Quantitative characterization of fracture network in large sheath-fold: field and UAV-based digital outcrop model analysis (Ligurian Alps, Italy)

Predictive models for the origin and distribution of tectonic fractures in regions with multiple and heterogeneous deformation events are still considered as characterized by a high degree of uncertainty. In fact, it is known that fracture networks development is dependent by many factors, such as composition, diagenesis, structural position, fold bending and changes in the tectonic stress history. However, several studies noted that the high-curvature regions of large folds often show a good correlation between the measured and theoretical geometry of the fracture sets. This paper presents a field- and digital outcrop model-based structural investigation of refolded km-scale sheath fold (Caprazoppa fold) and associated fractures host in a metamorphic carbonate sequence (Ligurian Briançonnais, Western Alps) that experienced multiple regional deformation phases. The orientation of the fracture sets along the 3D fold structure are broadly consistent with the network theoretically expected as due by the main sheath fold curvature. The subsequent, less intense ductile phase produced significant change of the structural patterns only in the schistose lithologies, whereas the more competent rocks mostly recycled the early foliations and fracture sets. Even the post-metamorphic brittle faulting/fracturing selectively reactivated the pre-existing fractures favorably oriented. We infer that, when strain is high, the strongly curved sheath folds exert a major control in the development of the fracture network, the geometry of which is preserved through the subsequent ductile/brittle deformation stages.