Modelling of seismic anisotropy in the lowermost mantle with rheologically constrained geodynamic setup

Seismic anisotropy is an observation that is believed to yield information on the flow pattern in the mantle. There are many studies of anisotropy in the upper mantle; however, the lower mantle is still underexplored, due to problems in seismic imaging and complexities of modelling of flow laws of
different minerals. In this study, we modelled the radially anisotropic behavior of two different geodynamic setups, one is the rising of a mantle plume from the core-mantle boundary to the surface, and another is subduction of a slab reaching the lowermost mantle. We use ASPECT for modelling large scale mantle flow and ECOMAN to simulate the development of lattice preferred orientation of mantle fabric. We use the slip system of Bridgmanite following the previous experimental study by Mainprice et al. (2008). We then couple the results from ASPECT to ECOMAN for modelling the radial anisotropy and maximum shear wave splitting direction. We show that in the part of the lowermost mantle surrounding the plume horizontally polarized shear waves (Vsh ) are faster than the vertically polarized ones (Vsv ) while the inside of the plume tail shows opposite signature. However, Vsh becomes greater than Vsv when the plume flattens out at the surface. We also find that the maximum splitting direction is horizontal outside the base of the plume and it becomes vertical inside the plume tail and again becomes horizontal at the surface. This result corroborates previous seismic observations (Wolf et al., 2019) of the Iceland plume at the core-mantle boundary. Moreover, our result for the slab setup reveals that as the slab reaches the lowermost mantle, Vsh becomes higher than Vsv and maximum splitting is horizontal at the base of the slab.

 

References
Wolf, J., Creasy, N., Pisconti, A., Long, M.D., Thomas, C., 2019. An investigation of seismic anisotropy in the lowermost mantle beneath Iceland. Geophys. J. Int. 219, S152–S166.

Mainprice, D., Tommasi, A., Ferré, D., Carrez, P., Cordier, P., 2008. Predicted glide system and crystal preferred orientations of polycrystalline silicate Mg-
perovskite at high-pressure: implicaitons for the seismic anisotropy in the lower mantle. Earth Planet. Sci. Lett. 271, 135–144.