The Marsili Ridge (Southern Tyrrhenian Sea, Italy): An island-arc volcanic complex emplaced on a ‘relict’ back-arc basin

Marsili Seamount (< 1 Ma; Southern Tyrrhenian Sea, Italy) is classically interpreted as the spreading ridge of the about 2 Ma old Marsili oceanic back-arc associated to the Southern Tyrrhenian Sea–Calabrian Arc subduction setting. High resolution bathymetric data show that the ridge extends along a NNE–SSW strike and consists of 4 sectors and 11 major segments. Seafloor failures and central-type activity from an overpressurized sill-like reservoir characterize the central sector, whereas fissural volcanism and passive magma ascent take places at the edges. Cones indicative of high effusion rates concentrate in the axial zone, whereas flat-top shields associated to lower rates are located at the northern tip. Along-axis, lateral magma migration due to a local deepening of the elastic–brittle thickness of the crust develops from the central sector toward the northern tip. The opening of the Marsili back-arc basin vanished from about 1 Ma and the Marsili Ridge formed by passive magma ascent along pre-existing fractures inherited by early spreading activity. The values of the long-term output rate, the ratio between magmatic pressure and tectonic stress, and the morphological features of the lava flows and dikes are consistent with those found in volcanic arc subduction settings. Marsili Ridge represents a volcanic arc edifice emplaced on an older, ‘relict’ back-arc. The formation of the Marsili Ridge marks the transition from an extensional subduction setting to a compressive one.