Tectono-metamorphic evolution of a post-variscan mid-crustal shear zone in relation to the Tethyan rifting (Ivrea-Verbano Zone, Southern Alps)

The Ivrea-Verbano Zone, in the Italian Southern Alps, samples a complete section of middle to lower continental crust, which records the transition between the Variscan and the Alpine Tethys rift-related tectonics. We present a structural-metamorphic characterization of one of the most important structures, the Forno-Rosarolo shear zone which drove the post-Variscan tectonic evolution of the Ivrea-Verbano Zone. The Forno-Rosarolo shear zone is a NE-SW-oriented, subvertical shear zone, mainly made of metapelites, amphibolites, calc-silicates and granulites involved in anastomosed proto-to ultra-mylonite layers enveloping weakly deformed lenses. Mylonitic deformation developed on an already folded and sheared metamorphic (Variscan) complex but predates brittle fracturing and faulting, locally associated with pseudotachylites. In present day orientation, the kinematic indicators point to a sinistral sense of shear. Removing the Alpine ∼90° tilt of the IVZ crustal section, this kinematic results in a former extensional shear zone. Investigations on the kinematic of the flow in the mylonites revealed a non-coaxial deformation with a major component of pure shear acting together with simple shear. Metamorphic conditions of the wall rocks vary from upper amphibolite (SE, footwall) to granulite facies (NW, hanging wall). Estimations from the mineral assemblages within the mylonites point to amphibolite facies conditions during deformation (∼650 °C and ∼5.5 kbar). In absence of precise geochronological constraints, we discuss the role of the Forno-Rosarolo shear zone within the Tethyan rifting system in comparison with recent reconstructions. Kinematic data and metamorphic conditions allow to constrain the development of the Forno-Rosarolo shear zone mylonitic deformation during the intermediate phase of the Tethyan rift known as “thinning mode”. This stage was characterized by general shear conditions (pure shear between 70% and 50%) suggesting a phase of transition from a symmetric to an asymmetric configuration of rift.