Pre to syn orogenic evolution of the Piedmont-Ligurian oceanic covers: clues on the Flysch units of the Western Ligurian Alps (CARG project – Ormea sheet 244). 

Abstract

The evolution of the European margin before and during the collisional phase of the alpine orogenesis is still a debated topic. The Western Ligurian Alps are a complex key area that can help to better understand this tectonic evolution. Here the contact between two different domains crops out: Briançonnais domain (European passive margin) and the Piedmont-Ligurian sedimentary covers (oceanic domain). These last units are characterized by several deformation stages, presented by many thrust sheets and non-cylindrical folds that make difficult to understand their relationships and their three-dimensional setting. Moreover, they are characterized by low-grade metamorphism that often masks their sedimentary structures and features, resulting in a challenging reconstruction of the pre-orogenic stratigraphic and structural setting. Previous works have hypothesized that these turbidite systems have been deposited in an abyssal plain resulted from the Piedmont-Ligurian Oceanic rifting and spreading. They are characterized by a lower part of basal complexes with thin bedded and very-thin bedded turbidites and often containing olistostromes. These basal complexes are overlain by sand- or carbonate-rich turbidite systems (Decarlis et al. 2014; Lanteaume et al.1990) that are interpreted as trench environment deposits (Di Giulio, 1992; Mueller et al. 2017). During the progressive advance of the accretionary wedge towards the European foreland, these units have migrated and stacked in reverse order, with the oldest one in the topmost part. The aim of the study is to review and integrate the previous works with new data following a multidisciplinary approach with a particular focus on the basal complexes of the flysch units. The CARG project is focused on the detailed fieldwork mapping that is already in progress with the aim of realizing the geological map of Ormea Sheet 244. During this activity, samples are collected for laboratory analysis. Specifically, petrographic characterization of samples collected in the basal complexes is currently carried out to better understand the source area of the sediments. Geochemical analyses are also in progress on basalt clasts found in the chaotic bodies. Another aim is to investigate the metamorphic grade by analysing fluid inclusions and vitrinite reflectance. Geochronological analysis will be performed with U/Pb analytical techniques on zircons to compare the results with surrounding crystalline basements to put an additional time constrain to the poor biostratigraphic data. REFERENCES Decarlis A., Maino M., Dallagiovanna G., Lualdi A., Masini E., Toscani G., Seno S., 2014. Salt tectonics in the SW Alps (Italy-France): from rifting to the inversion of the European continental margin in a context of oblique convergence. «Tectonophysics» 636, 293-314 Di Giulio A., 1992. The evolution of the Western Ligurian Flysch Units and the role of mud diapirism in ancient accretionary prisms (Maritime Alps, Northwestern Italy) «International Journal of Earth Sciences (Geologische Rundschau)» 81, 655-668 Lanteaume M., Radulescu N., Gavos., Feraud J., 1990. «Notice explicative, Carte Géol. De France (1/50000), feuille Viève-Tende» 948, Orleans, BRGM. 139 pp. Mueller P., Patacci M., Di Giulio A., 2017. A Hybrid event beds in the proximal to distal extensive lobe domain of the coarse-grained and sand-rich Bordighera turbidite system (NW Italy). «Marine and Petroleum Geology» 86, 908-93