The Masuccio Nappe, consisting exclusively of amphibolite-facies Variscan basement, is the lowermost unit of the Campo Nappe Complex. It overthrusts the Bernina Nappe and is located in the footwall of the Late Cretaceous Mortirolo Normal Fault, a transtensional top-E directed extensional fault that became folded around the Pass d’Ur Antiform during Cenozoic folding. In the south, the Masuccio Nappe is in a subvertical orientation and part of the Southern Steep Belt (or root zone) located north of the Tonale Fault, where it is juxtaposed with the structurally much higher Tonale Nappe, located further south and directly adjacent to the Tonale Fault.