Love-wave propragation in oceanic upper mantle: constraints on radial anisotropy and implications for dynamics of the asthenosphere
DESCRIPTION: The mechanisms that enable plate-like behavior on the Earth's surface and the processes that control plate motion are not fully understood. This study uses earthquake-generated seismic waves that were recorded by seafloor seismometers deployed for year in the central Pacific to probe the structure, particularly near the base of the plate. Using known relationships between deformation-induced mineral alignment and its effect on seismic signature, the degree of coupling between the plate and the underlying mantle will be evaluated. The question of how a rigid tectonic plate differs from the underlying mantle and whether or not these materials move in unison at the base of the plate, or not, has long intrigued Earth scientists. It is at the heart of understanding plate tectonics. The graduate student supported by this award will receive training in forefront marine seismic data analysis and have the opportunity to work with a unique dataset. Strong azimuthal seismic anisotropy in the Pacific lithosphere is consistent with observations of olivine alignment found in ophiolites, and it constrains models of ocean spreading center dynamics. In contrast, high-amplitude radial anisotropy observed in the Pacific asthenosphere provides evidence for a highly deformed and/or partially molten layer beneath the plate that may decouple the plate from the underlying mantle. A 600x400 km ocean bottom seismometer (OBS) array, located on ~70 Ma lithosphere, provided high-quality broadband seismic data, sufficient to characterize anisotropy with resolution (in depth and laterally) that is unattainable from global analyses. Rayleigh-wave velocities indicate extremely strong azimuthal anisotropy developed during formation of the lithosphere, but notably weaker azimuthal anisotropy is indicated in these data for the underlying asthenosphere. Determining the corresponding depth distribution of radial anisotropy requires detailed analysis of Love waves. Using a novel analysis of the wavefield, Love wave fundamental- and higher-mode phase velocities will be measured across the OBS array. Combined with the existing azimuthal anisotropy constraints, the resulting estimates of anisotropy will allow us to explicitly test whether flow-induced olivine fabric is consistent with the observations, or whether oriented melt is required to explain the observations.