The Canavese zone (internal Western Alps): a distal margin of Adria

The Canavese zone is situated in the westernmost Southern Alps bordering the Austroalpine Sesia zone along the internal Western Alps. It represents the southwestern (in present-day coordinates) continuation of the northwestern distal continental margin of Adria whose relics are exposed in the Austroalpine-South Pennine boundary zone (Err nappe) of Graubünden (eastern Switzerland). Although the stratigraphic successions of the Canavese zone are badly exposed and have been dismembered by Alpine ductile shear zones and brittle faults, the signatures of Early to Middle Jurassic rifting are still preserved in the rock record. Two types of basement, separated by tectonic contacts, can be distinguished: (1) a migmatic basement of gneisses associated with anatectic leucogranites and inclusions of mafic granulites similar to those of the nearby Ivrea zone; (2) an amphibolite-grade basement, composed of amphibolites, gneisses and micaschists with shallow intrusions of Permian granitoids, obviously representing a higher crustal level comparable to the Strona-Ceneri zone of the Southern Alps. A first phase of rifting is testified by tectono-sedimentary breccias (Macchia Vecchia) and neptunian dykes of Liassic age cutting across the Triassic pre-rift carbonate platform sediments. Further rifting during the Toarcian (?) to Middle Jurassic exposed Variscan basement rocks and Permian granitoids at the seafloor, providing clasts for matrixpoor or -free, non-fossiliferous, polymictic debris-flow breccias and for sandstone turbidites interbedded with black shales (late syn-rift sediments). The polymictic breccias are dominated by granite clasts; however, they contain also fragments derived not only from the upper but also from the lower crust, and clasts of fault-rocks (phyllonites and cataclasites) most probably derived from exposed Jurassic fault planes. The occurrence of breccias and sandstone turbidites in the Radiolarite (Middle to Upper Jurassic) and Maiolica Formations (Lower Cretaceous) yielding clasts from the continental basement indicate the persistence of a submarine relief along the margin. The Middle Jurassic late syn-rift sediments of the Canavese zone are conspicuously similar in terms of facies association, sedimentary structures and clastic content to the analogous syn-rift sediments (Saluver Formation) of the lower Austroalpine Err nappe (Graubünden). The occurrence of clasts of upper and lower crustal rocks in the polymictic breccias shows that the two types of basement of the Canavese zone were juxtaposed and exposed to the sea floor already in Middle Jurassic times, most probably along low-angle extensional detachment faults as observed today along the present-day west-Iberian margin. This scenario would match those developed for the ocean-continent transitions along the Austroalpine-South Pennine boundary zone of Graubünden, the southern prolongation of the Canavese zone in the external Apennines, and – last not least – the Cretaceous west-Iberian margin.