On the influence of pressure, phase transitions, and water on large-scale seismic anisotropy underneath a subduction zone

Seismic anisotropy mainly originates from the crystallographic preferred orientation (CPO) of minerals deformed in the convective mantle flow. While fabric transitions have been previously observed in experiments, their influence on large-scale anisotropy is not well documented. Here, we implement 2D geodynamic models of intra-oceanic subduction coupled with mantle fabric modelling to investigate the combined effect of pressure (P)- and water-dependent microscopic flow properties of upper mantle and upper transition zone (UTZ) minerals, respectively, on large-scale anisotropy. Our results for the upper mantle correlate well with observations, implying that the P-dependence of olivine fabrics is sufficient to explain the variability of anisotropy. Meanwhile, a dry UTZ tends to be near-isotropic whereas a relatively wet UTZ could produce up to 1% azimuthal and ∼ 2% radial anisotropy. Because water facilitates CPO development, it is therefore likely a requirement to explain the presence of anisotropy in the transition zone close to subducting slabs.