Laboratory measurements of the viscous anisotropy of olivine aggregates

Measurements of the viscous anisotropy of highly deformed polycrystalline olivine find it to be approximately an order of magnitude larger than that predicted by grain-scale simulations; the maximum degree of anisotropy is reached at geologically low shear strain, such that deforming regions of the Earth’s upper mantle should exhibit significant viscous anisotropy. Viscous anisotropy of the mantle The viscous anisotropy of the rocks of the Earth's mantle strongly affects many tectonic-scale processes. Here, Lars Hansen et al . present measurements of the viscous anisotropy of highly deformed polycrystalline olivine, a dominant mineral in the Earth's crust, and find that the anisotropy is approximately an order of magnitude larger than that predicted by grain-scale simulations. The maximum degree of anisotropy is reached at geologically low shear strain, such that deforming regions of the upper mantle should exhibit significant viscous anisotropy. The discrepancy between numerical simulations and laboratory experiments highlights the limitations of current models of anisotropy, and these results provide important constraints for future geodynamic simulations. A marked anisotropy in viscosity develops in Earth’s mantle as deformation strongly aligns the crystallographic axes of the individual grains that comprise the rocks. On the basis of geodynamic simulations, processes significantly affected by viscous anisotropy include post-glacial rebound 1 , 2 , foundering of lithosphere 3 and melt production above subduction zones 4 . However, an estimate of the magnitude of viscous anisotropy based on the results of deformation experiments on single crystals 5 differs by three orders of magnitude from that obtained by grain-scale numerical models of deforming aggregates with strong crystallographic alignment 6 , 7 , 8 . Complicating matters, recent experiments indicate that deformation of the uppermost mantle is dominated by dislocation-accommodated grain-boundary sliding 9 , a mechanism not activated in experiments on single crystals and not included in numerical models. Here, using direct measurements of the viscous anisotropy of highly deformed polycrystalline olivine, we demonstrate a significant directional dependence of viscosity. Specifically, shear viscosities measured in high-strain torsion experiments are 15 times smaller than normal viscosities measured in subsequent tension tests performed parallel to the torsion axis. This anisotropy is approximately a