Constraints on the Upper Mantle Structure Beneath the Pacific From 3-D Anisotropic Waveform Modeling

Seismic radial anisotropy is a crucial tool to help constrain flow in the Earth's mantle. However, Earth structure beneath the oceans imaged by current 3-D radially anisotropic mantle models shows large discrepancies. Here, we provide constraints on the radially anisotropic upper mantle structure beneath the Pacific by waveform modeling and subsequent inversion. Specifically, we objectively evaluate three 3-D tomography mantle models which exhibit varying distributions of radial anisotropy through comparisons of independent real data sets with synthetic seismograms computed with the spectral-element method. The data require an asymmetry at the East Pacific Rise (EPR) with stronger positive radial anisotropy xi = VSH2VSV2 = 1.13-1.16 at similar to 100 km depth to the west of the EPR than to the east (xi = 1.11-1.13). This suggests that the anisotropy in this region is due to the lattice-preferred orientation of anisotropic mantle minerals produced by shear-driven asthenospheric flow beneath the South Pacific Superswell. Our new radial anisotropy constraints in the Pacific show three distinct positive linear anomalies at similar to 100 km depth. These anomalies are possibly related to mantle entrainment at the Nazca-South America subduction zone, flow at the EPR and from the South Pacific Superswell and shape-preferred orientation (SPO) of melt beneath Hawaii. Radial anisotropy reduces with lithospheric age to xi < 1.05 in the west at similar to 100 km depth, which possibly reflects a deviation from horizontal flow as the mantle is entrained with subducting slabs, a change in temperature or water content that could alter the anisotropic olivine fabric or the SPO of melt.

Automated summary

In this study, constraints on the radially anisotropic upper mantle structure beneath the Pacific were provided through waveform modeling and inversion. The data show asymmetry at the East Pacific Rise, suggesting that anisotropy in the region is due to shear-driven asthenospheric flow beneath the South Pacific Superswell. Additionally, distinct positive linear anomalies at around 100 km depth in the Pacific may be related to mantle entrainment at subduction zones, flow at the East Pacific Rise, and shape-preferred orientation of melt beneath Hawaii.