The influence of water and LPO on the initiation and evolution of mantle shear zones

We present data from the Josephine Peridotite (SW Oregon, USA) that constrain the underlying physical processes responsible for the initiation of shear localization and the evolution of ductile shear zones in Earth's mantle. Field measurements of narrow (2-60. m wide) ductile shear zones in harzburgite were used to construct strain profiles, which have maximum shear strains ranging from γ=5.25 to γ>20. Measurements of pyroxene water concentrations from harzburgite samples within and immediately adjacent to the shear zones indicate that gradients in water concentration exist on a 10-100. m scale, even after exhumation. Water concentration measurements are correlated with olivine lattice-preferred orientation (LPO), corroborating experimental results on the influence of water on slip system activity. Using empirical olivine flow laws and the diffusivity of water in olivine, we model initiation of a ductile shear zone through localized water weakening. We demonstrate that this mechanism can readily generate spatial perturbations in both effective viscosity and strain. However this model is not able to reproduce both the observed shear strain gradients and water concentration data from the Josephine shear zones. We evaluate other plausible localization mechanisms, which may amplify this initial strain perturbation. The most relevant at these conditions is the development of viscous anisotropy associated with the evolution of olivine LPO. Using recent experimental results, we demonstrate that progressive rotation of olivine LPO into the shear plane enhances deformation within a shear zone. We conclude that feedback between at least two microphysical processes is needed to account for observed outcrop-scale shear localization.