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

John Keith Magali, Christine Thomas, Estelle Elisa Ledoux, Yann Capdeville, Sebastien Merkel

Seismic anisotropy mainly originates from the crystallographic preferred orientation (CPO) ofminerals deformed in the convective mantle flow. While fabric transitions have been previously observed inexperiments, their influence on large‐scale anisotropy is not well‐documented. Here, we implement 2Dgeodynamic models of intra‐oceanic subduction coupled with mantle fabric modeling to investigate thecombined effect of pressure (P)‐and water‐dependent microscopic flow properties of upper mantle and uppertransition zone (UTZ) minerals, respectively, on large‐scale anisotropy. We then compare our anisotropymodels with anisotropic tomography observations across the Honshu subduction zone. Our results for the uppermantle correlate well with observations, implying that the P‐dependence of olivine fabrics is sufficient toexplain the variability of anisotropy. Meanwhile, a dry UTZ tends to be near‐isotropic whereas a relatively wetUTZ could produce up to 1% azimuthal and ∼ 2% radial anisotropy. Because water facilitates CPOdevelopment, it is therefore likely a requirement to explain the presence of anisotropy in the transition zoneclose to subducting slabs.