The St. Marcel Valley, Western Alps: metaophiolites, metasedimentary sequence and tectonic setting

In the Southern Aosta Valley, the St. Marcel Valley metaophiolites consist of mainly metavolcanics and their sedimentary cover metamorphosed under HP subduction-related metamorphism. A detailed geological map carried out in the St. Marcel Valley reveals that the metasedimentary cover, although transposed by the Alpine tectonics, is essentially made of three main terms, that are Mn-rich metaquartzites, marble, and calcschists. The metasedimentary sequence is quite comparable with the unmetamorphosed sequence made of radiolarian cherts, Calpionella limestones and Palombini shales covering the Ligurian ophiolites. The St. Marcel Valley metaophiolites represent the upper crustal section of the Mesozoic Tethyan ocean and its pelagic sedimentary cover, overthrusting the serpentinite unit of the Mount Avic, to the East. Within the Piemonte nappe stack, the St.Marcel metaophiolites are located at the higher structural level.