Tectonic map and structural architecture of the "Briançonnais Zone" (Western Alps)

In the Southwestern Alps, tectonic units derived from the Briançonnais passive margin are exposed in two domains (Michard et al., 2022 and references therein): (i) the “Briançonnais Zone s.l.”, dominated by Mesozoic sequences either overlying Permo-Carboniferous deposits or Variscan basement, described by the early French authors (e.g., Termier, 1903), and (ii) the pre-Mesozoic basement-dominated “Internal Crystalline Massifs” (ICM). We present a new tectonic map covering a large portion of the diverse units of the “Briançonnais Zone s.l.” and immediately adjacent units. These comprise, from W to E: (1) Briançonnais, Subbriançonnais and Dauphiné units and far-travelled Ligurian Helminthoid units, structurally located in the footwall of the out-of sequence Penninic Frontal Thrust (PFT), (2) a stack of Briançonnais s.str. units in the immediate hangingwall of the PFT dominated by W-directed fore-thrusting (e.g., the classical “1.-4. écailles” of Termier, 1903), (3) a  complex intermediate zone characterized by large-scale backfolds leading to (4) a pile of backthrusted units, comprising units referred to as “Acceglio-type” and “Prepiemonte" units, and (5) units derived from the Piemont-Liguria Ocean. Units (4) and (5) tectonically overlie ICM (Michard et al., 2022 and references therein; Dana et al., 2023). We use existing geological maps and literature combined with own fieldwork data to construct a tectonic map, and associated geological cross-sections, of the above-mentioned units between Modane (north of Briançon) and the Argentera-Mercantour Massif. Our compilation aims at revealing the structural architecture of the “Briançonnais Zone s.l”, allowing for a classification of these units over longer distances along strike on the basis of structural and metamorphic criteria, avoiding as much as possible the traditional mixing of paleogeography and tectonics and preventing the use of a wealth of multiple names for one and the same unit. These units have been deformed by a polyphase structural evolution (from D1 up to D3) associated with Alpine metamorphic conditions ranging from greenschist to blueschist facies. Particularly, we highlight the frequently underestimated importance of backfolding and backthrusting (D3) and its effect on the structures associated with the previous deformation phases linked to subduction and fore-thrusting (D1-D2). A comprehensive review in the form of a map and profiles of the “Briançonnais Zone s.l.” tectonic units is essential for future and more detailed studies, on the one hand, and for better understanding of the highly non-cylindrical large-scale structure of the Western Alps arc and its transition into the E-W striking Central Alps.

 

 

Dana, D., Iaccarino, S., Schmid, S.M., Petroccia, A., & Michard, A. (2023). Structural and metamorphic evolution of a subducted passive margin: insights from the Briançonnais nappes of the Western Alps (Ubaye–Maira valleys, France–Italy). Swiss Journal of Geosciences, 116, 18.

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): review and new data on the tectonometamorphic evolution of the Briançonnais distal margin. Swiss Journal of Geosciences 115, 19 https://doi.org/10.1186/s00015-022-00419-8

Termier, P. (1903). Les montagnes entre Briançon et Vallouise: (écailles briançonnaises, terrains cristallins de l’Eychauda, massif de Pierre-Eyrautz, etc.), 182 pp. https://hal-insu.archives-ouvertes.fr/insu-00848081