Are Estimates of Lower Continental Crust Bulk Composition Biased Due to Seismic Anisotropy?

Abstract Most seismic inferences on the bulk composition of the lower continental crust are based on P‐wave velocities inferred from seismic wide‐angle data, for which the direction of propagation in the target region is sub‐horizontal. Given the likely presence of anisotropy in many parts of the lower continental crust, these estimates could be biased towards the mafic side. To assess the likelihood and importance of such a bias, we generate 1D stochastically layered models based on laboratory measurements on samples from the lower continental crust exposed in the Ivrea‐Verbano Zone. These models are designed to maximize lower crustal seismic anisotropy and, hence, are also expected to maximize any associated mafic biases. Considering a comprehensive range of volume fractions of the prevailing metapelitic and metamafic lithologies, we estimate the effective horizontal and vertical velocities. Our results indicate that the anisotropy is dominated by intrinsic contributions related to presence of preferentially aligned anisotropic minerals, while the contribution of layering is small. Based on the inferred effective anisotropic lower crustal velocity models, we generate synthetic wide‐angle seismic data using a finite difference solution of the anisotropic elastodynamic equations. Isotropic ray tracing of the corresponding arrivals demonstrates that the estimated lower crustal velocities largely correspond to the effective horizontal velocities. A comparison of the mineralogical SiO 2 content of our lower crustal models with that predicted from the horizontal P‐wave velocities shows that the largest mismatches are unrelated to anisotropy but rather due to the presence of significant amounts of garnet in granulite facies metapelites.