Modulation of Plate Motions by Viscosity Anisotropy in the Asthenopshere

Shear deformation within Earth's low-viscosity asthenosphere accommodates differential motion between plate motions and viscous flow within the Earth's mantle. Depending on whether mantle flow leads plate motions or vice-versa, the shear tractions that are transmitted through this viscous layer exert either a driving or resisting force on tectonic plate motions, the magnitude of which depends on the viscosity of the asthenosphere. However, asthenospheric minerals, particularly olivine, have been shown to exhibit effectively anisotropic viscous behavior that should influence the interaction between plate motions and mantle flow. In particular, the rotation of olivine a-axes into the direction of asthenospheric shear induces a reduced effective viscosity in the direction of shear compared to the viscosity that applies for deformation perpendicular to the shear direction. This reduction of viscosity in the direction of asthenospheric shear should affect the response of plate motions to perturbations to the tectonic forces that drive the plates. To determine the timescales and magnitudes of plate motion changes that are possible above an anisotropic asthenosphere, we tracked the crystallographic axes of olivine grains within a simple model of a shearing asthenosphere, and computed the resulting time-dependent anisotropic viscosity. We found that anisotropic viscosity develops only after accumulated strains exceed 100-200%, which requires tens of millions of years or more depending on magnitude of shear and the ratio of asthenospheric viscosity in the shear direction compared to that in perpendicular directions. Because changes in plate motion involve deformation perpendicular to the low-viscosity direction, the presence of anisotropic viscosity tends to stabilize plate motions to changes in plate tectonic driving forces. Gradual changes in the anisotropic structure of the asthenosphere can lead to slow evolution of plate motions over tens of millions of years, although more rapid variations may be possible for cases in which the shear direction is non-uniform beneath the area of the plate. Non-uniform shear is expected if patterns of mantle flow are not uniformly aligned with surface plate motions, or if the shear direction varies with depth within the asthenosphere due to the presence of pressure-driven (Poiseuille) flow. As both of these conditions have been demonstrated in numerical models of mantle flow constrained by observations of seismic anisotropy, we speculate that the complexity of global mantle flow and plate motions permits relatively rapid changes in plate motions despite the presence of anisotropic viscosity in the asthenosphere.