The Memory of the Mantle: The Influence of a Time Varying Flow Field on Present Day Observations of Seismic Anisotropy
Authors: Ward James , Walker Andrew , Nowacki Andy , Panton James , Davies Huw 
Published: February 2023

Seismic anisotropy in the lowermost mantle is thought to be caused by the non-random alignment of anisotropic crystals from texturing from the mantle flowfield. Therefore, seismic anisotropy observations are commonly interpreted in the context of mantle flow. It is unclear, however, how much of an influence the history of mantle convection has on lowermost mantle seismic anisotropy and whether the present-day flowfield is sufficient for interpretation. &#160;

We investigate this by comparing the predicted anisotropy from an Earth-like mantle convection model, which includes plate motion histories from 600 Ma and a Rayleigh number of approximately 108. Therefore, these models should contain structures on similar length scales and in similar locations to the Earth. We create maps of anisotropy 50 km above the CMB using the present-day flowfield in one case and allowing the flowfield to change with time in another. For each point, we model the texture development of 500 post-perovskite crystals on their journey through the mantle to the location of interest. We then use single-crystal elastic constants to compute the full elastic tensor from the texture. To investigate what influences material properties have on the memory of mantle texture, we use three different deformation systems where we vary how easily texture can develop.&#160;

We compare the two maps by taking the difference between radial anisotropy parameters &#958; = VSH2/VSV2 and &#966; = VPV/VPH as this is what is often analysed from seismic tomography. We also present the difference in the final elastic tensors at each location because observations such as from shear wave splitting will be sensitive to more of the full elastic tensor. We find that no matter the deformation model, some regions show very different radial anisotropy strength (>10 % difference). Outside of these regions, there is little effect of a time-varying flowfield (<1 % difference) when assuming post-perovskite is easy to texture. If post-perovskite is hard to texture, the influence of mantle flowfield history has a greater effect on the final texture and therefore the anisotropy (>1 % difference). We find a similar pattern when comparing the full elastic tensors, though most regions do show some small differences. Comparing the most complex paths and quantifying the memory of the mantle shows varying results depending on the deformation models of post-perovskite and the flowfield sampled. Assuming an easy-to-deform material, the memory of the mantle was approximately 10 Ma along some paths. However, along other paths, the final texture is sensitive to flow it sampled at 125 Ma. These results show that, while a time-varying flowfield makes a significant difference along complex paths with difficult-to-texture minerals, a time-varying flowfield produces similar results to those when assuming the present-day flowfield. This work represents progress toward an understanding of the relationship between lower mantle seismic anisotropy and mantle convection. &#160;