Velocity contrasts in the mantle: changing the paradigm of the Apennines

The slab retreat concept is widely used to explain the evolution of the Apennines in Italy. Since pioneeristic studies in the late '80, several models were proposed that are based on the eastward retreat of the Adria/Ionian slab, whose presence in southern Tyrrhenian area is undoubted due to the presence of intermediate-depth to deep seismicity and tomographic evidences. Anyway, the evolution of northern Apennines seems more complex and the retreat process was contradicted by most data and evidences gathered during the RETREAT project. In this study, a huge set of teleseismic receiver functions is used to resolve the velocity contrasts in the upper mantle along swaths that cross the northern Apennines digging down to about 160 km depth. Velocity contrasts show that the west-dipping high Vs Adria mantle terminates at depth of about 80-100 km and is bottomed by an oppositely dipping lithosphere. Conversely to expectation, no evidence of asthenospheric upraise was found in the Tyrrhenian back arc region still arguing against slab retreat models. The new structure revealed for the Tyrrhenian and Apennines mantle change our vision on the Alpine tectonics. We propose that the lithosphere that subducted to form the northern Apennines, since the past 16 Myr, is continental and sinks via delamination. This process was subsequent to the collision of the European and Adria plates and followed the end of the subduction of the Tethys Ocean in the Alpine domain.