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

In 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 inner sectors. In both the units the Alpine metamorphism does not exceed the epizone. A structural regional character that has to be evidenced concerns the attitude of thrust surfaces. While in the inner sectors of the chain these surfaces gently dip toward the hinterland, in the external sectors their dip changes toward the foreland. All the units show a regional foliation parallel to the main thrusts. This deformation (D1) also produced south-verging folds, up to isoclinal, always confined within each unit. Moreover, in the Ormea Unit the same D1 event has developed minor thrusts that cause repetitions of the stratigraphic succession, particularly evident when Mesozoic covers are involved. These duplications are frequently accompanied by the sudden elimination of whole lithostratigraphic units both in the direction of tectonic transport and at 90° to it. A peculiar configuration of the examined sector is represented by the fact that, over wide areas, the upper nappes (Caprauna-Armetta Unit, in turn covered by Piedmont rocks) lie directly on the inverted portions (Permian volcanics) of the Ormea Unit. This overthrust is older than all the ductile post-nappe deformation events and might have been generated by an out-of-sequence thrust, in turn linked to the outward transport of the Helminthoid Flysch set of tectonic units. These nappes, having by-passed the still active Penninic front, might have represented an obstacle to the usual path of thrust-development. Besides, it must be underlined that, as it observed in inner sectors of the chain, also in the outer side more than one post-nappe deformation events are well exposed. Among these ones, the most widespread and pervasive is marked by open north-verging folds, associated with a locally very well developed and closely-spaced crenulation cleavage. Finally, the last phase has produced asymmetrical «dome and basin» interference patterns