The Ivrea-Verbano tectonic evolution: The role of the crust-mantle interactions in rifting localization

This work aims to review and discuss with new interpretations the tectonic evolution of the Ivrea Verbano Zone (IVZ), to highlight how inheritance from previous orogenic cycles may have driven the Mesozoic Alpine Tethys rifting processes. After the Mesozoic rifting, the IVZ was located at the base of the continental crust of the Adriatic distal margin, before being exhumed at the surface during Alpine orogenesis. Different compositions of the IVZ peridotite massifs, mostly lherzolitic and harzburgitic, reflect upper mantle processes developed since late Proterozoic-early Paleozoic, when a sub-Gondwanian lherzolitic mantle slice (Balmuccia-type) coexisted with a depleted harzburgitic raft (Finero-type). After the buildup of the Variscan chain, in the Carboniferous, the overthickened crust might start to delaminate at its base. Whereas the Balmuccia body behaved as a passive witness of the Paleozoic mantle differentiation, the Finero mantle section underwent pervasive K-metasomatism in late Carboniferous to early Permian, triggered by melting of delaminated lower crustal slabs. Asthenosphere mantle decompression in the early Permian generated basic magmas emplaced in the lower crust. They led to a significant lateral variation of the crustal rheology, which probably drove the subsequent Triassic to Jurassic extensional tectonic pulses to delocalize along weaker zones. The Finero peridotite was characterized by a relatively low density, which was enhanced by the K-metasomatism, thereby promoting its exhumation near one of these weak zones, during a late Triassic-early Jurassic extensional pulse. Thus, the Alpine rift and, in general, magma-poor rift localization can be considered as the final product of deep processes predating the onset of rifting extensional tectonics s.s., which may have promoted crustal excision and mantle exhumation at specific locations.