We test models for the origin of intraplate volcanic ridges and gravity lineations on young seafloor west of the East Pacific Rise using Rayleigh wave dispersion measured in the Gravity Lineations and Intraplate Melting Petrology and Seismic Expedition (GLIMPSE) seismic experiment. The excellent azimuthal distribution of teleseismic sources recorded over a 12-month period provides resolution of phase velocities at periods up to 100 s. The average phase velocities for the study area reveal a pronounced low-velocity zone reaching a minimum shear velocity of ∼3.95 km/s. The negative velocity gradient defining the base of the lithosphere, observed at 40 ± 15 km, abruptly reverses at 70 km depth. The underlying positive gradient changes slope at ∼125 kin. We attribute these changes in gradient to the onset of incipient partial melting of upwelling mantle in the presence ofwater at 125 km, followed by increased melt production at 70 km that leads to dehydration of the residual matrix and migration of melt to the surface spreading center. Rayleigh wave tomography shows that there are anomalously low shear velocities extending to at least 50 km depth beneath the Sojourn Ridge and the Hotu Matua volcanic complex, with relatively high velocities between these volcanic chains. These observations are not consistent with passive models for the origin of the volcanic ridges involving lithospheric extension or thermoelastic cracking. Dynamic models invoking flow in the asthenosphore in the form of small-scale convection or viscous fingering instabilities may explain the observed pattern of seismic velocity anomalies.