A MANTLE PLUME HEAD TRAPPED IN THE TRANSITION ZONE BENEATH THE MEDITERRANEAN: A NEW IDEA

Abstract

We compare subduction and plume models for the Apennines and the Western Mediterranean. The impingement of a hot plume on the base of the Mediterranean lithosphere is ruled out due to the lack of distinctive signatures, such as excess topographic elevation, deep thermal regime and extensive flood basalts. However, a progressively eastward-grown plume head confined within the transition zone (between the 670 km and 410 km discontinuities), fed from the core-mantle boundary and controlled by the circulation in the upper mantle is considered to be a reasonable hypothesis. A simple area balance methodology is used in order to predict the size of such a plume along a regional transect extending from the Gulf of Lyon to Calabria. Equivalence is assumed to exist between the area of the plume head that enters within the transition zone and the area of the asthenosphere that is pushed laterally. The eastward-expanding asthenosphere drives the stretching of the overlying lithosphere and, therefore, the amount of net lithospheric extension (lfinal-linitial) is a direct expression of the amount of lateral asthenospheric expansion. The past sizes of the plume, before the onset of the Tyrrhenian phase (about 16 Myr ago) and at the beginning of the Ligurian-Provençal phase (about 30 Myr ago), are evaluated using the same balancing criteria. The possible shapes of the Mediterranean plume at the different deformation stages are supposed and depicted integrating the balancing technique with inputs from the literature on the geometry of numerically modelled plume heads trapped within the transition zone. The Apennine-Maghrebian fold-and-thrust belt is interpreted as an indirect plume product due to rift push forces which developed at the outer border of the Mediterranean stretched and thinned lithosphere. The peculiar geochemical and isotopic composition of the Italian Quaternary high-K and ultra-alkaline magmatic products is interpreted as strictly related to the enrichment of the mantle source by metasomatising fluids released within the asthenosphere by the plume head. The large-scale tomographic velocity pattern detected in the Mediterranean region might be compatible with the proposed trapped plume model if interpreted in terms of chemical variations rather than of thermal variations.