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

Abstract:

Seismic anisotropy provides unique insights into the organization of convective flows in the upper mantle. Its interpretation is however not straightforward and is related to deformation in the flow rather than to the flow itself. Seismic anisotropy in the upper mantle is mainly due to the lattice preferred orientation (LPO) of olivine crystals. Mechanical models of plastic deformation, and/or dynamic recrystallization can be used to calculate the LPO evolution of crystal aggregates, but complete forward modeling is usually numerically highly demanding, especially for 3D and time varying flows. Proxies for the relationship between seismic anisotropy and mantle flow are thus useful to interpret seismic anisotropy at a large scale, or to identify the zones in the flow where a more detailed and complete calculation of anisotropy is required. To determine such a proxy, 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 fast axis 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. Once combined these results provide an efficient method to model seismic anisotropy at low computational cost. Anisotropy aligns with the flow direction only when the product of the inverse strain rate with the time scale of ISA rotation within the flow is smaller than 0.1, which will occur in only restrictive parts of the upper mantle, making the use of the previous proxies more relevant.