A Natural Example of Olivine LPO Variation With Shear Strain

Understanding mantle rheology, seismic anisotropy, and strain localization requires knowledge of the evolution of olivine lattice preferred orientations (LPOs) as a function of strain and melt content. The Josephine Peridotite in southwest Oregon is ideal for analysis of the interaction of deformation and melt, due to the presence of shear zones with associated melt migration structures. We present results on the variation of olivine LPO with shear strain in these shear zones. The LPO evolution with proximity to the shear zone center provides a link between experimental data and LPO evolution models, which can be applied to the interpretation of mantle seismic anisotropy and to models of melt extraction at mid-ocean ridges. Results for harzburgites from the largest Josephine shear zone, which is 100m wide and contains syn-deformational dunite, indicate that the olivine LPO rotates in the shear zone so that the [100] maxima lies parallel to the shear direction. Outside of the shear zone, the harzburgites have a pre-existing LPO. Shear strain for the Josephine samples is calculated from the deflection angle of the regional pyroxene foliation. By ~100% shear strain, the olivine LPO evolves so that the [100] axis is aligned with the flow direction. These results on olivine LPO evolution during shear agree with the experimental data of Zhang and Karato (1995). They extend observations of olivine LPO to much higher strains and lower stresses. At face value, the Josephine shear zone data also agree with predictions from polycrystal plasticity models that incorporate the effects of dynamic recrystallization (Ribe and Yu, 1991; Wenk and Tomé, 1999). The variation of olivine LPO during simple shear is important for correctly interpreting seismic anisotropy. Our results indicate how much strain is necessary for olivine LPO to align in the flow direction. In conjunction with modelling studies, this information can be used to interpret the kinematics of deformation in regions where mantle anisotropy varies spatially.