The high-pressure metamorphic front of the south Western Alps (Ubaye-Maira transect, France, Italy)

André Michard*, Dov Avigad, Bruno Goffé, Ch Chopin

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

46 Scopus citations

Abstract

The Ubaye-Acceglio transect (south Western Alps) is a unique rock section showing the roughly progressive evolution of metamorphism from low grade, high-pressure greenschist facies to carpholite-quartz (blueschist) facies to low-temperature eclogite facies. The section, corresponding to the Middle Penninic Briançonnais zone, is exposed along the upper Ubaye, Varaita and Maira valleys, and is ca. 10 km thick. Our P-T estimates indicate that from the petrological point of view the section exposed corresponds to a pressure interval of ca. 0.7 GPa. Depending on rock density this interval is expected to represent a 20 km thick portion of the Alpine orogen. Although an extensional discontinuity was not detected by us along the transect, its thickness is significantly reduced. This feature is particularly intriguing as post-peak metamorphism backthrusting dominates the structure of the section, so that excess thickening is expected. We present a structural and petrologic study of the transect, including for the first time a metamorphic map of mineral occurrences. Along the transect, the Briançonnais zone consists of a number of rock slices (nappes) comprising duplexes that were piled through the operation of décollement in Lower Permian, Upper Scythian and Carnian levels. NW-directed D1 deformation, associated to the duplex piling, occurred during the subduction of the Briançonnais lithosphere. This was followed by an oblique shortening (D2) with longitudinal, reverse sinistral strike- slip faults and coeval NW-trending major folds. Further tightening (D3) carried the external Briançonnais onto the frontal part of the Upper Penninic prism, i.e. the Helminthoid Flysch nappes (Briançonnais Front), and contemporaneously resulted in strong nappe backfolding and backthrusting. Both D2 and D3 occurred in greenschist-facies conditions. Contractional piling was followed by horizontal kink bands that may record late orogenic collapse. At the top (west) of the pile, the external Briançonnais units equilibrated at ca. 0.6 GPa, 300 °C. Carpholitequartz and/or lawsonite-glaucophane assemblages (ca. 1.1 GPa, 350 °C) occurs in the internal Briançonnais-external Ultrabriançonnais units as well as in the Schistes Lustrés of the Acceglio D3 syncline. The most internal Ultrabriançonnais units of the Acceglio-Longet stripe and the underlying Schistes Lustrés at the base (east) of the section display jadeite-quartz and/or zoisite-jadeite-glaucophane assemblages (ca. 1.3 GPa, 430 °C). The external Briançonnais units, including Middle-early Late Eocene flysch, reached their maximum depth not before ca. 35 Ma. Assuming 80 km for the restored width of the Briançonnais plateau and a subduction angle of ca. 45°, the most internal Briançonnais units potentially reached >60 km depth then, but petrological and geochronological data indicate that they accreted to the upper plate and equilibrated at ca. 40 km depth at 36-38 Ma. Exhumation began at the end of D1 within the subduction channel-accretionary edifice, carrying deeply metamorphosed units onto, or closer to less metamorphic ones. Further decompression and cooling during the D2-D3 shortening phases is interpreted as the result of extension in the upper levels, and erosion on top of the Schistes Lustrés wedge during the Oligocene Alpine collision.

Original languageAmerican English
Pages (from-to)215-235
Number of pages21
JournalSchweizerische Mineralogische und Petrographische Mitteilungen
Volume84
Issue number3
StatePublished - 2004

Keywords

  • Blueschists
  • Exhumation
  • High-pressure metamorphism
  • Subduction
  • Western Alps

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