A STRUCTURAL TRANSECT ACROSS THE IVREA-VERBANO ZONE NEAR VARALLO SESIA, NORTHERN ITALY

The Ivrea-Verbano Zone (IVZ) is part of a basement high within the Southern Alps of the Cretaceous-Tertiary Alpine orogenic belt. This zone and adjacent lithotectonic units (Strona-Ceneri and Val Colla Zones) are commonly interpreted as part of a crustal section through the late Paleozoic crust of the Southern Alps. An important phase of the geologic history of the IVZ is late Paleozoic magmatism that post-dated an earlier history of contraction related to the Variscan orogeny. In the IVZ, this magmatism is in part manifested by the emplacement of a composite, ~10-km-thick, under/intraplated Early Permian Mafic Complex that was subsequently tilted to subvertical.

We have made a detailed structural transect across the IVZ, extending from the Cossato-Mergozzo-Brissago (CMB) Line on the SE to Insubric Line (IL) on the NW. Strain in the host rocks of the Mafic Complex is heterogeneously distributed. In this light, three structural domains have been recognized from the SE to NW. Domain 1 includes the eastern IVZ and extends from the CMB Line until an abrupt transition zone into Domain 2 (i.e., high-T shear zone). Domain 1 contains the earliest deformational history of the IVZ (i.e., Variscan contractional deformation), but is locally overprinted by later deformations--both pre-Alpine and Alpine. This domain is entirely within the amphibolite-facies metamorphic regime; thus it represents the lowest grade rocks that occur in the IVZ. Domain 2 is interpreted as a zone of general shear that developed during the progressive emplacement of the Mafic Complex. The westernmost domain is Domain 3 that is chiefly within the granulite-facies metamorphic regime, and lithologically this domain contains abundant intrusive plutonic rocks (ultramafic to mafic) in addition to paragneisses, mafic granofelses, and scarce metacarbonate rocks. The westernmost portion of Domain 3 is locally overprinted by penetrative, low-grade deformation (greenschist facies) and pseudotachylyte development related to the complex movement history along the IL. A fundamental conclusion that emerges from our structural transect across the IVZ is that this lithotectonic zone is composed of elements of variable age, and their structural evolution is polyphase and variable across the zone. Thus to view the IVZ as a “typical example” of deep continental crust disregards its obvious complex, polygenetic evolution.