A LATE EOCENE-EARLY OLIGOCENE SNAPSHOT OF NORTHERN APENNINES GEODYNAMICS AT THE ALPS/APENNINES BOUNDARY: STRUCTURE AND KINEMATICS OF THE OTTONE-LEVANTO LINE IN EASTERN LIGURIA, ITALY

In the Northern Apennine, the structural setting is represented by a pile of tectonic units issued from different palogeographic domains during the closure of the Ligure-Piemontese oceanic basin and the following continental collision. The northernmost sector of this belt is of key importance because it represents the junction between the Apennines and the Ligurian Alps. Here the tectonic pile is dissected by several lines, i.e. high-angle shear zones of regional extent, showing different kinematics, age and geodynamic role. Among these lines, the Ottone-Levanto line is regarded in literature as one of the most important structural element that played a key role in the geodynamic evolution of the Northern Apennine-Western Alps boundary. In this paper, fault rocks exposures representative of the Ottone-Levanto, and cropping out in the Sturla valley of Liguria are described. The field appearance of the Ottone-Levanto line consists of 5 to 15 m shear zone marked by foliated cataclasites, associated with meter thick bodies of unfoliated cataclastic serpentinites. The structural features indicate the predominance of brittle deformation involving components of contraction with top-to-the-NE thrusting and contemporaneous top-to-the-SW sinistral shearing and dip-slip shearing involving components of contraction with topto- the-NE thrusting and contemporaneous top-to-the-SW sinistral shearing and dip-slip shearing. These structures, that are all broadly contemporaneous, indicates highly heterogeneous, structural patterns that have been mathematically described and modelled as inclined transpression zones by Jones et al. (2004), and that has been described in regions dominated by triclinic transpressional strain (Holdsworth et al., 2002). Moreover, the collected data and the map-scale and regional evidences indicate that the Ottone-Levanto line can be regarded as a sinistral traspressional shear zone developed in the Late Eocene-Early Oligocene time. A detail review of the previous interpretation of this tectonic line is also presented: the coupling of the data from literature with those from the present study allows correlating the OL line with other lines acting at the Alpine-Apennine junction, such as the Insubric Fault, the Sestri-Voltaggio line and the Central Corsica shear zone, and, consequently, to draw a geodynamic picture of the Western Alps-Apennines boundary in the Late Eocene-Early Oligocene time span. In our interpretation, all these lines developed during collisional tectonics, resulting in both east- and westward thrusting of the internal zone of the Alpine-Apennine system onto the continental margin domains, coeval with the northward displacement of the Adria plate.