Geology of the Eastern Ligurian Alps: a review of the tectonic units

The Alpine and Apennine belts come into contact in Central Liguria, in an extremely complex structural arrangement; this junction is usually referred to as the Ligurian Knot. The geological mapping for the 213-230 "Genova" and 212 "Spigno Monferrato" 1:50,000 quadrangles necessitated the updating of the stratigraphic, structural and metamorphic descriptions of this area, and a redefinition of some of the units and geological ensembles already established in Central Liguria, i.e. the Voltri Group, the Sestri-Voltaggio Zone and the Val Polcevera Unit. We propose a new definition of tectonic and tectono-metamorphic units, grouped in three main associations, as follows: a) units derived from oceanic crust and mantle; b) units derived from a continental margin; c) Flysch units derived from the sedimentary cover of an oceanic basement. In group a), we propose to keep the already established Palmaro-Caffarella, Cravasco-Voltaggio, Varazze and Figogna units and to establish a new Voltri tectono-metamorphic Unit, including all the rocks showing a metamorphic climax in eclogite facies conditions, with a variably developed green schist facies overprint. In group b) we propose to keep the already established Gazzo-Isoverde Unit and to define a new Angassino-Terma tectono-metamorphic Unit, encompassing limited outcrops of quartzite and dolomitic limestone associated with the Voltri rocks. In group c) we propose to replace the former Val Polcevera Unit with new Ronco, Montanesi and Mignanego tectonic units. We propose to abandon the term "Voltri Group", for the term "Voltri Massif" that can be used to define a set of units, including the Voltri, Varazze, Palmaro-Caffarella and Angassino-Terma units, irrespective of their different lithology and/or paleogeographic derivation and/or metamorphism. For the Sestri-Voltaggio Zone, we propose to use this term only for the Cravasco-Voltaggio and Gazzo-Isoverde units (and not the Figogna Unit), to emphasize their common tectono-metamorphic evolution.