Extent of the South Pacific Superswell

The South Pacific Superswell is known as a broad area beneath French Polynesia characterized by numerous volcanic chains and very shallow seafloor compared to the depth predicted for its age by classical seafloor subsidence models. So far, its exact extent has not yet been established. Thanks to better bathymetric coverage and a specially adapted filtering method, we present here a new, complete, and precise mapping of the superswell. We show that the superswell covers a region broader than previously expected. It extends between latitudes 10 degrees N and 30 degrees S and longitudes 130 degrees W and 160 degrees W and has a maximum amplitude of 680 m. It is composed of two branches that display different characteristics when compared to other geophysical observations. Under the southern branch a dip in the geoid is observed which could be linked to upwelling in a convective mantle where the low-velocity zone is located immediately below the lithosphere as indicated by tomography models. Under the northern branch, there is a 12 m high in the geoid surface anomaly that may be explained by isostatic compensation of a mass deficiency located in the mantle at 300 km depth. The northern branch could also be related to another superplume that is now going down but which was rising up 30-90 m.y. ago, when it initiated the secondary plumes which created the Line Islands.