Thermochronological constraints on the multiphase exhumation history of the Ivrea-Verbano Zone of the Southern Alps

Abstract

The Ivrea-Verbano Zone of the western Southern Alps (NW Italy) exposes a well-preserved tilted section across the lower continental crust, making it a key region for studying deep crustal and exhumation processes. This paper refines the cooling and exhumation history of the Ivrea-Verbano Zone using K/Ar dating of mica and illite-rich fault gouges as well as zircon fission track and (U-Th)/He thermochronology. The adjacent Strona-Ceneri Zone, Sesia-Lanzo Zone and Lower Penninic nappes are included in the study to derive a broader picture of the low-temperature history of the area.In the Strona profile of the Ivrea-Verbano unit the biotite K/Ar, zircon fission track and (U-Th)/He geochronometers show well preserved, but unusually wide partial retention zones. The youngest ages, representing the formerly deepest position, are situated along the Insubric Line.The main foliation of the Ivrea-Verbano Zone is vertical. The exhumation of the Ivrea-Verbano Zone, which section has a horizontal position on the surface now – took place in three steps. During Jurassic time the Ivrea-Verbano Zone was exhumed to a shallow to mid-crustal position by continental-scale extension. In this displacement the Pogallo Line probably played a dominant role. The studied section occupied an oblique position with a calculated angle of ca. 15 to 23° in the Jurassic. Later the Ivrea-Verbano Zone experienced a minor cooling event in the Late Eocene (~38Ma zircon fission track ages) that was probably related to thrusting and erosion. The final exhumation towards the surface took place in the mid-Miocene as documented by the ca. 14Ma zircon (U-Th)/He ages and a 12.8MaK/Ar fault gouge age. The magnitude and the high rate of final exhumation suggest orogen-parallel extension as a driving force, which is widespread in the Alps in the Lower to Middle Miocene and is most probably connected to orogenic collapse.