Effect of olivine anisotropic viscosity in advancing and retreating subduction settings

Lattice preferred orientation (LPO) of olivine crystals occurs due to deformation in the mantle. Different parts of the upper mantle can undergo a large variety of deformation paths. During simple processes, such as simple shearing below oceans due to the movement of tectonic plates, the LPO will reflect the direction of the movement of tectonic plates. On the other hand, in areas, such as around subduction zones, the mantle undergoes more complex deformation paths, resulting in a less easily predictable LPO. Seismic anisotropy has been used as a proxy for mantle flows and the LPO formed in the mantle. To interpret the seismic anisotropy observations more accurately, we need to understand how LPO forms in different regions of subduction.

LPO has been implemented in many numerical modelling tools to predict seismic anisotropy, which places constraints on mantle dynamics. However, a few recent studies have linked olivine texture development to viscous anisotropy, resulted from the summed effect of individual crystals that are deforming anisotropically. Anisotropic viscosity can affect deformation and in turn the resulting LPO. To study the effect of anisotropic viscosity (AV) and LPO evolution in geodynamics processes, it is important to know the role of AV on LPO and the differences between the numerical methods that calculate them.

We choose three methods of olivine texture development to examine in this study. D-Rex is a polycrystal LPO model that is relatively balanced in computational efficiency and accuracy. From previous studies, D-Rex has been shown to produce faster texture development and stronger texture compared to other methods, including our second choice, the modified director method (MDM). The MDM parameterizes the olivine LPO formation as relative rotation rates along the slip systems that participate in the rotation of olivine grains due to finite deformation. We also couple the MDM with a micromechanical model for olivine AV (which makes our third choice MDM+AV), to allow the anisotropic texture to modify the viscosity and in turn affect the formation of LPO.

Here we compare the LPO evolution under subduction settings with a slowly advancing trench and a retreating trench, with and without the effect of AV. Since the mantle flow pattern in subduction zones is not homogeneous, different particles experience a variety of deformation paths. We place 60 particles in each subduction model around the slab to track the deformation and resulting olivine texture. We compute olivine texture using the above-mentioned three different methods (D-Rex, MDM, MDM+AV). With the particles, we can identify characteristic textures developed in key regions such as the mantle wedge, sub-slab area, and lateral slab edge. We then run a statistical analysis on the texture parameter and anisotropic properties of the particles from both retreating and advancing subduction models, to study where anisotropic viscosity has the largest effect on the mantle flow. We expect AV to have a larger effect in a retreating slab setting since the mantle flows feeding material to the sub-slab region is more intensive.