The problematic origin of the Pancherot-Cime Bianche-Bettaforca unit (PCB) in the Piemonte zone (Western Alps)

Leonsevero Passeri (*), Gloria Ciarapica (**) & Giorgio Vittorio Dal Piaz (***)
(*) Past prof of Sedimentology, Perugia University, Italy. (**) Past prof of Regional Geology, Perugia University, Italy, passed away on March 14, 2017. (***) Past prof of Geology, Padova University, Italy. Corresponding author e-mail: leonsevero.passeri@gmail.com.

The Pancherot-Cime Bianche-Bettaforca (PCB) is an ophiolite-free decollement cover unit with a greenschist facies imprint, continental affinity and problematic provenance. It is located in the ophiolitic Piemonte zone which marks the suture in the north-western Alps of the Mesozoic Liguria-Piemonte ocean (western Tethys). A new stratigraphic reconstruction of this unit is presented. It includes the following main lithofacies: Quartzitic Schists and metaconglomerates (Permian), Yellowish Quartzite (Early Triassic), Bedded Dolostone and Marbles (Middle Triassic), Tabular White Quartzite (Carnian), Massive Dolostone (Norian), Carnieules and Brown Marble (Rhaetian-Jurassic). Ages are hypothetical because no fossil has been found. Comparison with the Prepiemontese and Austroalpine sedimentary successions shows major affinity with the Austroalpine cover units. The continental basement of this exotic unit is unknown. Suitable candidates can be considered the Mt Emilius and other Lower Austroalpine eclogitic outliers, as these continental slices are missing of a sedimentary cover and are located inside the ophiolitic Piemonte zone, near the structural level of the PCB unit. In this view, these basement and cover units were decoupled during subduction due to their contrasting metamorphic features.

Piemonte zone, decollement cover nappe, stratigraphy, paleostructural reconstruction, Valtournenche, Aosta valley, Western Alps.