Multi-scale anisotropy development in viscous flow due to fabric evolution: Numerical modelling, upscaling, and application for strain localization

Viscous flow controls large parts of tectonic deformation. Viscous strain localization and associated softening mechanisms are important for subduction initiation and the generation of tectonic nappes. However, viscous flow of geologic materials can have a complex behaviour due to their evolving microstructure, such as an evolving anisotropy due to fabric development or a crystallographic preferred orientation, or due to other evolving microstructure, like, e.g., grain size or dynamic recrystallization.

In this contribution, we focus on strain localization in viscous rock due to the generation of anisotropy resulting from fabric evolution. Particularly, we focus on multi-scale anisotropy evolution in shear zones with many strong or weak inclusions, representing for example porphyroclasts. The shape change and relative alignment of the inclusions during shearing generates an anisotropy on the scale of the inclusions, termed here macroscale. We spatially resolve this macroscale anisotropy in the numerical simulations. Additionally, we consider the evolution of a microscale anisotropy in the shear zone matrix, representing the formation of a mylonitic foliation. We do not spatially resolve this microscale anisotropy but model it with an anisotropic flow law that involves different normal and tangential viscosities. We calculate the finite strain ellipse during shearing and use its aspect ratio as proxy for the anisotropy that governs the ratio of normal to tangential viscosity. To track the orientation of the anisotropy during deformation we apply a director method.

We perform numerical simulations with the two-dimensional state-of-the-art thermo-mechanical code MDoodz (Duretz et al. 2021). We evaluate the impact of micro- and macroscale anisotropy on strain softening and localization in shear zone up to shear strains in the order of ten. We further discuss the quantification of effective anisotropies that can be used for upscaling, for example for lithospheric scale numerical models. Moreover, we compare the numerical results to the analytical solution and the numerical results of Dabrowski et al. (2012). A particular feature of some simulations is the formation of buckle folds in regions with highly stretched weak inclusions.

 

Bibliography

Duretz T., R. de Borst and P. Yamato (2021), Modeling Lithospheric Deformation Using a Compressible Visco-Elasto-Viscoplastic Rheology and the Effective Viscosity Approach, Geochemistry, Geophysics, Geosystems, Vol. 22 (8), e2021GC009675

Dabrowski, M., D. W. Schmid, and Y. Y. Podladchikov (2012), A two-phase composite in simple shear: Effective mechanical anisotropy development and localization potential, J. Geophys. Res., 117, B08406, doi:10.1029/2012JB009183