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 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 with laboratory measurements of olivine aggregates. This
model characterizes the time-dependent coupling between large-scale
formation of lattice-preferred orientation (i.e., texture) and changes
in asthenospheric viscosity for a series of simple deformation paths
that represent 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. Our models indicate 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 could impede changes to
the plate motion direction for 10s of Myrs. However, the same
well-developed asthenospheric texture may foster subduction initiation
perpendicular to the plate motion and deformations related to transform
faults, as shearing on vertical planes seems to be favored across a
sub-lithospheric olivine texture. 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.