Late Alpine tectonics in the Ligurian Alps: constraints from the Tertiary Piedmont Basin conglomerates

The Tertiary Piedmont Basin (TPB) is a syn-tectonic Neoalpine-Apennine basin with all upper Eocene-upper Miocene sedimentary succession. It has all inner position with respect to the arcuate belt of the Western and Ligurian Alps, in northwestern Italy. In the area north of Genova, the Scrivia Valley separates two distinct sectors where the TPB basal conglomerates display a differentiated clast provenance: in the western sector conglomerates are fed by the metamorphic units of central Liguria, whereas in the eastern sector most of the conglomerate clasts come from the Antola flysch Unit. Observations on clast lithology demonstrate that the transition between the two types of conglomerates is abrupt and roughly coincides with the Scrivia River Valley. An explanation of the different clast provenance as due to a progressive deepening of the erosional level appears to be possible in the eastern sector, but it is not feasible in the western sector, where the flysch units appear to have been protected from erosion. We propose that the northwards thrusting of the Ligurian Alps metamorphic units onto the Antola flysch units, made the latter not available to erosion and led the Scrivia Fault and other equivalent tectonic lineaments to act as tear faults. Field data suggest that the Tertiary geodynamics of the Ligurian Alps was dominated by the Corsica-Sardinia anticlockwise rotation, that caused the northwards backthrusting of the Ligurian Alps, accommodated by a complex strike-slip fault system, that segmented the TPB, affecting the sedimentation and the clast provenance of the conglomerates. Copyright (C) 2008 John Wiley & Sons, Ltd.