POSSIBLE COLD SUBDUCTION INITIATION BENEATH A CONTINENTAL MARGIN IN CALABRIA, SOUTHERN ITALY

In some intraoceanic settings, a new subduction zone is thought to initiate in young and hot oceanic lithosphere, leaving a distinct high-pressure, high-temperature sheet, or metamorphic sole, at the base of the upper plate ophiolite (“hot initiation”). As this involves subduction of very young crust, the igneous protolith and amphibolite-facies metamorphic overprint have similar ages. This HT event is sometimes followed by a blueschist-facies overprint, as continuing subduction lowers the geothermal gradient. Several cases are known from around the world.

In contrast, there are no known examples from the rock record of subduction initiation beneath a continental margin. This may be because subduction along continental margins begins with, and beneath, old and cold crust, in low geothermal gradient settings. These cool conditions prevent formation of the distinctive HT event seen in intraoceanic settings, making recognition of this process difficult. Instead, the first evidence of subduction zone metamorphism may be a blueschist-facies metamorphism of crust that was very old, and cold, at the time of subduction (“cold initiation”).

The Cenozoic accretionary wedge in Northern Calabria, Southern Italy, emplaced beneath the former continental margin of the Corsica-Sardinia microplate, meets these criteria. The structurally highest unit affected by later HP Alpine metamorphism, the blueschist-overprinted garnet-amphibolite Zangarona Schist, has given an amphibole Ar/Ar age of 302±4 Ma, consistent with a late Hercynian metamorphic event. Below it, the blueschist-facies Tethyan ophiolite unit is through to be Jurassic in age, based on the overlying Calpionella limestone. Since both units were affected by a blueschist-facies Alpine event, estimated by other workers to be ~40 Ma in age, subduction must have begun in crust at least 100 Ma old. Initiation of subduction in such old crust, plus the lack of an initial HT Alpine event, suggests that subduction began in relatively cold conditions, unlike the hot subduction initiation sometimes seen in intraoceanic settings. Preservation of evidence for subduction initiation here may be aided by the unique lack of continental collision in this part of the Alpine Orogen.