The effect of deformation history on the texture and seismic anisotropy of the upper mantle: Results from experiments and numerical modeling

Abstract:

Olivine crystallographic preferred orientation (CPO) is the main cause for the seismic anisotropy observed in the upper mantle. Numerical models of texture evolution are frequently used to interpret seismic anisotropy or to predict the seismic anisotropy generated by a particular pattern of mantle flow. However, the relationship between mantle flow and seismic anisotropy is non-trivial in regions with complex deformation kinematics, for example along subduction-zones and mid-oceanic ridges. Here we examine the effect of deformation history on the process of texture and seismic anisotropy development from both experiments and numerical models.

Samples from the Åheim dunite, which has a relatively strong pre-existing CPO, were deformed in axial compression using a Griggs apparatus at T = 1200°C, P ~1 GPa. Three different sample orientations were investigated, with the compression axis perpendicular, oblique, or parallel to the foliation plane. Olivine preferred orientation was determined by EBSD and quantified using three elements of texture: strength, symmetry, and orientation. We have found that that the evolution of olivine CPO depends strongly on the initial sample orientation. In addition, none of these experiments show the development of a clear steady-state texture and mechanical behavior.

The experimental results are compared to numerical models of texture development using the viscoplastic self consistent approach (VPSC). In these models we used the initial texture measured from the Åheim dunite rotated into three different orientations, to reproduce the starting conditions of the laboratory experiments. We find some similarities between the model and the experiments, although there are subtle and important differences. Future modeling work will attempt to define a set of parameters that allow numerical models of texture evolution to reproduce more precisely experimental results.