the paleogeography of this area. Onshore, there were no signs of Lower-Pliocene marine deposits nor Gilbert deltas. The absence of such sedimentary edifices, which are characteristic of the Pliocene refilling of the Mediterranean basin are clues about significant post-rift vertical movements in the Tyrrhenian sea.