Evolution of the Josephine Peridotite Shear Zones: 2. Influences on Olivine Cpo Evolution

Seismic anisotropy arises in the upper mantle due to the alignment of olivine crystal lattices and is often used to interpret mantle flow direction. Experiments on the evolution of olivine crystal-preferred orientation (CPO) have found that the texture that develops is dependent on many factors, including water content, differential stress, preexisting CPO, and deformation kinematics. To evaluate the role of these factors in naturally deformed samples, we present microstructural transects across three shear zones in the Josephine Peridotite. Samples from these shear zones exhibit a mixture of A-type textures, which have been associated with dry conditions and primary activation of the olivine [100](010) slip system, and of E-type textures, which have been associated with wetter conditions and primary activation of the [100](001) slip system. CPOs with characteristics of both A-type and E-type textures are also present. CPO type does not evolve systematically as a function of either strain or water content. We used a micromechanical model to evaluate the roles of preexisting texture and kinematics on olivine CPO evolution. We find that the preexisting texture controls CPO evolution at strains up to 5 during simple shear. Kinematics involving a combination of simple shear and pure shear can explain the olivine CPOs at higher strain. Hence, preexisting CPOs and deformation kinematics should be considered in the interpretation of CPOs measured in naturally deformed rocks and of large-scale patterns in upper-mantle seismic anisotropy.