Multistage accretion and exhumation of the continental crust (Ivrea crustal section, Italy and Switzerland)

The Ivrea crustal section exposes in map view all levels of the southern Alpine continental crust, from ultramafic, mafic, and felsic granulite facies rocks of the deep crust (Ivrea-Verbano Zone), through medium-grade basement rocks (Strona-Ceneri Zone and Val Colla Zone), to unmetamorphosed Permo-Mesozoic sediments. The oldest part of the crustal section is preserved in the medium-grade basement units, which are interpreted to be the overprinted remains of an Ordovician (440-480 Ma) magmatic are or forearc complex. During Variscan subduction this are was tectonically underplated by a Carboniferous accretion-subduction complex(320-355 Ma) containing metasediments and slivers of Rheic oceanic crust presently found in the Ivrea-Verbano Zone. During the late stages of Variscan convergence (290-320 Ma), lithospheric delamination triggered magmatic underplating and lead to polyphase deformation under amphibolite to granulite facies conditions. This was broadly coeval with extensional exhumation and erosion of the Variscan-overprinted Ordovician crust presently exposed in the Strona-Ceneri and Val Colla Zones. Post-Variscan transtensional tectonics (270-290 Ma) were associated with renewed magmatic underplating, mylonitic shearing, and incipient exhumation of the lower crust in the Ivrea-Verbano Zone. This coincided with the formation of elongate basins filled with volcanoclastic sediments in the upper crust. Early Mesozoic, Tethyan rifting of the southern Alpine crust (180-230 Ma) reduced crustal thickness to 10 km or less. In the lower crust, most of this thinning was accommodated by granulite to retrograde greenschist facies mylonitic shearing. The lower crust was exhumed along a large, noncoaxial mylonitic shear zone that was linked to asymmetrical rift basins in the upper crust. The composite structure resulting from this complex evolution is probably typical of thinned, late Variscan continental crust on the passive margins of western Europe. Alpine faulting and folding (20-50 Ma) fragmented the crustal section. The originally deepest levels of the crustal section in the Ivrea-Verbano Zone as well as some segments of the basement-cover contact were steepened, whereas other parts of the crustal section, particularly the Shona-Ceneri Zone, underwent only minor to moderate Alpine rotation.