The Maira-Sampeyre and Val Grana Allochthons (south Western Alps): review and new data on the tectonometamorphic evolution of the Briançonnais distal margin

Abstract Here we describe the structure, high-pressure, low-temperature (HP-LT) metamorphism and tectonic evolution of Briançonnais distal margin units from south Western Alps. The studied area extends southwest Dora-Maira (U)HP basement east-southeast classical nappes. A new structural map accompanied by geological profiles shows thrusting oceanic nappes (Monviso Queyras units) onto (D1 D2 late Eocene deformation phases) under blueschist-facies conditions. Subsequent during Early Oligocene (D3 phase) took place greenschist-facies conditions was associated with back-folding -thrusting in overlying massif exhumation related to normal reactivation former thrusts within latter massif. Two large cover units, detached their basement, are redefined as Maira-Sampeyre Val Grana Allochthons (shortly: Maira-Grana = MGA) including, (i) unit involving Lower Middle Triassic formations, seemingly subduction process, (ii) Middle-Upper Early-Middle Jurassic which probably correlates “Prepiemonte units” known Ligurian Alps Swiss Prealps. Three major shear zones mélanges continental rocks at base Grana, Dronero testify an early phase channel front Adria plate. We present a metamorphic based on published petrological data, including thermometric data obtained Raman spectroscopy carbonaceous material (RSCM). T RSCM values range ~ 400 °C > 500 °C, going most external schists uppermost unit. During Late Triassic, width s.l. domain can be restored 100 km, whereas it reached 150 km after rifting. significant, second rifting event affected Cretaceous-Paleocene, forming Longet-Alpet chaotic breccias, deserve further investigations.