Discussion: “Non-hotspot volcano chains originating from small-scale sublithospheric convection” by Ballmer et al. (2007)

In their introduction, Ballmer et al. [2007] state: “The most fundamental prediction of the hotspot hypothesis is a linear age progression of the volcanic edifices along the chain; however, some ridges – such as the Marshall, Line, and Cook-Austral Islands – display highly irregular age-distance relationships … and therefore require another mechanism.” The authors proceed to describe modeling results that might well explain anomalous volcanism inconsistent with the hotspot hypothesis. However, one of the three examples cited, the Cook-Austral Islands, may indeed be consistent with the hotspot hypothesis. The isotopic dating of Koppers and Staudigel [2005] combined with those compiled by Clouard and Bonneville [2005], provide a remarkable record of intraplate volcanism in the south-central Pacific.

Fig. 1 illustrates hypothetically restored data points and loci of isotopic dates from the Gilbert, Tokealu, Cook, and Austral seamount and island chains of the South Pacific in Google EarthTM. For each sample location from these chains, the oldest isotopic date was selected, on the assumption that it is closest in age to the initial eruption due to the sub-plate melting anomaly [e.g., Pilger, 2007]. The data points were rotated back to their inferred location relative to the Hawaiian hotspot frame at the time of inferred eruption. Since there is uncertainty in not only the isotopic age itself, but in the actual initial eruption (the inaccessible base of the volcanic pile), loci were also calculated plus/minus five m.y. (light-colored line segments). The plate hotspot model of Wessel et al. [2006] was used to make the reconstruction calculations.

From the reconstructions (Fig. 1), note that there are two concentrations of data points and intersecting loci, near the inferred Macdonald-Tokelau (approximately 27°S, 141°W), and Rurutu-Gilbert (approximately 23°S, 152°W) hotspots. The more northerly trending loci are restored locations from the Tokelau and Gilbert chains. The more westerly trending loci are from the Cook and Austral chains. Older loci from the Austral chain which cluster adjacent to the inferred Foundation-Austral to the southeast. Similarly, dates from the western Pacific cluster near the inferred Samoan hotspot to the west (80 to 90 Ma dates, when restored, are southeast of the younger dates from the Samoan chain; Wessel et al. [2006], note that their model for the Late Cretaceous is less well-constrained than the model since 70 Ma.).

The present alignment of the Cook and Austral chains is an apparent coincidence, in that the Gilbert and Tokelau chains can be explained as originating from the same melting anomaly responsible for the Cook and Austral chains. The older chains (Gilbert and Tokelau) parallel each other, but are nowhere near co-linear; they predate the observed alignment for dates since 50 Ma.

The model of Ballard et al. does not explain two separate chains which subsequently overlap; the evidence implies that the Gilbert-Cook and Tokelau-Austral (younger) chains originate from distinctly different long-lived melting anomalies. Sparser data from the western Pacific and Samoan chains suggest an additional, long-live melting anomaly, while the Foundation-Austral (older dates) reflect still another melting anomaly.

The modeling of Ballard et al. may be applicable to other anomalous volcanic chains, but cannot be readily applied to the Cook-Austral chains. Conceivably, sublithospheric convection could enhance the melting anomalies, but the kinematic evidence indicates the melting anomalies predate the convection mechanism.
Update: See newer, related posts: Long-lived melting anomalies and "Hotspots" of the South Pacific.

Acknowledgements
The original data points and their restorations are posted as part of a global compilation on the GoogleEarthTM Community website (bbs.keyhole.com) for a number of plate hotspot models; search for username: “rex_pilger”. Fig. 1 was prepared using Landmark Graphics’ GeoGraphix Discovery.
References
Ballmer, M. D., J. van Hunen, G. Ito, P. J. Tackley, and T. A. Bianco (2007), Non-hotspot volcano chains originating from small-scale sublithospheric convection, Geophys. Res. Lett., 34, L23310, doi:10.1029/2007GL031636.

Clouard, V., A. Bonneville (2005), Ages of seamounts, islands and plateaus on the Pacific Plate. In: Foulger, G., Natland, J., Presnall, D., Anderson, D. (Eds.), Plates, plumes and paradigms, Geol. Soc. Am., Special Paper 388, 71–90.

Koppers A. A. P., and H. Staudigel (2005), Asynchronous bends in Pacific seamount trails: a case for extensional volcanism? Science, 307, 904-907.

Müller, R. D., W. R.Roest, J.-Y. Royer, L. M. Gahagan, and J. G. Sclater (1997), Digital isochrons of the world’s ocean floor: J. Geophys. Res., 102, 3211–3214, doi: 10.1029/96JB01781.

Pilger, R. H. (2003), Geokinematics: Prelude to Geodynamics, Springer-Verlag, Berlin, 338 p. (Obtain from Amazon)

Pilger, R. H. (2007), The Bend: Origin and significance, Bull. Geol. Soc.Am., 119, 302-313.

Wessel, P. (2001), Global distribution of seamounts inferred from gridded Geosat/ERS-1 altimetry, J. Geophys. Res., 106(B9), 19,431-19,441; dataset: ftp://ftp.soest.hawaii.edu/pwessel/Global_Seamounts_JGR2001.dat.bz2.

Wessel, P., Y. Harada, and L. W. Kroenke (2006), Toward a self-consistent, high-resolution absolute plate motion model for the Pacific, Geochem. Geophys. Geosyst., 7, Q03L12, doi:10.1029/2005GC001000.


Figure 1 (above). Data points (present and restored) and restored loci, Cook, Austral, Gilbert, Tokelau, and Foundation island and seamount chains, South Pacific. Present data points (gray diamonds), restored data points (solid circles) and loci (line segments; calculated for ages plus or minus 5 m.y. from the data point age at 2.5 m.y. intervals) restored according to the plate-hotspot model of Wessel et al. (2006), and interpolated using spline methods described in Pilger (2003). Magnetic isochrons and fracture zones from Müller et al. (1997); seamounts (small triangles) from Wessel et al. (2001). Dotted circles approximate possible underlying “hotspot” locations. Equatorial equidistant projection. Graticules at 15° intervals.