Mantle exhumation and sequence of magmatic events in the Magnaghi-Vavilov Basin (Central Tyrrhenian, Italy): New constraints from geological and geophysical observations

The Tyrrhenian basin opened in the Neogene following the E-SE retreat of the Appenines-Calabrian subduction system and the subsequent back-arc extension of an orogenic crust. The resultant crustal structure includes a complex distribution of continental, back-arc magmatism, and mantle-exhumation domains. A clear example of this complex structure is found in the central and deepest part of the basin (i.e. Magnaghi-Vavilov sub-basin) where geophysical data supported that the bulk of the basement is composed of partially serpentinised peridotite representing exhumed mantle rocks, and intruded by basalts forming low ridges and volcanic edifices. However, those data sets cannot univocally demonstrate the widespread presence of serpentinised mantle rocks, let alone the percentage of serpentinisation. Here, we use S-wave arrivals and available geological information to further constrain the presence of mantle serpentinisation. Travel times of converted S-waves were used to derive the overall Vp/Vs and Poisson's ratio (?), as well as S-wave velocity of the basement in the Magnaghi-Vavilov Basins. This analysis reveals Vp/Vs ? 1.9 (? ? 0.3) that strongly supports a serpentinised peridotite forming the basement under the basins, rather than oceanic-type gabbro/diabase. P-wave velocity models is later used to quantify the amount of serpentinisation from fully serpentinised (up to 100%) at the top of the basement to < 10% at 5-7 km deep, with a depth distribution similar to continent-ocean Transition zones at magma-poor rifted margins. Seismic reflection profiles show normal faulting at either flank of the Magnaghi-Vavilov Basin that is potentially responsible for the onset of serpentinisation and later mantle exhumation. These results, together with basement sampling information in the area, suggests that the late stage of mantle exhumation was accompanied or soon followed by the emplacement of MOR-type basalts forming low ridges that preceded intraplate volcanism responsible for the formation of large volcanoes in the area.