Mantle transition zone anomalies beneath Iberia and NW Maghreb

The 410 and 660 discontinuities are predicted to be the result of isochemical phase changes in olivine. The differing Clapeyron slope of the reactions, though, leads to opposite 410 and 660 behaviour for a same temperature variation, with cold and hot mantle anomalies resulting in a thicker or thinner transition zone (MTZ), respectively. Here we use more than 56500 high-quality P-wave receiver functions obtained from 881 broadband seismic stations to locate anomalies in the MTZ beneath Iberia and NW Maghreb. We obtained robust maps of the 410 and 660 discontinuity depth thanks to the combined measurements of several 3D depth migrations using regional and global P-wave tomography models, and used these maps to calculate the MTZ thickness. Our results reveal several large-scale anomalies in the region mostly linked to the thermal effects of cold subducted slabs, but we also found evidence for significant chemical heterogeneity in the MTZ. The Gibraltar-Alboran and Alpine-Tethys slabs cause a continuous MTZ thickening along the Mediterranean coasts. Accompanying the slab anomalies are up to three areas with a low-velocity layer (LVL) located at the top of the 410 discontinuity, which provide evidence for partial melting coinciding with an MTZ enriched in water due to slab dehydration reactions. A similar LVL is also found at the top of the lower mantle where the Alpine-Tethys slab pushes the 660 discontinuity downwards. Mantle upwelling occurs at the back of the Gibraltar-Alboran slab, where we find the thinnest MTZ in the region. Upwelling hot materials seem to travel SW following a toroidal flow along the southern boundary of the slab and cause the 410 discontinuity to deepen significantly. Even though the MTZ thickness remains near-standard, the 410 also deepens in a more discontinuous manner beneath the Atlas Mountains. The active anorogenic volcanism in the Western Mediterranean correlates remarkably well with the LVL on top of regions with sunken 410, possibly pointing at a MTZ source for the melts.