Observations of Small-Scale Heterogeneity in the Upper Mantle Beneath Old Oceanic Lithosphere

We present tomographic images of the velocity structure of the upper mantle beneath old ((Formula presented.) Ma) oceanic lithosphere, derived from arrival time measurements of teleseismic body waves recorded by ocean-bottom seismometers. Differential travel time measurements on P, S, and SKS body waves across a wide range of frequency bands show consistent geographic and backazimuthal variations that highlight substantial velocity heterogeneity in the upper mantle beneath the array. A joint inversion of all data sets shows that the information in the P and S times are consistent with each other. The amplitude of apparent velocity heterogeneity and the scaling between independently constructed (Formula presented.) and (Formula presented.) tomographic models is consistent with either the presence of melt and (Formula presented.) (Formula presented.) K thermal anomalies or, if melt free, (Formula presented.) K thermal anomalies. (Formula presented.) and (Formula presented.) tomographic models both show statistically meaningful elongation of structure along an approximately NW-SE axis, subparallel to the orientation of free-air gravity lineations and absolute plate motion. We interpret the overall structure as evidence for convective processes associated with the delamination of old lithosphere. Our work adds to a growing body of observational evidence that highlights strong heterogeneity in the oceanic upper mantle, potentially suggesting widespread small-scale convection.