Structural evolution of different tectonic units across the Austroalpine–Penninic boundary in the middle Orco Valley (Western Italian Alps)

A structural study along the Orco Valley, which represents a geological section through the Alpine belt (western Italy), is reported. A Lower Tectonic Element, composed of units with eclogite facies relics (Gran Paradiso and Zermatt–Saas-like Unit) and an Upper Tectonic Element, made of greenschist facies units (Combin-like Unit and Gneiss Minuti Complex of the Sesia–Lanzo Zone) have been distinguished. These two elements show a different main Alpine tectono-metamorphic event (D1) and are separated by a mylonitic contact (D2) developed under greenschist facies conditions (Orco Shear Zone). This regional scale shear zone was responsible for the exhumation of the deeper eclogitic units, and has been inferred as the continuation of the Combin Fault south of the Aosta Valley. From D3 onward, the same tectono-metamorphic evolution was recorded in all units. The following extensional crenulation cleavage (D4) occurs mostly along a NNE–SSW deformation zone about 1 km thick at the base of the Upper Tectonic Element which leads to a further exhumation of the Lower Tectonic Element.