Implementing 3D anisotropic viscosity calculations into ASPECT

<p>Olivine, the main rock-forming mineral of Earth's mantle, responds to tectonic stress by deforming viscously over millions of years. This deformation creates an anisotropic (direction-dependent) texture that typically aligns with the mantle flow direction. According to laboratory experiments on olivine, we expect this texture to also exhibit anisotropic viscosity (AV), with deformation occurring more easily when it is parallel to, rather than across, the texture. However, the direction dependency of lithospheric and asthenospheric viscosity is rarely addressed in geodynamic models.</p><p> The open-source modeling package ASPECT can address AV in a 2D setting using the director method, where AV is present due to shape preferred orientation created by dike intrusions (Perry-Houts and Karlstrom, 2019). We have adapted this implementation for current versions of ASPECT and benchmarked it against similar Rayleigh-Taylor instability models by Lev and Hager (2008).</p><p>Unfortunately, a 2D method is inappropriate to address AV related to olivine crystallographic preferred orientation (CPO or texture), as, by default, olivine has three independent slip systems on which deformation can occur. Integrating anisotropic viscosity into 3D models would also allow us to use the actual laboratory-based parametrizations of the olivine slip system activities and texture parameters when describing the evolution of CPO and AV. One of the biggest challenges in addressing AV in a 3D setting is to find the full, rank 4, viscosity tensor, which can be done with a method similar to the one for the fluidity tensor in Király et al., (2021).</p><p>Here, we present the initial results of simple geodynamic setups (shear box, corner flow), where 3D olivine CPO develops, based on the D-Rex method (Fraters and Billen, 2021), and this CPO creates AV based on the micromechanical model described in Hansen et al., (2016).</p><p>Our goal is to create a tool within ASPECT that allows for CPO to develop and affect the asthenospheric or lithospheric mantle’s viscosity to improve modeling a wide range of geodynamic problems.</p><p> </p><p>References listed:</p><p>Fraters, M.R.T., and Billen, M.I., 2021, On the Implementation and Usability of Crystal Preferred Orientation Evolution in Geodynamic Modeling: Geochemistry, Geophysics, Geosystems, doi:10.1029/2021GC009846.</p><p>Hansen, L.N., Conrad, C.P., Boneh, Y., Skemer, P., Warren, J.M., and Kohlstedt, D.L., 2016, Viscous anisotropy of textured olivine aggregates: 2. Micromechanical model: Journal of Geophysical Research: Solid Earth, doi:10.1002/2016JB013304.</p><p>Király, Á., Conrad, C.P., and Hansen, L.N., 2020, Evolving Viscous Anisotropy in the Upper Mantle and Its Geodynamic Implications: Geochemistry, Geophysics, Geosystems, v. 21, p. e2020GC009159, doi:10.1029/2020GC009159.</p><p>Lev, E., and Hager, B.H., 2008, Rayleigh-Taylor instabilities with anisotropic lithospheric viscosity: Geophysical Journal International, doi:10.1111/j.1365-246X.2008.03731.x.</p><p>Perry-Houts, J., and Karlstrom, L., 2019, Anisotropic viscosity and time-evolving lithospheric instabilities due to aligned igneous intrusions: Geophysical Journal International, doi:10.1093/gji/ggy466.</p>