Peak metamorphic temperatures in the Alpine tectonic wedge, south Cottian Alps, Italy
Abstract
In the south Cottian Alps, the metamorphic wedge of the Western Alps is widely exposed. It includes, from
SW to NE, the Briançonnais internal border (Acceglio units), the ophiolitic Schistes lustrés nappes (Piemonte-
Liguria Ocean) and the southern Dora-Maira units derived from the Briançonnais s.l. basement (Michard et al.,
2022, and references therein). Located between the Acceglio and Dora-Maira units, the Val Maira-Sampeyre
and Val Grana Allochthons (MGA) consist of a Permo-Triassic to Mid Jurassic “Prepiemonte”-type sequence
detached from the Briançonnais s.l. basement. The peak metamorphic grade of these tectonic complexes
evolves from blueschist- to coesite-eclogite (Brossasco-Isasca unit of Dora-Maira). Here we report new peak
temperatures by carrying the RSCM geothermometry approach (see Lahfid et al., 2019) on most of the south
Cottian transect with emphasis on the MGA and on the intervening mélange shear zones. TRSCM values
range from ~400°C to >500°C, going from the most external Val Grana unit and overlying Queyras schists
to the uppermost Dora-Maira unit. We present a new metamorphic map and profiles based on published and
new data, including our new thermometric data. We note an overall fit between our TRSCM results and those
inferred from the mineral assemblages, where available. The thermal anomaly linked to the rifting evolution
that gave birth to the Piemonte-Liguria Ocean was recognized in the poorly metamorphic Adriatic margin SE
of the Periadriatic line (Beltrando et al., 2015), but cannot be evidenced in the studied Briançonnais-derived
units due to their high metamorphic grade.
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Sampeyre and Val Grana Allochthons (south Western Alps): review and new data on the tectonometamorphic evolution
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