Pre-Alpine structuration of the Briançonnais basement: lithostratigraphy, geochronology and tectono-metamorphic reconstruction of three Alpine massifs.

Paleozoic European crust – i.e., crust formed or reworked before the Permian and the onset of the Alpine orogenic cycle, is exposed in several massifs in the Briançonnais domain of the Western Alps. These Briançonnais crustal rocks (or 'basement', hereafter) show many similarities with other basement rocks accross Europe, particularly those affected by the Variscan orogeny. While the successive Variscan tectono-metamorphic events have been studied in many European basement massifs to unravel the orogenic and paleogreographic organization of Europe during the Paleozoic, not much has been done in comparison on the Briançonnais basement rocks.

Though partly transposed and re-equilibrated during the Meso-Cenozoic Alpine cycle, older fabrics and mineral remnants enable reconstructing part of their pre-Permian history. Our study is focused on three basement massifs of the Briançonnais domain, namely the Ambin, Vanoise and Ruitor massifs. These massifs are largely made of Cambrian to pre-Cambrian metasedimentary units cut by intrusives with variable chemical affinities, interpreted as related to the Cambro-Ordovician bimodal volcanism event. We herein present preliminary results on the (1) lithostratigraphy of this basement in the three different areas, (2) zircon U/Pb geochronology of key formations (e.g., metasediments and intrusives) and (3) the structuration of this basement crust despite the strong Alpine imprint.