Abstract

Asthenospheric shear causes some minerals, particularly olivine, to develop anisotropic textures that can be detected seismically. In laboratory experiments, these textures are also associated with anisotropic viscous behavior, which should also be important for geodynamic processes. To examine the role of anisotropic viscosity for asthenospheric deformation, we developed a numerical model of coupled anisotropic texture development and anisotropic viscosity, both calibrated according to laboratory measurements of olivine aggregates. This model characterizes the time-dependent coupling between large-scale formation of LPO textures and changes in asthenospheric viscosity for a series of deformation paths that are representative of upper-mantle geodynamic processes. We find that texture development beneath a moving surface plate tends to align the a-axes of olivine into the plate motion direction, which weakens the effective viscosity in this direction and increases plate velocity for a given driving force. We demonstrate that the effective viscosity increases for shear in the horizontal direction perpendicular to the a-axes. This increase should slow plate motions and new texture development in this perpendicular direction, and can impede changes to the plate motion direction for 10s of Myrs. However, the same well-developed asthenospheric texture should foster both subduction initiation and lithospheric gravitational instabilities as vertical deformation is favored across a sub-lithospheric olivine texture, and the sheared texture can quickly rotate into a vertical orientation. These end-member cases examining shear-deformation in the presence of a well formed asthenospheric texture illustrate the importance of the mean olivine orientation, and its associated viscous anisotropy, for a variety of geodynamic processes.