Impact of Foliation and Fractures on Seismic Velocity Anisotropy in Lower Crustal Rocks

Using a comprehensive data set from ICDP‑DIVE borehole 5071_1_B in the Ivrea‑Verbano Zone in Northern Italy, we explore how foliation and fractures affect seismic velocity anisotropy in lower crustal rocks. The borehole intersects alternating metasediments, known as kinzigites, and amphibolites that show strong foliation and abundant brittle deformation. A two‑step modelling framework is applied: first, intrinsic anisotropy is computed from alignment of mineral axes along the foliation; second, fracture‑induced anisotropy is incorporated by using a linear‑slip model approach. Six representative structural scenarios are defined along the borehole, each with distinct foliation and fracture geometries. Our results indicate that intrinsic P‑wave anisotropy ranges from roughly 17–22%, and S‑wave anisotropy from 26–37%, with no systematic difference between the two dominant lithologies. Fracture‑induced anisotropy is generally smaller but variable, reaching up to ∼17% for P‑waves and ∼29% for S‑waves, depending on fracture density and orientation. In most scenarios, the embedded fractures significantly rotate the fast and slow wave directions, but their impact on the overall anisotropy magnitude differs between scenarios. Overall, the results demonstrate that both mineral alignment and fractures exert strong, scenario‑dependent controls on seismic velocities in these lower crustal rocks, and must be jointly considered when interpreting seismic velocities.