Fluid-like movements in the Earth’s mantle largely depend on the rheological behaviour of olivine, the main rock-forming mineral in the upper mantle. The most effective deformation mechanism in the uppermost mantle is dislocation creep, which orients olivine crystals into a direction parallel to the flow, forming a crystallographic preferred orientation (CPO) (Karato and Wu, 1993). We know that olivine CPO is abundant because its presence can be detected using a variety of seismological observations. Indeed, such observations are important indicators of mantle deformation associated with moving lithospheric plates and vigorous flow around subduction zones (Long and Becker, 2010). Importantly, rock mechanics experiments have demonstrated that olivine crystals are also anisotropic in their viscous properties (Durham and Goetze, 1977; Hansen et al., 2016, 2012), and hence when the crystals align with each other the upper mantle's viscosity should be direction-dependent. Anisotropic viscosity (AV) should significantly affect a variety of geodynamic processes by accelerating or slowing their development depending on the CPO in the mantle.
Simplified numerical models showed that the evolution of olivine CPO and anisotropic viscosity are inter-linked (Király et al., 2020). When CPO is well developed we can have fast deformation parallel to its primary orientation, but if stresses are acting perpendicular to it, deformation can significantly slow down. This translate to for example, a large dependency of plate velocity on the CPO and stress orientations in the asthenosphere underneath the tectonic plates.