Relationships between magmatism and lithosphere-asthenosphere structure in the Western Mediterranean and implications for geodynamics

Shear-wave (V _S ) tomography along transects across the Western-Central Mediterranean area reveals heterogeneous lateral and vertical physical characteristics in the lithosphere-asthenosphere system (LAS). A 50 km thick low velocity layer (LVL), with V _S ∼ 4.0–4.2 km/sec, typical of low rigidity fluid-bearing mantle material, is observed at a depth of about 70–120 km from offshore Provence, to Sardinia and the Central Tyrrhenian Sea. This LVL, enclosed between higher velocity mantle rocks, rises to a depth of less than 30 km below the recent and active volcanoes of Central Italy and the Southern Tyrrhenian Sea, where a maximum in the heath flow is observed. The LVL is absent beneath Southeastern France and the northern border of the African foreland. In the Balearic Sea-Sardinia-Central Tyrrhenian section, the depth of LVL corresponds to pressure conditions of minimum temperature of peridotite+CO₂+H₂O solidus, consistent with conditions where fluid loss from the slab and mantle flow over the subducting plate favor significant melt generation above steep, west-dipping subduction zones. It is suggested that LVL in the Balearic-Tyrrhenian domains is the result of mantle contamination and melting left behind by the eastward retreating Adriatic-Ionian subducting plates from Oligo-Miocene to present. This layer also marks a discontinuity between the lithospere and underlying mantle behind the subduction zone, favoring detachment and westward drift of the lithosphere, and consequent opening of backarc basins. These data support the hypothesis that the orogenic Oligocene to Quaternary volcanism in the Western Mediterranean area is the effect of shallow mantle processes, and argue against the presence of deep mantle plumes. A shallow-mantle origin is also suggested for the EM1-type Plio-Quaternary anorogenic magmatism in Sardinia and for the FOZO-DMM-type magmatism on the northern margin of the African foreland.