Modal depth anomalies from multibeam bathymetry: Is there a South Pacific superswell?

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A region west of the southern East Pacific Rise (SEPR), between the Marquesas and Austral Fracture Zones has previously been found to exhibit anomalous depth-age behavior, based on gridded bathymetry and single-beam soundings. Since gridded bathymetry has been shown to be unsuitable for some geophysical analysis and since the area is characterized by unusually robust volcanism, the magnitude and regional extent of depth anomalies over the young eastern flank of the so called ‘South Pacific Superswell’ are re-examined using a mode-seeking estimation procedure on data obtained from several recent multibeam surveys. The modal technique estimates a representative seafloor depth, based on the assumption that bathymetry from non-edifice and edifice-populated seafloor has a low and a high standard deviation, respectively. Flat seafloor depth values are concentrated in a few bins which correspond to the mode. This method estimates a representative seafloor value even on seafloor for which more than 90% of coverage is dominated by ridge and seamount clusters, where the mean and median estimates may be shallow by hundreds of meters. Where volcanism-related bias is moderate, the mode, mean and median estimates are close.Depth-age results indicate that there is only a small anomaly (< 200 m) over 15–35 Ma Pacific Plate seafloor with little age-dependent shallowing, suggesting that the lithosphere east of the main hot-spot locations on the ‘superswell’ is normal. An important implication is that, in sparsely surveyed areas, depths from ETOPO-5 are significantly different from true depths even at large scales (∼ 1000 km) and thus are unsuitable for investigations of anomalies associated with depth-age regressions. We find that seafloor slopes on conjugate profiles of the Pacific and Nazca Plates from 15 to 35 Ma are both slightly lower than normal, but are within the global range. Proximate to the SEPR, seafloor slopes are very low (218 m Myr−12) on the Pacific Plate (0–22 Ma) and slightly high (∼ 410 m Myr−12) on the Nazca Plate (0–8 Ma); slopes for older Pacific seafloor (22–37 Ma) are near normal (399 m Myr−12). Seafloor slopes are even lower north of the Marquesas Fracture Zone but are highly influenced by the Marquesas Swell. We find that the low subsidence rate on young Pacific seafloor cannot be explained by a local hot-spot or a small-scale convective model exclusively and a stretching/thickening model requires implausible crustal thickness variation (∼ 30%).