1B - alpine metamorphism

organisation and contact:

Philippe Agard philippe.agard@upmc.fr

Michel Ballèvre michel.ballevre@univ-rennes1.fr

 

This field trip in the internal domains of the Western Alps focuses on the metamorphic remnants of the former (Liguro-Piemontese) Alpine ocean in order to discuss subduction processes, particularly well-recorded in this part of the orogen due to the lack of significant post-subduction, collisional-related heating.

Eclogitized portions of the Liguro-Piemontese oceanic lithosphere, in particular, are found along 200 km, from the Zermatt-Saas area in the north to the Monviso area in the south, and further to the SE in Voltri, and constitute some of the largest and deepest ophiolitic slices detached from a subduction zone worldwide.

The Liguro-Piemontese domain can be divided in two main tectono-metamorphic domains: (1) dominantly blueschist facies sedimentary units with minor crustal lenses such as the Schistes Lustrés complex are located to the west, whereas (2) large, mainly eclogitic oceanic crust bodies (such as those found in Monviso and Zermat-Saas units) mainly crop out to the east.

During day 1 we will focus on the Schistes Lustrés domain s.s. in the Queyras area. After a few stops in the Briançonnais domain, we will investigate this fossil accretionary wedge, where m- to hm-sized blocks and lenses of mafics and ultramafics are embedded in blueschist-facies, km-thick Late Mesozoic metasedimentary rocks, and show how metamorphic P-T conditions progressively increase eastward.

Day 2 will be devoted to a tour around the Monviso ophiolite, which forms the structural base (mantle and crust) of the Liguro-Piemontese domain and is separated from the adjacent Schistes Lustrés s.s. domain by a major ductile normal fault. The Monviso ophiolite (closely associated in space with the continental Internal Crystalline Massifs, such as Dora Maira) preserves km-scale slices of oceanic lithosphere with remarkably constant P-T conditions of ~24-27 kbar 540-550°C as well as evidence for past earthquake ruptures.