The seafloor swells and Superswell in French Polynesia

Abstract

Abstract. High-quality ship soundings obtained since 1967 in French Polynesia are used to better define the pattern of depth anomalies in French Polynesia and to test the accuracy of previous results based on ETOPO5 gridded data. The modes of the differences between the ship data and ETOPO5 demonstrate that the latter database is accurate to _+250 m for nearly 70% of the observations in the region, although there is a tendency for ETOPO5 to be of the order of 35 m too shallow. The ship depth versus age distribution shows that the South Pacific's unusually shallow seafloor, termed the "Superswell," cannot simply be described as the cooling of a hotter or thinner thermal plate. Rather, the seafloor rises between the ages of -40 and 80 million years before resuming thermal subsidence. The Superswell appears to be bounded to the north by the Marquesas Fracture Zone, and the depth anomaly tapers down to smaller values south of the Austral Fracture Zone. Its modal-depth-versus-age relation cannot be mimicked easily by classical steady state thermal cooling models, leading us to prefer dynamic explanations for the depth anomaly. Modal depth analysis aids in the isolation of the individual swells superimposed on the Superswell. Whereas the Marquesas swell, just north of the Superswell, is similar to other swells worldwide in its width (> 1000 km) and amplitude (--1 km), both the Society and Austral swells are narrower and lower than global norms. We explain the small size of the Society and Austral swells as the reduced thermal buoyancy of these hotspots entrained in a broader mantle upwelling.•

“Plate‐like” subsidence of the East Pacific Rise–South Pacific superswell system

“…At a ''medium'' scale there is at least one oceanic plateau (e.g., Tuamotu [e.g., Talandier and Okal, 1987]) and a cluster [McNutt and Judge, 1990] of isolated [Sichoix et al, 1998] hot spot swells (e.g., Society, Marquesas and Austral). These topographic features do not contribute to the superswell [McNutt and Fischer, 1987;McNutt, 1998;Sichoix et al, 1998] and hinder its isolation because they make it difficult to determine a ''regional'' depth, which represents the upper surface of the superswell, from the bathymetry.…”

“…Dubbed a ''superswell'' by McNutt and Fischer [1987], the depth anomaly reportedly has a maximum amplitude of !1 km [McNutt and Sichoix, 1996;Sichoix et al, 1998], a width of approximately 3000 km [McNutt, 1998], and covers some 15 million km 2 of the seafloor [McNutt and Fischer, 1987].…”

“…MFZ, Marquesas Fracture Zone; AFZ, Austral Fracture Zone; dashed grey lines are volcanic chains. Circles indicate clusters of recent volcanism [Clouard and Bonneville, 2001] or ''hot spots'' [e.g., Fowler, 1990], which, except possibly Pitcairn [Binard et al, 1992], are associated with bathymetric swells [McNutt and Fischer, 1987;Sichoix et al, 1998;McNutt and Bonneville, 2000], commonly called hot spot swells.…”

Hotspot Concept: The French Polynesia Complexity

“…They propose that the Superswell could be an artifact of the poor sampling and gridding of ETOPO5, which in oceanic regions corresponds to the SYNBAPS database. Subsequent studies using original ship depth soundings (McNutt et al, 1996;Sichoix et al, 1998) confirm that the seafloor is indeed 1 km shallower. The precise description of the location and extent of the superswell remained approximate for a while.…”

Seamount detection and isolation with a modified wavelet transform

“…median and mode) were therefore utilized [e.g. McNutt & Fischer 1987;Smith, 1990;McNutt & Sichoix, 1996;Levitt & Sandwell, 1996;Sichoix, 1998]. These significantly mitigate this problem, but still have some tendency to pass through, rather than beneath, seamounts where many seamounts co-exist [Hillier & Watts, 2004] or occur down trenches (Figure 12).…”