Tertiary age and paleostructural inferences of the eclogitic imprint in the Austroalpine outliers and Zermatt-Saas ophiolite, western Alps

The Austroalpine Sesia-Lanzo inlier and upper Austroalpine Dent Blanche, Mt . Mary and Pillonet outliers occur on top of the western-Alpine orogenic we dge and, as a whole, override the structurally composite ophiolitic Piemont e zone. Instead, the Mt. Emilius, Glacier-Rafray, Etirol-Levaz and other lo wer Austroalpine eclogitic outliers are inserted within the Piemonte zone, between its upper (Combin) and lower (Zermatt-Saas) tectonic elements, or w ithin the latter. Rb-Sr dating on phengitic micas show that the eclogitic i mprint in the lower Austroalpine outliers, conventionally regarded as Late Cretaceous by comparison with the Sesia-Lanzo inlier, is of Eocene age (49- 40 Ma), like the underlying Zermatt-Saas ophiolite (45-42 Ma) between the A osta valley and Gran Paradiso massif. Ar-40-Ar-39 plateau ages on the same mica concentrates of the ophiolitic Zermatt-Saas nappe (46-43 Ma) are consi stent with Rb-Sr dating, whereas that on the Austroalpine Glacier-Rafray kl ippe (92 Ma) is influenced by argon excess. The lower Austroalpine outliers underwent the subduction metamorphism concurrently with the Zermatt-Saas n appe, 20-25 Ma later than the eclogitic Sesia-Lanzo inlier and blueschist P illonet klippe. The temporal gap and present intra-ophiolitic position mean that the lower Austroalpine outliers were probably derived from an intraoc eanic extensional allochthon (Mt. Emilius domain) stranded inside the Piemo nte-Ligurian ocean far from the Dent Blanche-Sesia domain and Adriatic marg in.