Nappe stacking and syn-nappe folding in the northern Dora-Maira Massif (Western Alps)

The internal structure of the Dora-Maira Massif is of key importance for understanding exhumation mechanisms of continental-derived HP-UHP rocks in the Western Alps. Numerous petrological-geochemical studies have been done on the world-famous UHP Brossasco-Isasca Unit in the southern Dora-Maira Massif, and recent syntheses have provided an updated view of its metamorphic (Groppo et al. 2019) and structural (Michard et al., 2022) history.

By contrast, the northern Dora-Maira Massif has been much less explored. We are presently undertaking a multidisciplinary project aimed at better constraining its geometry and history. We studied in detail an area comprised between the Germanasca and the Chisone rivers. The first results are as follows:

• The nappe stack comprises, from bottom to top, the Pinerolo (Carboniferous metasediments intruded by dioritic and granitic plutons), Chasteiran (UHP), Muret (polycyclic unit, made of a Variscan basement overprinted during Alpine HP metmorphism) and Serre (Permian rhyolitic to granitic rocks, and the associated epiclastic rocks; slices of Mesozoic cover) Units.
• The pre-Alpine history of the Muret Unit (6-7 kbar, 650 °C) dated at 324 Ma (U-Pb LA-ICP-MS on monazite inclusions in garnet) is well preserved in undeformed volumes (Nosenzo et al., 2022).
• A new, colder (garnet + Fe-rich chloritoid +coesite), UHP unit (the Chasteiran Unit) has been discovered (Manzotti et al., 2022 and this meeting). This Unit, located in the immediate hangingwall of the Pinerolo Unit, occupies the same structural position than the UHP Brossasco-Isasca Unit, but records temperatures 200°C lower.

Geological mapping, structural data, and petrological investigations provide new constraints on the geometry and kinematics of this part of the Dora-Maira Massif. The main foliation D₁ developed at different peak PT conditions in the different units. During their stacking, a new foliation D₂ developed associated with kilometer-scale, E-W trending folds. Final doming of the Dora-Maira nappe stack (D₃) is associated to the westward displacement of the Adria mantle indentor. Detailed geological maps and cross-sections will be provided for illustrating the main steps of the history.

 

Groppo, C., Ferrando, S., Gilio, M., Botta, S., Nosenzo, F., Balestro, G., Festa, A., & Rolfo, F. (2019). What’s in the sandwich? New P–T constraints for the (U)HP nappe stack of southern Dora-Maira Massif (Western Alps). European Journal of Mineralogy, 31, 665–683.

Michard, A., Schmid, S.M., Lahfid, A., Ballèvre M., Manzotti, P., Chopin, C., Iaccarino, S., Dana, D. (2022). The Maira-Sampeyre and Val Grana Allochthons (south Western Alps): a review and new data on the tectonometamorphic evolution of the Briançonnais distal margin. Swiss Journal of Geosciences, 115, in press.

Manzotti, P., Schiavi, F., Nosenzo, F., Pitra, P., Ballèvre, M. (2022). A journey towards the forbidden zone: a new, cold, UHP unit in the Dora-Maira Massif (Western Alps). Contributions to Mineralogy and Petrology, in press.

Nosenzo, F., Manzotti, P., Poujol, M., Ballèvre, M., & Langlade, J. (2022). A window into an older orogenic cycle: P-T conditions and timing of the pre-Alpine history of the Dora-Maira Massif (Western Alps). Journal of Metamorphic Geology, 40, 789-821.