Multiscale heterogeneity control on the nucleation of a crustal shear zone: petro-structural investigation and U-Pb titanite dating from the Anzola shear zone (Ivrea-Verbano Zone, Southern Alps)

The Ivrea-Verbano Zone (IVZ, Southern Alps) is a fossil exhumed passive margin section of the pre-Alpine middle to lower continental crust that escaped Alpine subduction. Following the Variscan orogeny, the IVZ was affected by Permian post-orogenic extension and Triassic-Jurassic polyphasic rifting stages. Rift-related deformation was accommodated by several km-scale shear zones active at different crustal levels (e.g., Beltrando et al., 2015). Due to the intrinsic importance of these tectonic structures, a detailed characterization of their compositional, metamorphic and structural patterns, as well as the timing of activity may provide key information for the models of shear development in relation to the evolution of the regional tectonics.

In this contribution, we investigate one of these major extensional structures - the Anzola shear zone - with the aim to assess the conditions that promoted the strain localisation. We also provide U-Pb dating on titanite as attempt to constrain the timing of the high-temperature crystal-plastic deformation occurred within the shear zone. Recent field and meso-structural investigations revealed that the Anzola shear zone overprinted basement rocks characterized by inherited lithological and structural heterogeneities (Corvò et al., 2022). Gabbroic rocks and migmatites define the hanging wall and footwall, respectively. According to the petrography and geochemistry (ultra-)mylonitic rocks developed at the expense of a multi-lithological sequence showing amphibolite to granulite facies metamorphic conditions and deformation features related to pre-shearing event. Estimated P-T conditions indicate that mylonitic deformation started at high temperature (~820°C) with presence of melt and continued as solid-state deformation down to amphibolite facies (~650°C). As regard the timing, we show preliminary petrochronological results from titanite of the mylonitic amphibolites that recorded recrystallization event under amphibolite facies at about 185 Ma, which is coeval to deformation occurred at different crustal levels in the IVZ (Simonetti et al. 2021).

On the base of our findings, we argue that the shear zone development was promoted by the rheological contrasts derived from the inherited compositional and structural patterns. Moreover, we emphasizes evidence of syn-deformational partial melting and small amounts of free fluids localized in certain layers that enhanced the viscosity contrasts within the multi-lithological complex. Melts/fluids played a key role in both weakening mechanisms controlling the strain localization, as well as the syn-tectonic growth-recrystallization processes of the titanite, resulting in a strong influence of the U-Pb petrochronology results. Finally, our results are discussed in the framework of the geodynamic evolution of IVZ.