Constraints on Olivine Deformation Mechanisms from SKS Shear-Wave Splitting beneath the High Lava Plains and Northern Basin and Range

Abstract:

Shear-wave splitting observations of SKS and SKKS phases have been used widely to map upper mantle anisotropy. However, the weak depth resolution led to ambiguous interpretations on the origin of the observed anisotropy, and it impeded further constraints on the lattice-preferred orientation of minerals. The non-vertical incidence of the core phases can reduce this ambiguity, since it is expected to cause an azimuthal variation of splitting parameters that can serve as additional constraint on the type of anisotropy. We have shown previously that single-layer cases of asthenospheric and lithospheric anisotropy can indeed be distinguished, if enough events have been recorded. The different orientation of the shear plane, and the associated effective anisotropy, produces a phase shift in the azimuthal variation of fast orientation. However, deformation conditions in the upper mantle affect the fabric of olivine-rich rock in the mantle, and we need to distinguish especially A-, C-, and E-type olivine. Differentiating between these cases can help to constrain the water content in the upper mantle. In this study, we thus use the above-mentioned approach to predict the azimuthal variation of splitting parameters for A-, C-, and E-type olivine, and match them with observations from the High Lava Plains and the Northern Basin and Range region, an area known for a consistent pattern of fast orientation, and increased splitting delay in the back-arc of the Cascadia Subduction Zone. Comparing expected and observed variation renders C-type olivine unlikely; a differentiation between A- and E-type olivine remains difficult though. However, the conformance in the amplitude of the azimuthal variation, and the potential to explain larger splitting values, suggest the occurrence of E-type olivine in a hydrated upper mantle.