Assessing the Impact of Seismic Anisotropy on Estimates of Lower Continental Crust Bulk Composition: Insights from the Ivrea-Verbano Zone

Constraining the bulk composition of the lower continental crust is important for understanding the evolution and dynamics of Earth's lithosphere. Lower crustal P-wave velocities inferred from seismic wide-angle profiles tend to be significantly higher than their upper crustal counterparts, which, in turn, points towards a rather mafic composition of the lower continental crust. There is, however, also geochemical evidence to suggest that the bulk composition of the lower continental crust could be intermediate to felsic, which would imply that the seismic evidence is biased towards the mafic side. A likely reason for such a potential bias could be the fact that the interpretation of seismic wide-angle data tends to ignore the effects of anisotropy in the lower continental crust.  To explore this question, we have constructed canonical models of Phanerozoic lower continental crust based on the comprehensive cross-section exposed in the Ivrea-Verbano Zone (IVZ). These models simulate a 1D stochastic interlayering of the prevailing metapelitic and metamafic lithologies, to which we assign anisotropic P- and S-wave velocities based on published laboratory measurements. The effective elastic properties of these stochastically layered sequences are calculated using a Backus averaging scheme accounting for intrinsic anisotropy in conjunction with a Monte Carlo procedure to comprehensively explore the model space. Our analysis reveals that seismic anisotropy is primarily governed by the alignment of anisotropic minerals, such as mica and hornblende, while the associated influence of macroscopic layering is rather benign. Synthetic wide-angle seismic data generated using a finite-difference solution of the anisotropic elastodynamic equations show that isotropic interpretations of such data essentially provide the effective horizontal P-wave velocities of the underlying 1D layered lower crustal models. We find that the SiO₂ content inferred from these effective lower crustal velocities generally agrees well with the actual values based on the underlying samples. Quite interestingly, the most significant discrepancies in terms of the predicted and observed SiO₂ content, which are on the order of 4%, seem to be largely unrelated to the prevailing seismic anisotropy. Our results therefore indicate that, despite the rather pronounced intrinsic anisotropy of the metapelitic lithologies prevailing in the IVZ, estimates of lower crustal bulk composition based on seismic wide-angle measurements are unlikely to be systematically biased towards mafic side.