The nature of petrophysical anisotropy in the continental crust: a case study from the Serre Massif

In this study the relationships between fabric, seismic and magnetic anisotropy have been determined on a suite of lithotypes that are representative of a continental type crust exposed in the Serre Massif (Southern Italy). In particular, we selected two main lithotypes viz. granitoids (granodiorite/tonalite) and a metagabbro, which are respectively considered to be representative of upper/middle crust and the lower crust., By taking into account the fabric elements of the rock (e.g. lineation, foliation) we measured, seismic properties up to 400 MPa confining pressure with a triaxial multi-anvil apparatus as well as we carried out calculations on the basis of modal content and mineral composition. In granitoids, intrinsic (i.e. fracture-free) seismic velocities are around 6.2 km/s for compressional waves, whereas average Vs are ~ 3.6 km/s. In the same rocks, the Poisson’s ratio ranges from 0.240 to 0.257, in accordance with the modal proportions of feldspars. In metagabbro, average Vp is 6.9 km/s and average Vs is 3.7 km/s. Results showed that velocity distribution within the studied lithotypes depends on progressive alignments of anisotropic mineral such as biotite, amphibole and pyroxene, which define the rock fabric. Indeed, at 400 MPa, after all of the microcracks are closed, the maxima seismic velocity values are localised within the foliation plane. Seismic anisotropy (Vp- and Vs- related) has been compared with magnetic anisotropy (Km, the bulk magnetic susceptibility, that ranges between 136 and 15800 x 10-6 SI units), highlighting the different role of the constituting minerals over the petrophysical properties as a function of rock fabric. Km of the granitic rocks is < 300 x 10-6 SI units indicating that paramagnetic mineral phases such as biotite and amphibole control the intensity of magnetic anisotropy. Moreover, a positive correlation between measured seismic and magnetic anisotropy has been recognized in these granites. This indicates that the modal proportions as well as arrangement of biotite and amphibole contributes to the petrophysical and textural crustal anisotropy in the middle crust.. In contrast to the above, in the metagabbro, which represents the lower crust, has a very high Km (15800 x 10-6 SI units). Magnetomineralogical investigations reveal that the minor mineral ferromagnetic s.l minerals (magnetite and pyrrhotite) play an important role in controlling the AMS of the metagabbro despite the fact that the main minerals present in the sample are paramagnetic.. Obtained results yielded useful constraints for the comprehension of the petrophysical behavior of the continental crust and also for integrating and constructing the geophysical models of the lithosphere structure beneath southern Calabria.