Influence of anisotropic viscosity on mantle flow patterns near plate boundaries-- implementation for general, flow-induced olivine texture

The elastic effects of flow-induced lattice-preferred orientation of mantle minerals (seismic anisotropy) have received a lot of attention but the corresponding rheologic effects have received much less attention. In this study we assess the scale of feedbacks between upper mantle flow pattern and rheologic anisotropy such as is predicted to develop via deformation along specific flowlines. We are developing an iterative approach that starts with an initially isotropic viscosity tensor and reference flow field. Based on textures predicted for this scenario, we compute the associated viscosity of the mineral aggregate (directly, no simplifying assumption about directionality) and update the viscosity tensor. In low strain rate portions of the flow field, the deviation from the isotropic viscosity tensor is small but at strain rates typical for points of inflection in the flow (e.g. the ‘corner’ associated with a spreading center axis) notable rheologic anisotropy is predicted. For our initial tests, we determine the viscosity tensor for textured aggregates from representative areas within the flow field. These representative tensors are used to update the viscosity tensor in each respective regions and flow field iteration continues. Tests of both this basic assessment of scale as well as of our eventual setup, where viscosities will be updated at every node point in the finite-element model based on deformation along flowlines leading up to that point in the model, are underway at present. At the meeting, we will present a series of results illustrating the feedback between texturing and local flow pattern. Based on the magnitudes of rheologic anisotropy for isotropically-based flow field, we expect these will not be negligible in the high velocity gradient regions near the plate boundary.