Defining a Proxy for the Interpretation of Seismic Anisotropy in Non-Newtonian Mantle Flows

Seismic anisotropy can provide unique insights on convection in the upper mantle. Here we study the link between seismic anisotropy and mantle flow using a non-Newtonian rheology consistent with deformation by dislocation creep. Using analytical first-order flow models underneath a ridge and in subduction zones, we find that finite strain ellipsoid (FSE) is a robust proxy of seismic anisotropy, both in terms of orientation and strength, for natural strains smaller than ≈ 1. At larger strains, anisotropy aligns with the “infinite strain axis” (ISA), defined as the orientation of the long axis of the FSE in the limit of infinite strain, and its percentage reaches a plateau. Anisotropy aligns with the flow direction only when the product of the inverse strain rate with the timescale of ISA rotation within the flow is smaller than 0.1.