The role of the Ottone-Levanto Line in the geodynamic evolution of the Northern Apennine: evidences from the high Sturla Valley, Liguria, Italy

In the Northern Apennine, the structural setting is represented by a pile of tectonic units assembled from different pelogeographic domain during the closure of the Ligure-Piemontese oceanic basin and the following continental collision. This pile is cut by several high-angle shear zones of regional extent, showing different kinematics, age and geodynamic role. Among these lines, the Ottone-Levanto line (Elter and Pertusati, 1973) is regarded in literature as one of the most important structural element that played a fundamental role in the geodynamic evolution of the Northern Apennine, at its junction with the Western Alps. Despite its importance, no meso- and microstructural data are available in order to constrain the kinematics of the Ottone-Levanto line. A shear zone representative of the deformation along the Ottone-Levanto line has been identified in the high Sturla valley (Liguria). In the field, the shear zone is represented by 5 to 15 m thick foliated cataclasites, associated with meter thick bodies of unfoliated cataclastic serpentinites. The foliated cataclasites originated from the brittle dismemberment of an assemblage of peridotites, gabbros, granites. They display a crude mesoscopic layering defined by the alternation of mm- to cm-thick bands of different colours, from green to dark grey. Bands display locally an anastomosing network, and are often characterized by a sharp thickness reduction along their length. The dark grey bands show a fine-grained, homogenous granular texture, whereas the green ones consist of matrix-dominated rocks where mm to cm size angular fragments of serpentinites, limestones and gabbros are floating in a fine-grained matrix with well-developed penetrative foliation. Foliation surfaces dip toward the SW, and show a strike ranging from N30°/N40° to N80°/N90°, as a result of a deformation by NW-SE trending folds with gently dipping axial plane. More over, the foliated cataclasites show an internal deformation by open to close folds with E-W trending axes that does not affect the surrounding lithotypes. In the NW-SE striking and SW dipping foliated cataclasites, the facing of these folds indicate a top-to-the SE sense of shear. The cataclastic unfoliated serpentinites occur as meter-thick dark green bodies characterized by a complex network of quartz veins. These bodies, even if massive, reveal a rough banding of thick levels, featuring relics of pyroxene, and a poorly modified peridotite primary structure, alternating with levels showing a strong grain size reduction. The structural analysis at meso-scale indicates a deformation involving components of contraction with top-to-the-NE thrusting, and contemporaneous top-to-the-SW sinistral shearing and dip-slip shearing. At the micro-scale, the foliated cataclasites are characterized by elongated, angular to sub-rounded clasts of serpentinite, and subordinately calcite, both deriving from the rock types characterizing the hanging wall and footwall units. Serpentinite clasts are generally asymmetric with δ-type shapes, and the asymmetry is evidenced by pressure shadows at clasts edges defined by re-crystallization of micro-crystalline quartz. In oriented samples, the clasts asymmetry is indicative of a sense of shear toward the eastern sectors. Calcite in the clasts is characterized by thin and thick, regular straight twins, ascribable to the Type I and II twin morphology classes of Burkhard (1993) and Ferrill et al. (2004). The foliation of the cataclasites is a planar anisotropy defined by the millimeter-scale alternation of phyllosilicate layers and granoblastic quartz-rich layers. Phyllosilicates layers are made by chlorite crystals preferentially oriented parallel to banding to define the foliation. Granoblastic layers are characterized by re-crystallized fine- to very fine-grained quartz crystals that are frequently associated with microcrystalline serpentine and calcite. Chlorite also occurs as an alteration product of the serpentinite clasts, whereas calcite is also found in veins possibly related to alteration and fracturing of the serpentinite. Quartz is dominantly affected by intracristalline deformation (mainly undulose exctintion), but commonly shows as well evidence of dynamic recrystallization. These data coupled with regional evidences suggest that the Ottone-Levanto line can be regarded as a sinistral traspressional shear zone developed in the Late Eocene-Early Oligocene time. The line can be thus correlated with others lines of the Alpine-Apennine system, as the Insubric Fault, the Sestri-Voltaggio line and the Central Corsica shear zone. All these lines developed during the collisional tectonics resulting in both east- and westward thrusting of the internal zone of the appennine-alpine belt onto the continental margin domains, coeval with the northward displacement of the Adria plate.