On the age of deep crustal extensional faulting in the Ivrea zone, northern Italy

Summary High temperature fault zones in the Valle d’Ossola region of the Ivrea zone in northern Italy appear to accommodate extension of the deep crustal section. 40 Ar- 39 Ar age spectra indicate that this extensional event occurred prior to 280 Ma, and initiated a period of crustal thinning and cooling. Interpretation of the closure temperature in this system is dependent on grain size and cooling rate.