Quantitative 3D microstructural analysis of naturally deformed amphibolite from the Southern Alps (Italy): microstructures, CPO and seismic anisotropy from a fossil extensional margin

Abstract The anisotropy of a rock is intimately related to the development of shape-preferred orientations (SPOs) and crystallographic-preferred orientations (CPOs). Quantifying the three-dimensional (3D) CPOs and SPOs in natural rocks is therefore critical for understanding the processes underlying the development of anisotropy. In this work, we present a CPO study of six amphibolite samples from the western Southern Alps (Italy) that have been characterized previously. Quantitative texture analyses using neutron diffraction data provided 3D CPOs for amphibole and plagioclase and were used to calculate seismic properties. We describe the relations between mesoscopic foliation and lineation, crystallographic fabrics and seismic anisotropies for lower–middle crust amphibolites. Based on these relations and in the context of lower–middle crust within fossil extensional margin, we suggest that seismic profiles should display large-scale geological features commonly present in extensional tectonics, such as folds and shear zones, rather than flat-lying structures. Moreover, from the integration of CPOs with geological data, we observe that samples from the Strona Ceneri boundary are characterized by a granulite to amphibolite facies transition while those from the Scisti dei Laghi only record the amphibolite facies evolution, supporting the idea of two independent tectono-metamorphic units pre-dating the amphibolite re-equilibration.