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Vavilov seamount: A mildly alkaline Quaternary volcano in the Tyrrhenian Basin

Abstract:

Vavilov seamount emerges from the Central Tyrrhenian plain, at 3600 m depth, and reaches 684 m below sea level. Four dives in the Cyana submersible showed that this voluminous volcanic edifice (350-400 km3) can be divided into two parts: (1) from its base to 1500 m deep, a pile of thick submarine basaltic flows; and (2) an upper part, above 1500 m, consisting of smaller radial lava flows related to a major central volcano (Vavilov s.s.) and other minor vents aligned north-south. The basal series is affected by normal faults, directed north to N40°, and strongly tilted westward, like the neighbouring Sardinian continental block. Radiometric ages of upper lavas range from 0.1 to 0.4 Ma. Other observations suggest that the late phase of construction has been subaerial, and lead to estimate the Upper Quaternary subsidence of the basin to at least 500 m (4-5 mm yr-1). Although Vavilov is located at the supposed centre of oceanic accretion, the characteristics of its lavas strongly differ from those of the underlying crust. The basalts fall into the mildly alkaline series and are strongly enriched in light REE. Among the products of passive margin volcanism, they may be compared with intermediate basalts between the olivine-tholeiite of the continental flood basalts and the P-type oceanic basalts; in terms of oceanic volcanism, they are very close to Oceanic Island Basalts (OIB). Vavilov appears as a structure independent of the oceanic crust which supports it, and does not represent a ridge related to the oceanic accretion as one might suppose. © 1987.