Development of Petrofabric whitin granitoid mylonites and insights for the seismic behavior of the shear zone

In most collisional mountain belts, naturally deformed rocks act as natural laboratories where it is possible to observe the relationships between microstructural and textural features and physical properties of the variously deformed rocks. A crucial point is that shear zones evolve from low anisotropy to high anisotropic fabric. Anisotropy is directly linked to the rock fabric (crystallographic preferred orientation of the minerals), whose shape, symmetry and intensity depends upon the type and amount of flow and deformation geometry. Therefore, it is fundamental to establish the seismic anisotropy from laboratory investigation by simulating the in situ conditions at which the rocks occur, in order to permit an accurate interpretation of geophysical surveys. In this study, we present the results obtained after laboratory investigation on lithotypes representative of two shear zones that occur in different tectonic settings. The first site is located near Montalto (Aspromonte Massif, Calabria, southern Italy), where an Alpine crustal-scale shear zone involved Hercynian orthogneiss (Cirrincione et al., 2010). This is an interesting and well preserved site showing changes in mylonitic features of leucogneisses characterized by progressively increasing strain, as documented by the development from coarser-grained to fine-grained quartz-rich fabric as well as mica strong alignment (Fazio et al., 2016). The other investigated site is the Kavala granitoid pluton (North East Greece), whose emplacement and deformation took place in the early Miocene, offering a peculiar opportunity for investigating the textural development related to the extensional shearing events which affected the evolution of the southern Rhodope Core Complex (Punturo et al., 2014). In both localities, we focused on the relationship between progressive deformation, and therefore correlated rock fabric and the distribution of seismic properties. In particular, we carried out a microstructural, compositional and petrophysical investigation on mylonitic rocks that testify as deformation occurred at various extents. Results showed that the microstructural changes that take place affect the velocity distribution within the mylonitic granitoids, since the highest P- and S-wave velocities become progressively oriented parallel to the foliation plane, whereas the lowest values are localised normal to foliation. Moreover, results testify the textural control on the seismic properties and this holds important implications in the behaviour of high strain zones as potential seismic reflectors together with the source of seismic anisotropy. In summary, the multidisciplinary investigation on regional shear zones based on structural, microstructural, petrographic, mineralogical, geochemical and petrophysical methodologies demonstrated to be a powerful tool in providing important contributions to geodynamic modeling of a whole region.