The effect of deformation history on the interpretation of seismic anisotropy in the upper mantle: experiments and numerical modeling

Abstract:

The main source of seismic anisotropy in the upper mantle is the deformation-induced crystallographic preferred orientation (CPO) of olivine. The interpretation of seismic anisotropy relies on models and experiments that predict certain relationships between olivine CPO and the deformation kinematics. Under some conditions, such as the interiors of oceanic plates, these relationships may be quite simple. However, near plate boundaries flow patterns are complex and the interpretation of seismic anisotropy is not straight-forward. In this contribution we describe the effect of deformation history on the re-orientation of olivine CPO as a function of strain. High pressure and temperature deformation experiments were performed on the Åheim dunite, which exhibits a pre-existing texture. Experiments were conducted in three different configurations with the pre-existing foliation at 0°, 45°, and 90° to the axis of compression, simulating three unique deformation histories. Deformation microstructures and texture are analyzed using electron backscatter diffraction (EBSD). The experiments show that up to strains of ~0.7 the three configurations evolve differently from one another, and from models initiated with random textures. Moreover, none of the models achieved the expected textural steady-state. The experiments results are then compared to numerical simulations using a Viscoplastic Self Consistent (VPSC) approach, and D-Rex. The input for the models is the Åheim dunite CPO in the three configurations used in the experiments. It is shown that, generally, texture symmetry evolves similarly in the models and the experiments although there are notable differences in texture strength. To achieve better agreement between experiments and models, new model parameterizations are proposed. Finally, we use the new parameterization of D-Rex to simulate a range of plausible deformation histories and associated seismic anisotropy in a variety of flow settings.