LEG 107

The Tyrrhenian Sea

The present day Mediterranean Sea basins result from the consumption of the former Mesozoic Mesogean and Tethyan ocean areas and from the subsequent opening of young arc basins. The Tyrrhenian Sea is also a rare example of a tectonic basin where there is interference between collision and subduction processes creating a marginal basin and, as such, provide a reference model for other collisional basins and the evolution of previous and future Mediterranean basins.

During Leg 107, a transect of seven sites (Site 650 to Site 656) was drilled from a young passive continental margin to an even younger oceanic-type basin. At the northwesternmost site, Site 654, located on the upper Sardinian margin, rifting began in the upper Tortonian and slowed significantly or stopped in the upper Messinian. On the lower Sardinian margin, rifting was initiated during the Messinian and probably ended in the lower Pliocene. Rifting and subsidence thus appear to be diachronous across the continental margin. By the Messinian, rifting and subsidence of the upper and middle Sardinian margin were sufficiently advanced to allow deposition of basinal evaporites, whereas rifting and subsidence on the lower margin was less advanced, the facies here being lacustrine or subaerial. The water depth at the upper margin is now 1.3 km shallower than at the lower margin; thus a drastic morphological reorganization of the ancestral basin occurred during and since the Messinian, possibly due to progressive rifting.

Neither the Vavilov or the Marsili basins seem to have been deep oceanic basins during the Messinian. Just as the locus of intense rifting migrated from northwest to southeast, a comparable shift in the location of crustal accretion occurred from the Vavilov to Marsili basins. In the Vavilov Basin, basaltic flows are overlain by sediments dated at 3.5 Ma and basement is composed of a basalt cap over peridotite. Basalts from the Marsili Basin were dated at < 2 Ma, thus injection of basaltic crust began at least 1.5 m.y. later in the Marsili Basin. The basalt was found to be very vesicular, implying that eruption occurred into water much shallower than its present 4,100-m depth and thus that the eastern part of the Tyrrhenian has subsided markedly since its formation.

Plio-Pleistocene stages were defined originally in land sections around the Tyrrhenian Sea but, prior to Leg 107, stratigraphic correlations between the enclosed Mediterranean and open-ocean records had been ambiguous. Continuous sediment successions are needed to calibrate the various techniques used to date marine sediments; at Site 653, ODP recovered two such sections to 5 Ma.