Abstract
AbstractSeismic 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 ξ = = 1.13–1.16 at ∼100 km depth to the west of the EPR than to the east (ξ = 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 ∼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 ξ < 1.05 in the west at ∼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.
Highlights
Earth's mantle structure has long been investigated through seismic tomography
We find that current tomographic models such as S362WMANI and SGLOBE-rani underpredict the magnitude of radial anisotropy in the young Central Pacific and to the east of the East Pacific Rise (EPR)
The inversion of the new surface wave phase misfit data presented here reveals that three linear anomalies of positive radial anisotropy along the west coast of South America, west of the EPR and around and south of Hawaii become more pronounced and defined than in the original models
Summary
Earth's mantle structure has long been investigated through seismic tomography. There is currently largescale agreement among 3-D isotropic mantle models, at least at shallow depths, such as low seismic wave velocities associated with oceanic ridges and high velocities linked with cratons (e.g., Chang et al, 2014).Due to the enormous expansion of seismic data sets and advances in computing technology we are able to image more complex and realistic properties than isotropy, such as anisotropy and attenuation.In particular, radial anisotropy, the difference between horizontally and vertically polarized shear waves, is a powerful tool to probe the direction of mantle flow. There is currently largescale agreement among 3-D isotropic mantle models, at least at shallow depths, such as low seismic wave velocities associated with oceanic ridges and high velocities linked with cratons (e.g., Chang et al, 2014). The alignment of mineral grains into a lattice-preferred orientation (LPO) by large-strain deformation such as mantle flow is thought to be the main mechanism behind large scale seismic anisotropy in the upper mantle Positive anomalies of radial anisotropy (VSH > VSV) currently observed beneath the Pacific oceanic lithosphere are associated to first order with horizontal flow (e.g., Chang et al, 2014).
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