Abstract

The structural and geological field-mapping of the Polcevera and Scrivia valleys (Genoa Province) has revealed the presence of four tectonic units located between the Sestri-Voltaggio Zone and the Antola Unit. These units will be hereafter referred as: Bric Montaldo unit, Serra Ricco unit, Vallecalda unit and Ciae unit. All these units consist of turbiditic sequences. In addition, the Serra Ricco unit is characterized by the presence of pebbly mudstone deposits. The pre-Oligocene structural evolution of Bric Montaldo, Serra Ricco, Vallecalda and Ciae units is characterized by a polyphase deformation pattern with strong similarities showing the deformation history recognized in the Internal Ligurid Units. Each tectonic unit is affected by at least three main pre-Oligocene deformation phases, with the development of multiple generations of folds and thrust surfaces. The first deformation phase (D <sub>1</sub> ) is marked by non cylindrical isoclinal to sub-isoclinal F <sub>1</sub> folds (axis trend N180E-N200E) associated with cataclastic shear zones. The S <sub>1</sub> axial plane foliation is markedly penetrative and well developed in fine grained lithology, where it can be classified as slaty-cleavage. The D <sub>1</sub> phase is characterized by a non-coaxial deformation history, with a top-to-west sense of shear. The metamorphic peak conditions were reached during this phase. The study of mineralogical parameters (mineral assemblages, "crystallinity", polytypism and b <sub>0</sub> of illite, as well as the "crystallinity" of chlorite) point to very low-grade conditions for all the units, with an increase of metamorphic grade from the highest to the lowest tectonic unit. The second deformation phase (D <sub>2</sub> ) consists of sub-cylindrical asymmetric folds (axis trend N160E-N180E), with approximately parallel geometry, associated with a crenulation cleavage. F <sub>2</sub> folds consistently show westward vergence. During this phase, all pre-existing structural elements including the thrusts between the tectonic units, are deformed. The third deformation phase (D <sub>3</sub> ) is characterized by large, open folds with subvertical axial plane roughly N-S trending. Lithostratigraphic features and structural evolution of Bric Montaldo, Serra Ricco, Vallecalda and Ciae units strongly support the interpretation of the successions from this units as palaeogeographically belonging to the Late Cretaceous turbiditic successions of the Internal Ligurid Domain, with a tectonic evolution developed in the same palaeotectonic setting. Particularly, lithostratigraphic features and structural stack of the Internal Ligurid Units from Valpolcevera show strong similarities with the tectonic units cropping out in the high Trebbia and Aveto Valleys.

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