Short‐wavelength geoid, bathymetry and the convective pattern beneath the Pacific Ocean

SUMMARY The geoid and bathymetry data in the Pacific Ocean are used to shed new light on the structure of upper mantle convection. At short wavelengths (200 km), the geoid pattern is characterized by parallel lineations. Lineations however appear less clearly at larger wavelengths (400 or 600 km) where the signal due to convective thermal anomalies might be expected. The most prominent features among the short‐wavelength lineations are associated with the well‐known chains of seamounts and islands. Other lineations observed in the geoid coincide with computed tracks of other volcanic centres. Although these tracks are not continuous, this suggests that long‐lived plumes are at the origin of the apparently short chains of seamounts. Apart from the lineations corresponding to the tracks of ‘classical’ hotspots, there are numerous similar lineations visible on the filtered geoid maps which have the same directions as hotspot tracks. These preferential directions and the gravity‐to‐bathymetry ratios seem to indicate that the lineations are due to magmas implaced just above or within the lithosphere. Convective plumes, fixed with respect to each other and much more numerous than the classical hotspots could be the source of these magmas. A geometrical test of this hypothesis is performed by comparing the lineation pattern with the computed tracks for a set of tentative new hotspots. In this way, numerous plumes, long‐lived and fixed with respect to each other are found to provide an explanation of the observed small‐wavelength geoid features.