UC Santa Barbara

Abstract We present tomographic images of the velocity structure of the upper mantle beneath old ( 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 and tomographic models is consistent with either the presence of melt and K thermal anomalies or, if melt free, K thermal anomalies. and 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. Plain Language Summary At several locations in the oceans, parallel, and alternating linear features can be observed in measurements of gravitational acceleration. Understanding the origin of such features enables us to better understand the evolution of tectonic plates. To study the Earth's interior at one of these locations, we acquired data from an array of seismometers placed there. Using measurements of the times at which seismic waves from distant earthquakes were observed at these seismometers, we developed three‐dimensional images of how fast seismic waves travel. We found significant variations in the speeds shown in these images, which also show oriented features roughly parallel to the gravity anomalies. These images provide evidence for rising and sinking material in the Earth's interior at this location. This evidence, along with previous work, supports a hypothesis whereby this convective process may be common, particularly beneath mature oceanic plates. Key Points High‐resolution body‐wave imaging of the upper mantle below old oceanic lithosphere in a region characterized by free‐air gravity lineations Substantial heterogeneity in the asthenospheric mantle implies thermal anomalies and potentially melt These results add to growing observational evidence of top‐driven small‐scale convection beneath oceanic lithosphere