Role of inherited compositional and structural heterogeneity in shear zone development at mid-low levels of the continental crust (the Anzola shear zone; Ivrea-Verbano Zone, Southern Alps)

Abstract

The formation of shear zones is crucial to understand the deformation of the crust and the evolution of rifted margins. However, despite their intrinsic importance, a detailed characterization of the compositional and structural patterns of shear zones is often lacking, resulting in poorly constrained models of shear initiation. In this contribution, we reconstruct the pre-shearing lithological, structural and metamorphic proprieties of rocks forming a major, rift-related shear zone with the aim to assess the conditions promoting the strain localization. We focus on the Anzola shear zone, a major extensional structure from one of the best-preserved cross-sections through the middle to lower continental crust of a fossil passive margin, the Ivrea-Verbano Zone (Southern Alps, Italy). Until now, the Anzola shear zone is believed to have developed within a rheologically hard and isotropic gabbro rather than in the surrounding weaker and anisotropic volcano-sedimentary metamorphic sequence. New geological mapping shows that significant pre-existing heterogeneities related to the composition and deformation structures, characterize the Anzola shear zone. Field relationships and geochemistry reveal that the (ultra-)mylonitic rocks overprinted a multi-lithological sequence that have already experienced Variscan deformation and late Variscan High-Temperature metamorphism, at the boundary between amphibolite and granulite facies. Our in-depth trace elements study is shown to be a powerful tool in reconstructing the pre-shearing relationships between wall rocks and mylonites and determining the protoliths of tectonites. 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), following a retrograde path. We argue that strain localization was promoted by the combination of rheological boundaries derived from pre-existing conditions, including: i) compositional and structural anisotropies of the volcano-sedimentary metamorphic sequence contrasted by ii) the close intrusion of a nearly isotropic gabbro and iii) the presence of melt within the metamorphic boundary depicted by the transition between granulites (dominated by anhydrous minerals) and amphibolite facies (dominated by hydrous minerals). Our findings finally suggest that pre-existing significant heterogeneities relate to rock composition, deformation and metamorphism represent the preferential loci for strain localization controlling the initiation and development of rift-related structures in the mid to lower crust of passive margins. • Field mapping and chemical analysis of an extensional shear zone from middle/lower crust. • Trace element analysis reveals composite protoliths of the mylonites. • Anisotropic and/or hydrated rocks are folded with isotropic and/or anhydrous layers. • Mylonites postdates folding and upper-amphibolite-granulite facies metamorphism. • Pre-existing contrasted rheological boundaries promoted strain localization.