Resolving Mantle Flow Beneath Italy: The Scientific Goals of the RETREAT Seismological Deployment, Northern Apennines, Italy

The Apennines mountains in Italy are associated with subduction by many researchers, motivated by uplift beside thick accretions of sediments in the Po River valley and the Adriatic Sea; deep earthquakes and volcanism in southern Italy, and a long tabular high-wavespeed feature that is observed in mantle tomography from the base of the Apennines to the transition zone. The objective of the RETREAT seismic array is to resolve the pattern of mantle flow associated with the inferred rollback of the Adriatic slab. Because both sides of the active orogen are continental, the Apennines differ from typical oceanic subduction zones. The descent of lithosphere has not, in the historical record, been accompanied by great thrust earthquakes. GPS estimates of convergence are small, no more than a few mm/year. It is not known how much of the crust of the downgoing plate descends with the mantle lithosphere, and how much accretes to the upper crust of the overriding plate. Our seismic results will allow us to resolve the Moho beneath the Apennines and the transition into the actively flowing asthenospheric mantle. Many researchers have argued that subduction of the Adriatic slab has induced a corner flow in the asthenosphere above the slab. Geodynamic modelling suggests that a complex double-cell corner flow is necessary to generate the observed extension of the overriding plate. RETREAT includes broadband seismometers in both 2-D- and linear-arrays that straddle the Apennines and its mantle high-velocity features. In several subduction zones (Kamchatka, Cascadia, Alaska) receiver functions detect P-to-S converted waves from both top and bottom of the subducted oceanic crust. Anisotropy near the top of the slab enhances the P-to-S conversion and suggests the presence of hydrous minerals. If subduction of the full lithosphere is occurring beneath Italy, we expect to observe P-to-S converted phases, with an anisotropic signature, from crust within the subducted lithosphere. Weak or absent P-to-S slab conversions will favor a scenario in which the upper crust does not subduct. The latter scenario would bolster the Rudnick model for continental composition, i.e., that detached lower crust recycles back into the mantle.