Cold and old: The rock record of subduction initiation beneath a continental margin, Calabria, southern Italy

We present data from northern Calabria, in southern Italy, which show that subduction may have initiated beneath a continental margin east of the Corsica-Sardinia-Calabria block during the Eocene. Calabria lacks ophiolites (oceanic rocks) within the upper plate, which are the strongest evidence for intraoceanic subduction initiation. The structurally higher nappes of the Calabrian subduction complex include continental crustal material, a feature that is also inconsistent with intraoceanic subduction initiation. In addition, Calabria lacks an amphibolite-facies metamorphic sole, regarded by some as a consequence of intraoceanic subduction initiation in young oceanic lithosphere. The age of blueschist metamorphism, compared with ages of arc volcanism in Sardinia, indicates preservation of rocks subducted and accreted during or shortly after subduction initiation, precluding significant subduction erosion. Peak subduction-related metamorphism reached blueschist facies, including rocks that apparently accreted early, during or shortly after subduction initiation. The protoliths of these rocks were ≥80 m.y. old at the time of subduction, indicating that subduction began in old and cold lithosphere along a continental margin. Subduction initiation here suggests that the serpentinization of the upper mantle, observed in the Tethyan rocks of Calabria, may have been important in weakening the oceanic lithosphere at the continental margin. Alpine subduction may have initiated in a similar manner along other reaches of the orogen.