Source tracing of detrital serpentinite in the Oligocene molasse deposits from the western Alps (Barrême basin): implications for relief formation in the internal zone.

Stéphane Schwartz (1) , Guillot Stéphane (1) , Tricart Pierre (1) , Bernet Mathias (1) , Jourdan Sébastien (1) , Dumont Thierry (1) , Montagnac Gilles (2)

1 Tectonique reliefs et bassins
2 ENS de Lyon - École normale supérieure de Lyon

We present the first contribution of tracing the source area of ophiolitic detritus in the Alpine molasses by Raman spectroscopy. The lower Oligocene molasse deposits preserved in the Barrême basin, in the SW foreland of the western Alpine arc, are known for the sudden arrival of the first "exotic" detritus coming from the internal Alpine zones. Among them, the pebbles of serpentinized peridotites have so far not been studied. We show that they only consist of antigorite serpentinite, implying that they originate from erosion of HT-blueschists. In contrast, the upper Oligocene/lower Miocene molasse, shows mixed clasts of serpentine including antigorite and lizardite without any evidence of chrysotile. This suggests that they were derived from a less metamorphosed unit such as the LT-blueschist unit. Taking into account the sediment transport direction in the basin and the varied metamorphic characteristics of the other ocean-derived detritus, we constrain the lithological nature of the source zones and the location of the relief zones, identified as the internal Alps, SE of the Pelvoux external crystalline massif. Available structural data and in situ thermochronological data allow reconstructing the Oligocene to early Miocene collisional geometry of the Paleogene subduction wedge. This phase corresponds to two major phases of uplift evolving from a single relief zone located above the Ivrea body during the early Oligocene and persisting up to the early Miocene; then during the late Oligocene/early Miocene a second relief zone developed above the Briançonnais zone. At that time, the internal western Alps acquired its double vergency.

Mots clés: serpentinite pebbles; Raman spectroscopy; source tracing; western Alps; Barrême basin

Domaines: Tectonique; Milieux et Changements globaux

Stéphane Schwartz, Guillot Stéphane, Tricart Pierre, Bernet Mathias, Jourdan Sébastien, et al.. Source tracing of detrital serpentinite in the Oligocene molasse deposits from the western Alps (Barrême basin): implications for relief formation in the internal zone.. Geological Magazine, 2012, pp.1-16. ⟨10.1017/S0016756811001105⟩. ⟨insu-00663347⟩