Age of the deformation phases in the Internal Liguride Units: evidences from Lower Oligocene Val Borbera Conglomerates of Tertiary Piedmont Basin (Northern Italy)

The Tertiary Piedmont Basin (Northern Italy) represents an episutural basin developed onto Alpine and Apenninic units previously deformed during the Late Cretaceous to Eocene tectonic events. In its eastern sector a thick pile of lower Oligocene conglomerates, known as Val Borbera conglomerate, crops out. The Val Borbera conglomerate consists of an up to 2500 m thick succession of prevalently massive to crudely bedded conglomerates. The source areas of these conglomerates were characterized by the occurrence of sedimentary and metamorphic rocks. In two beds from uppermost level of the Val Borbera conglomerate pebbles derived from very Low-grade metamorphic rocks have been recognized. Their petrographical features indicate that these rocks can be correlated with the lithologies of the Internal Liguride units. The petrographical and microstructural analyses of these pebbles suggest that almost two deformation phases affected the Internal Liguride units before their subaerial erosion to supply the detritic materials of the Val Borbera conglomerate. Consequently, these folding phases are older than the Val Borbera conglomerate (Early Oligocene), and predate the emplacement of the Liguride units onto the sub-Ligurian and Tuscan domains.