Rayleigh Wave Tomography Study of the Oceanic Upper Mantle Beneath Intraplate Volcanic Chains West of the East Pacific Rise

In order to investigate the origin of intraplate volcanic ridges in the GLIMPSE study area on the Pacific plate west of the East Pacific Rise and their possible association with cross-grain gravity lineations, we conducted a passive seismic experiment employing a year-long deployment of ocean bottom seismometers. We present Rayleigh wave tomography results for periods between 16 and 100 s, with more abundant observations below 60 s. The average phase velocity as a function of frequency in the study area falls between dispersion curves for 0 - 4 Ma and 4 - 20 Ma from previous studies (Nishimura and Forsyth, 1988), consistent with seafloor age in this region. Phase velocities decrease from 20 to 33 s, indicating the presence of a thin lithosphere. Velocities increase sharply at periods beyond 40 s as the bottom of the low velocity zone is detected. Two-dimensional phase velocity maps indicate anomalously low phase velocities beneath the eastern end of the Sojourn ridge extending eastward beneath the smaller, newly mapped Brown ridge. In this area of recent, off-axis volcanic activity, anomalous shear wave velocities in the mantle must extend nearly to the base of the crust. The velocities are much less anomalous beneath the other recently active region, the Hotu-Matua volcanic complex, where melting must be deeper and/or confined to a narrower region to avoid detection. High velocities are observed northwest of the Sojourn ridge associated with an offset in seafloor age across the Garrett fracture zone. High velocities are also observed between the Sojourn ridge and Hotu-Matua in the western part of the study area. Azimuthal anisotropy averaged over the study area is well resolved, with Rayleigh waves for periods between 16 and 40 s traveling 3.5{%} faster perpendicular to the ridge than parallel to it. With this degree of anisotropy, the anisotropic layer must be on the order of 200 km thick to explain the shear wave splitting delays.