Linking the Alps and Apennines subduction systems: New constraints revealed by high-resolution teleseismic tomography

We report a new model of the upper mantle structure beneath Italy obtained by means of P-wave teleseismic tomography. Besides the recent and remarkable development of the Italian Seismic Network, a high model resolution has been achieved improving the inversion method upon the ACH method used in previous investigations and picking high quality arrival times with the Multi-Channel Cross-Correlation technique. The finer details of our Vp model yield new insights into the heterogeneous structure of the Adria continental lithosphere involved in the collision between the Africa and Europe plates. A wide low Vp anomaly located in the northern Adria mantle, facing the Alpine high Vp slab, supports the idea that the Adria lithosphere has been hydrated and thinned during the Alpine subduction. We argue that this mantle softening may have played a key role in favoring the subsequent delamination of the Adria lithosphere in the northern Apennines. We hypothesize that delamination of continental lithosphere previously thinned in a back-arc setting may be considered a key process to favor subduction polarity reversal and recycling of continental material into the mantle circulation. Conversely, in the central-southern Apennines, the velocity structure is consistent with the existence of a deeper oceanic slab that flattens at the base of the upper mantle, in agreement with the widely accepted geodynamic evolution of the central Mediterranean by slab retreat and back-arc spreading. The oceanic slab is discontinuously detached from the surface plate, suggesting a different structure of the Adria lithosphere, which resists subduction instead of favoring delamination.