Taking previous speculations that the Gilbert Ridge and Tokelau Seamounts include a "bend" analogous to the Hawaiian-Emperor bend, K&S show that isotopic ages from the two chains are incompatible with the age of the H-E bend and with one another. Within their published figures, they show hypothetical loci generated by a Pacific hotspot model and demonstrate inconsistency with their newly acquired dates. Interestingly, for the Toekalu chain, they also show a locus anchored at Macdonald "hotspot" -- which better fits the new ages of the seamounts.
Comparison of K&S's inferred seamount chains and their included bends with Smith and Wessel's (1999) bathymetry suggests a certain selectivity in interpretation, especially given the short length of the "younger" portions inferred to mimic the younger part of the H-E chain. Putting such subjective observations aside, there is another way to look at the significance of the new data in light of a relatively fixed sub-Pacific hotspot reference frame.
Taking the new data points, along with previously published data, I've reconstructed each sample date back to its location when the isotopic clock started according to the plate-hotspot model of Raymond et al. (2000) as interpolated using my spline methods (Pilger, 2003). Given the age uncertainty, I've included data points +/- 5 m.y. around each data point at 2.5 m.y. intervals (producing a maximum of five data points), thereby defining loci segments.
The restored loci from the Gilbert and Tokelau chains (click thumbnail below) produce a fascinating pattern when combined with the restored segments from the Cook-Austral chain. The two older chains produce loci that intersect with younger segments close to (1) the inferred location of the Macdonald hotspot and (2) another inferred hotspot location.
Previously, available isotopic dates from the C-A chain implied a miinimum of two hotspots (three if the older part of the Foundation chain were included). K&S's new data strengthen this inference.
Additionally, then, the approximate alignment of the three hotspots cannot as readily be ascribed to intraplate stresses as might previously have been inferred. Why? Because the Gilbert and Tokelau chains are not themselves aligned. That is, the melting anomalies responsible for the two older chains are not attributable to a single locus of intraplate extension. The alignment of the "hotspots" post-47 Ma is largely a coincidence.
Thus, the new data of K&S don't strengthen the case for intraplate extension, they undermine it.
Perhaps the revised title could be: "New isotopic ages from Pacific seamount chains: Further evidence for sub-lithospheric hotspots?"
Cited: Koppers and Staudigel (2005) Science, 307, 904. Pilger (2003) Geokinematics: Prelude to Geodynamics. Raymond et al. (2000) AGU Geophys. Mon.121, 359. Smith and Sandwell (1997) Global seafloor topography from satellite altimetry and ship depth soundings, Science, 277, 1957.
Originally posted at: http://mesoplates.spaces.live.com/blog/cns!F9234EE50D713CDC!149.entry
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