Polyorogenic deformation history recognized at very shallow structural levels: the case of the Antola Unit (Northern Apennine, Italy)

The Antola Unit is a Ligurian Unit occurring at the top of the nappe pile of the Northern Apennines (Italy). Structural analysis indicates that the Antola Unit has been involved in a complex polyphase deformation history developed at a shallow structural level. The successive deformation phases belong to both the Alpine and Apenninic orogenic cycles. Therefore the Antola Unit is a good example of a polyorogenic tectonic unit deformed at a very shallow structural level. The older part of the structural evolution is assigned to the Meso-Alpine tectonic stage and consists of two deformation phases (D1 and D2) characterized by opposite tectonic transport directions. The D1 and D2 structures were sealed by the middle Eocene-Miocene deposits of the Tertiary Piedmont Basin. The younger part of the deformation history instead can be referred to the Northern Apennines tectonics, and consists of two phases (D3 and D4) both involving the Tertiary Piedmont Basin succession. The structural data indicate that the D1 deformations developed in mostly unlithified rocks, and that these structures can be related to the westward emplacement of the Antola Unit in the Alpine orogenic wedge. The D2 phase is characterized by folds and low-angle extensional faults, and is referable to a gravity-driven deformation. Both the D3 and D4 phases, characterized by folds and thrusts, are due to the compressive regime related to the development of the Northern Apennines.