An analysis of the altimetric geoid in various wavebands in the Central Pacific Ocean: constraints on the origin of intraplate features

Abstract

SEASAT profiles were filtered in three different wavebands and interpolated to elaborate maps of the very short (25–110 km) and short (80–320 km and 280–820 km) wavelengths of the geoid anomalies for the South Pacific. For the very short wavelengths, we mapped the geoid roughness i.e. the variations of amplitude of the anomalies. Two lines following present-day plate motions and separated by 800–1000 km, were identified. The first is formed by the Tuamotu Archipelago, the Pitcairn-Crough alignment and the Easter-Sala y Gomez chain. The second comprises the Cook-Austral Islands, a seamount cluster mapped close to the East Pacific Rise, south of 30°S and the Chile Rise. Alignments with a N150°E direction and corresponding to traces of old thermal activity, were mapped on lithosphere older than 40 m.y. These features are also spaced of 800–1000 km. The map for the 80–320 km waveband shows a pattern of 200 km wavelength undulations. Several trends are superimposed, but the N95°E direction is dominant in the area. We associate these geoid undulations to lithospheric phenomena, such as fracturations and/or small magmatic ridges. The 280–820 km waveband reveals linear anomalies, roughly following the direction of present-day absolute plate motions. The linear anomalies have a 400–500 km wavelength and are nearly continuous between the Pacific and the Nazca plates. The topography is located on the highs, but the anomalies generally precede the active ends of the volcanic chains. This result favours the hot-line hypothesis previously suggested by several authors. We associate these bands to thermal heterogeneities probably due to a convective regime. Since the main topographic features shall be emplaced over the ascending limbs of two neighbouring cells, the 800–1000 km spacing of the topography imposes a minimum size of 400–500 km for the cells. So, the convective cells cannot be limited to a thin layer beneath the lithosphere, but will affect part of the upper asthenosphere. The existence of hot lines aligned parallel to plate motions, can explain the intermittent magmatic activity observed for the volcanic lineaments of the South Pacific. Volcanic activity can be the result of the interaction between the hot lines and lithospheric zones of weakness and/or of instabilities in the convective cells. The regular spacing observed for topography formed before and after 40 m.y. suggests that the size of the cells is rather constant through time.