Deformation and Material Transfer in a Fossil Subduction Channel: Evidence From the Island of Elba (Italy)

We document an exhumed plate boundary shear zone—a subduction channel—developed at the contact between a fossil Cretaceous-Eocene accretionary prism (Ligurian Units) and the underlying continent-derived nappes of the Northern Apennines. The subduction channel, referred to as the Norsi-Cavo Complex (NCC), is continuously exposed for ∼10 km along strike on the island of Elba. The NCC consists of oceanic sediments and serpentinites, coupled at the base of the prism. In its northernmost exposures the NCC crops out as a serpentinite mélange with blocks of sedimentary rocks. There, evidence of pervasive fluid-rock interaction is present. We interpret this mélange as the result of material transfer from the upper plate to the subduction channel through tectonic erosion at the base of the prism. The southern part of the NCC preserves a series of tectonic slices of shales and limestones (Palombini shales) coupled with ultramafic rocks through serpentinite shear zones containing lenses of massive serpentinites, mafic rocks, and sediments. Since sedimentary slices preserve the same structures of the overlying Ligurian Units, produced by accretionary prism deformation, we interpret this complex as the result of material transfer from the prism to the subduction channel and its subsequent underplating at the base of the prism. Based on our interpretation, the NCC fossil subduction channel formed during E-verging deformation over the W-directed subduction of the Ligurian Ocean during the early development stages of the Apennines. This deformation terminated when the continental margin entered the subduction.