Sub-Lithospheric Small-Scale Convection Tomographically Imaged Beneath the Pacific Plate

Small-scale convection beneath the oceanic plates has been invoked to
explain off-axis non-plume volcanism, departure from simple seafloor
depth-age relationships, and intraplate gravity lineations. We deployed
thirty broadband OBS stations on ~40 Ma Pacific seafloor in a region
notable for gravity anomalies, measured by satellite altimetry, elongated
parallel to plate motion. P-wave teleseismic tomography reveals
alternating upper mantle velocity anomalies on the order of ±2%, aligned
with the gravity lineations. These features, which correspond to ~300-500
˚K lateral temperature contrast, and possible hydrous or carbonatitic
partial melt, are – surprisingly – strongest between 150 and 260 km depth,
indicating rapid vertical motions through a low-viscosity asthenospheric
channel. Coherence and admittance analysis of gravity and
topography using new multibeam bathymetry soundings substantiates
the presence of mantle density variations, and forward modelling predicts
gravity anomalies that qualitatively match observed lineations. This study
provides observational support for small-scale convective rolls beneath
the oceanic plates.