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 similar to 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 melange with blocks of sedimentary rocks. There, evidence of pervasive fluid-rock interaction is present. We interpret this melange 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.

Automated summary

This study documents the existence of an exhumed plate boundary shear zone, known as the Norsi-Cavo Complex (NCC), which developed during the early stages of the Apennines in Northern Italy. The NCC is composed of oceanic sediments and serpentinites and is continuously exposed on the island of Elba. The authors interpret the formation of the NCC as a result of material transfer from the upper plate to the subduction channel through tectonic erosion at the base of the prism. The findings suggest that the NCC formed during eastward subduction of the Ligurian Ocean and ceased when the continental margin entered the subduction.