Large-scale folding in the upper part of the Ivrea-Verbano zone, NW Italy

New geological mapping has led to a new interpretation of the large-scale superimposed folding in the upper part of the Ivrea-Verbano zone, Italian Alps. The region is widely held to represent an upended section through lower continental crust of northern Italy. The dominant fold structure, extending some 40 km along strike, is the Massone tight to isoclinal antiform, with a hinge line strongly curved through 115°. This folds pre-existing large-scale folds that formed during regional migmatization, probably during the Hercynian orogeny, to form a type-2 interference geometry. The region then suffered post-orogenic mafic magmatic underplating and other magmatism, accompanied by crustal stretching, with contact metamorphism and migmatization causing the imposition of the final pattern of metamorphic isograds. The Ivrea-Verbano zone was brought into contact with the overlying metamorphic rocks of the Serie dei Laghi on a major shear zone. Sub-solidus stretching continued though displacements on low-angle, high-temperature shear zones. Most of the Ivrea-Verbano zone was finally tilted to the vertical and emplaced into its present position after the Mesozoic era and probably during Alpine orogenesis, forming the vertical limb of a crustal-scale double kink.