The opening processes in the deep Tyrrhenian basins of Marsili and Vavilov, as deduced from magnetic and chronological evidence of their igneous crust

Shapes and chronology of magnetic anomalies of the mafic crust flooring the Marsili and Vavilov abyssal plains in the central Tyrrhenian Sea are discussed. In the Marsili deep basin the possible 3-phase evolution of a magmatic-tectonic process of spreading is considered. The early magmatic phase apparently took place at the time of the Olduvai magnetic event (1.87 to 1.67 Ma B.P.; Site 650 of the Ocean Drilling Project Leg 107). It appears to consist of spreading manifestations of a low magnetic intensity and a round shape, forming the igneous basement around the Marsili volcano. The late phase was characterized by a spreading process exhibiting a linear configuration and high intensity of the magnetic anomalies (organized spreading). This phase of the Brunhes epoch has given rise to the great Marsili volcano emplaced in the basin's central part. The intermediate phase shows reversed magnetic polarity and slightly elongated shapes. It appears to have developed in late Matuyama times (from 1.67 to 0.73 Ma ago). The intensities of the magnetic anomalies in the basin of Vavilov are, on the whole, lower than those present in the magnetic field of the Marsili. Also, in the Vavilov deep basin the magnetic anomalies associated with the early volcanic manifestations, which are of Late Miocene age (about 7.5 Ma), are characterized by low intensity and a round shape, whereas those associated with the effusions on the Vavilov volcano, whose age ranges from Late Pliocene to Late Quaternary (from 2.4 to less than 0.12 Ma), exhibit a high intensity and a linear shape. The two bathyal plains have inter-arc structural settings. From west to east, the Vavilov bathyal plain is bordered by the orogenic volcanic belt of the island of Sardinia and, maybe, a central Tyrrhenian arc; the latter and the arc of the Eolie, which is active in the present time, border the Marsili plain. The ages of the early spreading processes in the bathyal plains (respectively from about 10 to 6 Ma and from 1.9 to 1.3 Ma) appear to represent distinctive temporal markers: in fact, they may be coincident with the periods of quiescence of the volcanic activity that affected the adjoining arcs.