Anisotropy in the Pacific Upper Mantle from Inversion of a Combined Global and Regional Dispersion Dataset

Abstract:

Models of seismic anisotropy in oceanic regions have the potential to provide information about the geometry of strain and flow in the mantle, the specific nature of the lithosphere-asthenosphere boundary, and the possible presence of partial melt in the asthenosphere. In order to investigate these and other questions, we are developing a three-dimensional model of the anisotropic velocity structure of the Pacific upper mantle. We use measurements of fundamental-mode dispersion for Rayleigh and Love waves traversing oceanic paths. These observations are drawn from the waveform dataset used to construct the global dispersion model GDM52. To supplement this global dataset, we make additional measurements of surface-wave dispersion on waveform data from the NoMelt experiment, a deployment of broadband ocean-bottom seismometers on ~70 Ma lithosphere between the Clarion and Clipperton fracture zones in the central Pacific. The shorter oceanic paths sampled by this dataset help improve the resolution of the velocity model in the Pacific by providing a regional constraint on the larger plate-scale model. We invert phase-velocity maps from the combined dataset for velocity structure at depth. An oceanic reference model is used to compute the sensitivity kernels. We invert simultaneously for isotropic velocity structure and radial and azimuthal anisotropy beneath the Pacific Basin. The resulting anisotropic velocity model will improve constraints on olivine fabrics and strain geometries in the oceanic upper mantle.