Tuesday, February 15, 2011

What's missing in these hotspot volcanic models? Answer: geokinematics

Three recent "hotspot" papers (Ballmer et al., 2009, 2010, and Presnall & Gudfinnsson, 2011) share a common feature. They are missing something -- a piece of evidence which none of the three confront: geokinematics. In fairness, however, first, these papers are not unique; and, second, the authors could with equal validity claim that geokinematics papers neglect geochemical and geodynamic models, too.

Ballmer et al. (2009 and 2010) emphasize small-scale convection so as to explain what they describe as "complex age-distance patterns" in hotspot traces, especially the familiar chains of the South Pacific: Austral, Cook, and Samoan. Presnall and Gudfinnsson (2011) attempt explanation of hotspot traces in general in terms of propagating fractures, perhaps the longest-lived hypothesis for the origin of such chains (see, e.g., Dana et al., 1849, or "within" plate tectonics, Jackson et al., 1972).
Ballmer et al. acknowledge that the South Pacific "complex" chains may be related, genetically, to Cretaceous seamounts. However, they do not explicitly reference the kinematic models of, e.g., Wessel and Kroenke (2006, 2008) and Jackson et al. (2010; of course, this paper is quite recent), as applied to the Cretaceous-to-Present seamount chains: the Magellan, Gilbert, Tokelau, Samoa, Cook, Austral, Macdonald, and Foundation. The convective mechanism of Ballmer et al. may be relevant, however, the geokinematic evidence would also imply an additional constraint -- the source regions for each composite trace (see, e.g., this post) which are stable within the same, larger "hotspot" reference frame.
Presnall and Gudfinnsson, 2011, emphasize the geochemical evidence for relatively shallow and cool origin for hotspot volcanism, and argue that their models imply fracturing of oceanic plate accesses the source region. However, Presnall & Gudfinnsson do not touch on the two additional components of hotspot traces that any mechanism for their origin must incorporate: evidence for the melting anomalies existant within reference frames, of which there appear to be three (e.g., Norton, 2000; Pilger, 2003). It is difficult to expect progressive fracturing to be consistent in propagation rate, not only over a single plate (e.g., the Pacific), but also the Nazca and eastern Australian plates (e.g., Pilger, 1982, 2007). More critical, how does a simple propagating fracture mechanism explain hotspot traces which extend over multiple plates (e.g., Easter-Nazca-Tuamotu; Reunion-Chagos-Laccadive-Deccan, Duncan, ; Kerguelen-Ninetyeast; Great Meteor-New England: Pilger, 2003, 2007, and references therein)?
Yes, kinematic/reconstruction models for plate-hotspots need to incorporate geochemical and geodynamic models. But, given their simplicity, why are kinematic models seemingly ignored? Or are kinematic models simple indeed? Code for plate reconstructions is available from multiple sources, while their underlying algorithms were provided early in the development of plate tectonics. For example Pilger (2003) provides C-code which is digitally reproduced here. A collection of published plate reconstruction parameters can be found here.      
Finally, there is an ongoing problem with both plume and non-plume models for the origin of hotspot traces: Morgan's original hypothesis linked plumes and their fixity. However, it became apparent quite early that Atlantic-Indian Ocean hotspot traces are inconsistent with Pacific Ocean traces and both with respect to the Earth's spin axis (see references in Pilger, 2003). Geodynamic models of plumes now are largely independent of plume fixity, either relative to one another or to any "absolute" reference frame (e.g., O'Neill et al., 2005). Whether hotspots (hypothetical plumes) are fixed relative to one another is largely irrelevant to confirmation of plume models. However, there is strong evidence for three distinct hotspot reference frames which may represent plumes or, alternatively, some sort of fertile region which persists across plate boundaries (Pilger, 2003l, 2007).
In sum, fracture and small-scale convection models may be relevant to the origin of hotspots, but some sort of melting anomaly (plume, fertile region, ancient subduction zone...?) is still required. For more discussion, see herehere,  here, and here.
References
Ballmer, M. D., van Hunen, J., Ito, G. & Bianco, T. A., 2009, Intraplate volcanism with complex age-distance patterns: A case for small-scale sublithospheric convection, 10, No.6, 24, Q06015, doi:10.1029/2009GC002386, ISSN: 1525-202.
Ballmer, M. D., Ito, G., van Hunen, J., & Tackley, P. J., 2010, Small-scale sublithospheric convection reconciles geochemistry and geochronology of ‘Superplume’ volcanism in the western and south Pacific, Earth and Planetary Science Letters, 290, 224–23.
Dana, J. D., 1849, Geology, in Wilkes, C., ed., United States Exploring Expedition 10, Philadelphia, NewYork, Putnam, 756 p.
Jackson, E. D., Silver, E. A. & Dalrymple, G. B., 1972, Hawaiian-Emperor chain and its relation to Cenozoic circumpacific tectonics, Geological Society of America Bulletin, 83, 601-618.
Jackson, M. G., Hart, S. R., Konter, J. G., Koppers, A. A. P., Staudigel, H., Kurz, M. D., Blusztajn, J., & Sinton, J. M., 2010, Samoan hot spot track on a “hot spot highway”: Implications for mantle plumes and a deep Samoan mantle source, Geochemistry, Geophysics, Geosystems (G3), 11, Q12009, 24 p. doi:10.1029/2010GC003232.http://www.agu.org/pubs/crossref/2010/2010GC003232.shtm 
Norton, I. O., 2000, Global hotspot reference frames and plate motion, American Geophysical Union Geophysical Monograph 121, 339-357.
O’Neill, C., Müller, D., and Steinberger, B., 2005, On the uncertainties in hot spot reconstructions and the significance of moving hotspot reference frames: Geochemistry, Geophysics, Geosystems (G3), Q04003, 35 p., doi: 10.1029/2004GC000784. 
Pilger, R.H. 1982. The origin of hotspot traces: evidence from Eastern Australia, Journal of Geophysical Research, 87, 1825-1834.
Pilger, R. H., 2003, Geokinematics: Prelude to Geodynamics, Springer-Verlag, Berlin, 338 p.
Pilger, R. H., 2007, The Bend, Geological Society of America Bulletin, 119; 302–313, doi: 10.1130/B25713.1, Data Repository item 2007006, https://docs.google.com/leaf?id=0B4YxGFE9X3wSNjBiNWRiMGMtMTc0Ny00ZDJlLWE0NDYtYmZjNWJiMWViNWQ4&hl=en,.
Presnall, D. C., & Gudfinnsson, G., 2011, Oceanic volcanism from the Low-Velocity Zone without mantle plumes, Journal of Petrology, advance access published February 8, 2011.
Wessel, P., Harada, Y., & Kroenke, L. W., 2006, Toward a self-consistent, high-resolution absolute plate motion model for the Pacific, Geochemistry, Geophysics, Geosystems (G3), 7, Q03L12, doi:10.1029/2005GC001000.
Wessel, P., and L. W. Kroenke, 2008, Pacific absolute plate motions since 145 Ma, Journal of Geophysical Research, 113 (B06101), doi:10.1029/2007JB005499.

No comments:

Post a Comment