The anisotropic and rheological structure of the oceanic upper mantle from a simpe model of plate shear

We have developed a channel flow model that dynamically couples plate motion and mantle stress with a composite rheology (diffusion creep and dislocation creep) to study the rheological and anisotropic structures of the oceanic upper mantle. A semi-analytic approach is used to solve for mantle stress and viscosity, allowing fast calculations and exploration of a wide range of rheological parameters. Mantle stress in our model is due to shearing by a moving plate. By comparing mantle stress with a transition stress for dislocation creep, we identify regions where either diffusion creep or dislocation creep is active. Deformation by dislocation creep results in a mineral fabric that may be responsible for observed seismic anisotropy. Our study suggests that there is an important relation between plate motion, seismic anisotropy, mantle viscosity and transition stress. Using laboratory results for rheological parameters, we find that dislocation creep exists only in a layer at certain depths in the upper mantle. For a plate velocity of 10 cm yr^-1, an asthenospheric viscosity of 10¹⁹ Pa s and an asthenospheric transition stress of 0.1 MPa, our model predicts a ∼200 km thick dislocation creep layer, which is broadly consistent with the observations of seismic anisotropy. For a plate velocity of 10 cm yr^-1 and an asthenospheric transition stress of 0.1 MPa, the asthenospheric viscosity needs to be greater than 5 × 10¹⁸ Pa s to produce any dislocation creep deformation, and the asthenospheric viscosity needs to be larger for slower plate motion or larger transition stress. Slower plate motion leads to a thinner dislocation creep layer, which may partially explain the observed asymmetry in anisotropic structure in the East Pacific Rise.