Limited Extent of Fast Seismic Anomaly Beneath Northern Apennines Favors a Lithospheric Delamination Scenario

Tectonic evolution of the Apennines is generally understood in the framework of subducting slab rollback, with Adriatic lithosphere being consumed in the process. Recently, studies of upper mantle deformation and surface uplift patterns questioned the validity of a rollback scenario for the northern Apennines, at least in the last few million years. A key element of the rollback scenario is the presence of a slab that descends at an angle, and is connected to the lithosphere on the surface. The lower end of this slab should either cut through the transition zone or else be deflected within or above it. Past imaging efforts in central Italy identified the sub-vertical fast feature, although vertical resolution was not sufficient to establish either the continuity with the Adriatic lithosphere or the exact nature of interaction with the transition zone. Using data recorded by the 2003-2006 RETREAT network we perform P and S body wave tomographic imaging, and also carry out receiver function common conversion point stacking. In tomographic images based on nearly 11000 measurements of both P and S waves, we identify a very clear fast anomaly, ~50 km thick and descending vertically from the Apennines to depths ~300 km. Resolution of our model is sufficient to rule out the possibility of this feature extending into the transition zone. Also, the feature is truncated laterally, extending north-westward from 43°N. Results of common conversion point stacking of over 3200 individual receiver functions show that there is no thickening of the transition zone beneath northern Apennines, which suggests that there is no unusually cold material within it. Our results are consistent with a lithosphere delamination scenario proposed for the region, and are not consistent with a long-lived retreating subduction zone. The limited vertical extent of the cold anomaly likely implies a recent episode of delamination. It may, however, be an example of a "stagnant" lithospheric drip similar to those recently identified in western US.