Stratigraphic evolution in the Ligurian Alps between Variscan heritages and the Alpine Tethys opening: A review

The Ligurian Alps are the southernmost segment of the Alpine orogenic belt. Their formation is the result of a multistage process that comprises two orogenic cycles: the Variscan and the Alpine one. Since the Late Paleozoic onwards, superposed sedimentary successions deposited in this area, as a response to the changing tectonic setting in the frame of the Western Mediterranean evolution. The transition between the two orogenies was characterized by the formation of a net of sedimentary basins controlled by diffused extension, which is not correlated to a clear geodynamic context yet. As the fault network is severely affected by Alpine reworking, the multiple pre-Alpine tectonic stages experienced by the Ligurian sector can be unravelled only by the analysis of the well exposed stratigraphic succession, which offers very distinctive feature for each tectonic phase.From the present day situation of a nappe-pile orogen backward to the originary basinal setting, the aim of this work is to reconstruct the paleogeographic evolution of the Ligurian Alps between Permian and Jurassic, through a detailed stratigraphic analysis. Several sedimentary packages belonging to different tectonic units are analysed, grouped into specific domains and correlated. Five major steps in the evolution of the Ligurian Alps have been pointed out; they have been contextualized within the latest paleogeographic reconstructions of the Alpine sector, contributing to detail the role of the study area in the plate tectonic dynamics. The sedimentary record is referred to successive geodynamic stages, from the Pangea break-up and Variscan belt dismantling, through a Triassic diffused extension, to the Alpine Tethys rifting and finally to the spreading that generated the Piedmont-Ligurian oceanic branch. The stratigraphic reconstruction of the Ligurian sector also indicates the lacking of an ocean interposed between the European continent and the Alpine collisional wedge, thus representing the southward termination of the Valais basin.On the whole, the paleogeographic reconstructions provided in this work highlight that the Ligurian Alps were a domain in which for over a hundred million years orogenesis, rifting and oceanisation strongly affected the integrity of the upper crust. The heterogeneity of the crust that suffered multiple mantle uplifts, partial melting and extensive faulting over a long period is testified by field evidences and several effusive events. The last and most important rifting and the following Alpine Tethys oceanisation developed around a narrow, elongated area of crustal weakness generated by multiple geodynamic events.