Both European and Adriatic continental margins of the Alpine Piemont ocean were deformed and metamorphosed in the eclogite facies at ∼100 Ma, prior to the main stage of continental collision in the Eocene. We propose that the precollision (Eoalpine) high-pressure metamorphism 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 Piemont ocean. Concurrent oceanic subduction and tectonic erosion on the Adriatic side resulted in the Eoalpine high-pressure metamor- phism of the Sesia-Dent-Blanche system. Our model is consistent with the commonly accepted paleogeography of the western Alps, wherein the locus of the oceanic suture is given by the present structural position of the oceanic units. The precollisional setting that we propose for the western Alps is comparable to the tectonic configuration around the Gulf of Oman, where continent-continent collision has not yet occurred.

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