THE ROLE OF EARLY MIOCENE THRUST TECTONICS IN THE STRUCTURAL ARRANGEMENT OF THE VOLTRI GROUP (LIGURIAN ALPS, ITALY): EVIDENCE FROM THE BANDITA AREA

Abstract

Geological mapping and structural analysis of the Bandita area in the northwestern sector of the Voltri Group (Ligurian Alps) revealed the occurrence of ductile shear zones and brittle thrust faults involving metamorphic rocks. This paper focuses on the brittle thrust faults and their role in the structural arrangement of a large metalherzolite body belonging to the Erro-Tobbio Unit. Layers of fault rocks, mostly banded cataclasites and crush breccia, are associated with the brittle thrust faults and show a consistent top to the east-northeast sense of shear. The features of these brittle thrusts match those of structures related to the Early Miocene post-collisional tectonic phase known as the D4 phase in the Voltri Group and “fase ligure III” (3rd Ligurian phase). In this area a major D4-related thrust fault is the basal contact between the metalherzolites and the underlying serpentinites. Similar structural relationships of the metalherzolites to other Voltri Group rock types also occur in adjoining areas. However, at the regional scale, structural and metamorphic evidence supports an early coupling (i.e. pre-D4) of the lherzolite with the serpentinite, metabasite and metasediment, indicating that the D4-related thrust surfaces rework a pre-existing structural arrangement. We suggest that the Erro-Tobbio metalherzolites are frequently involved in the D4 thrusting because the pre-existing contacts with the surrounding rock types, due to the competence contrast, acted as preferential sites for the nucleation of the D4 shear surfaces.