TECTONIC MODEL FOR THE EVOLUTION OF THE WESTERN ALPS

Both European and Adriatic continental margins of the Alpine Piemont o cean were deformed and metamorphosed in the eclogite facies at approxi mately 100 Ma, prior to the main stage of continental collision in the Eocene. We propose that the precollision (Eoalpine) high-pressure met amorphism in the internal crystalline nappes of the western Alps (Dora Maira, Gran Paradiso, and Monte Rosa) was a result of the subduction of the European continental margin below the lithosphere of the Piemon t ocean. Concurrent oceanic subduction and tectonic erosion on the Adr iatic side resulted in the Eoalpine high-pressure metamorphism of the Sesia-Dent-Blanche system. Our model is consistent with the commonly a ccepted paleogeography of the western Alps, wherein the locus of the o ceanic suture is given by the present structural position of the ocean ic units. The precollisional setting that we propose for the western A lps is comparable to the tectonic configuration around the Gulf of Oma n, where continent-continent collision has not vet occurred.