Sub-Lithospheric Small-Scale Convection Tomographically Imaged Beneath
the Pacific Plate
Abstract
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 seafloor in the
equatorial Pacific, in a region notable for gravity anomalies measured
by satellite altimetry elongated in the direction of plate motion.
P-wave teleseismic tomography reveals alternating upper mantle velocity
anomalies on the order of ±2%, oriented parallel to the gravity
lineations. These features, which correspond to ~300-500
˚K lateral temperature contrast, and possible hydrous or carbonatitic
partial melt, are strongest between 150 and 260 km depth, indicating
rapid vertical motions through a low-viscosity asthenospheric channel.
Coherence and admittance analysis using new multibeam bathymetry
soundings substantiates the presence of asthenospheric density
variation, 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.