The CMB Line: the southern margin of the Ivrea–Verbano Zone (basement of Southern Alps, Italy): a re-appraisal

A new interpretation of the Cossato–Mergozzo–Brissago Line (CMB) is presented here. This tectonic discontinuity divides the lower crustal Ivrea–Verbano Zone from the middle crustal Serie dei Laghi. The association of the two units was considered by some researchers as a cross section of the continental crust. The CMB is here seen as a transpressional dextral shear zone about 6 km thick, probably connected to an Early Permian intra-Pangea dextral shear zone. Ivrea–Verbano and Serie dei Laghi were brought together only after the Variscan metamorphism. The CMB is the seat of an intense magmatic activity ranging from mafic dykes to large granitic plutons, probably triggered by an inversion of the sense of shear from dextral to sinistral. The sinistral Pogallo Line may have played a major role in this context.