Seismic anisotropy is caused mainly by the lattice-preferred orientation of anisotropic minerals. Major breakthroughs have occurred in the study of lattice-preferred orientation in olivine during the past ∼10 years through large-strain, shear deformation experiments at high pressures. The role of water as well as stress, temperature, pressure, and partial melting has been addressed. The influence of water is large, and new results require major modifications to the geodynamic interpretation of seismic anisotropy in tectonically active regions such as subduction zones, asthenosphere, and plumes. The main effect of partial melting on deformation fabrics is through the redistribution of water, not through a change in deformation geometry. A combination of new experimental results with seismological observations provides new insights into the distribution of water associated with plume-asthenosphere interactions, formation of the oceanic lithosphere, and subduction. However, large uncertainties remain regarding the role of pressure and the deformation fabrics at low stress conditions. 59

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We conducted a microstructural study of a high-strain mantle shear zone from the Josephine Peridotite, SW Oregon, USA. The goal of this study is to understand how microstructural evolution at large strains leads to transitions in rheological behavior. The shear zone we investigated exhibits higher strain and greater localization than previously studied shear zones in the Josephine Peridotite. The margins of the shear zone have a homogeneous microstructure, characterized by moderately strong olivine fabrics, fairly weak orthopyroxene fabrics, and grain sizes of 2^3 mm. The highly deformed samples from the center of the shear zone display two distinct microstructural domainsça relatively coarse-grained domain ($550 m) that contains only olivine and a finer-grained domain ($250 m) that contains both olivine and orthopyroxene. The coarse-grained domain has a strong E-type olivine lattice-preferred orientation (LPO). Within the fine-grained domain the olivine LPO is also E-type, but significantly weaker.The E-type fabrics are rotated slightly past the shear plane, providing the first field-based confirmation of similar experimental observations. The presence of E-type fabrics, which form in the presence of moderate quantities of water, also highlights the potential importance of water to shear zone evolution.The orthopyroxene in the fine-grained domains has no LPO, suggesting that a transition to grain-size sensitive deformation occurred. The microstructural transition in orthopyroxene may have resulted in a marked weakening of the rock, suggesting that orthopyroxene plays a critical role in shear localization. These samples provide a crucial microstructural link between moderately localized shear zones and highly deformed ultramylonites.

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