Structural setting of the External Briançonnais in the Maritime Alps | Assetto strutturale del Brianzonese esterno nelle Alpi Marittime

n the Ligurian sector of Alpine chain, the upper part of Tanaro valley, offers a spectacular three-dimensional picture of the brittle-ductile deformation of regional extension displaying a great number of tectonic units that form this orogenic belt. Units come from three main adjacent paleogeographic domains: the Piedmont-Ligurian ocean, the Briançonnais paleo-European continent and, in the middle, the Piedmont continental margin. The uppermost sedimentary cover of the oceanic magmatic crust (known as «Helminthoid Flysch of the Western Liguria») was detached and transported as far as the outermost part of the chain. It forms a set of anchimetamorphic nappes that cover the tectonic sub-horizontal contact between the outer Briançonnais and the underlying Dauphinois domains (SENO et alii, 2005). Besides the particular setting of the Flysch units, the order of nappe superposition generally follows the «rule» known all over the Western Alpine chain: the lower is the geometric position of a unit, the more external is its provenance. Some out-of-sequence nappes are present. The local names of the two external Briançonnais units here considered are «Ormea» (the lower one) and «Caprauna-Armetta». The stratigraphic series of the Ormea Unit starts with Upper Carboniferous continental deposits and ends with Eocene neritic limestones, while the Caprauna-Armetta Unit lacks all the terms older than the Upper Permian-Lower Triassic conglomerate. As usual for the whole Briançonnais domain, both the units are marked by a stratigraphic gap, which covers a time-interval always including the Lower Jurassic and increasing towards the originally