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   Pulsing plumes?

The pulsing Iceland plume: A personal evolution from believer to agnostic

Richard Hey

 

Hawaii Institute of Geophysics and Planetology, School of Ocean and Earth Science and Technology, University of Hawaii, Honolulu, HI 96822 USA, hey@soest.hawaii.edu

 

Full disclosure

Jason Morgan was my Ph.D. advisor when he proposed his mantle plume model and resurrected Tuzo Wilson’s hotspot hypothesis, which at the time was not generally accepted. The South Africans, for example, were convinced that the Walvis aseismic ridge could not be a hotspot track, groups at both Scripps and Oregon State insisted that non-hotspot models were required for the Galapagos area, and at the time no-one in Hawaii thought Hawaii was a hotspot. All of us at Princeton thought Jason was right of course – his track record was pretty incredible, beginning with discovering plate tectonics as a post-doc a few years earlier.

Assuming Jason was always right was a very good career move for me. My dissertation work showed how the seemingly fatal geometric arguments against the Cocos and Carnegie aseismic ridges being Galapagos hotspot tracks could be resolved, the key being ridge axis jumps toward the hotspot. This seemed like a plausible effect of a hotspot/plume, and I grew up thinking these concepts were correct. Fresh from this triumph for the embattled hotspot hypothesis, I gave a recruiting talk at the University of Hawaii that began “Good news. Despite what you’ve been led to believe Galapagos is a hotspot just like Hawaii”, and the entire faculty rose up enthusiastically and said “Hawaii’s not a hotspot”, so I had to quickly prepare a  second, more introductory talk. The controversy used to go the other way.

I assumed Iceland was a hotspot too, presumably overlying some kind of deep mantle convection plume, and in addition, Peter Vogt had shown it was a pulsing plume when he discovered the V-shaped basement ridges and scarps, commonly called VSRs, symmetrical about the Reykjanes Ridge south of Iceland (Vogt, 1971). He proposed that pulses of asthenosphere flowing away from an Icelandic plume would combine with seafloor spreading to produce the diachronous VSRs.

His model was so simple and elegant I thought it had to be correct. The VSRs proved that something in the plume-ridge system was variable, and if it wasn’t the ridge, which was known to be spreading symmetrically in a stable (although oblique) geometry, it must be the plume. The logic seemed inescapable. Thus when Gillian Foulger, Don Anderson, and their colleagues proposed their alternative model for the Iceland melt anomaly, a broad zone of unusually fertile upper mantle (Ed: see Iceland & the North Atlantic Igneous Province) (Foulger & Anderson, 2005), I thought it was demonstrably wrong, because a broad upper mantle heterogeneity would not pulse, and so could not produce the VSRs. I noticed their group never discussed this, and thought it was a fundamental problem for their model (Ed: I agree, it was, which is why we never discussed it). So when I was asked to review Gillian’s anti-plume book proposal (Ed: see Plates vs Plumes: A Geological Controversy) I declined because I thought I was too biased in favor of plumes to be objective.

I was aware that propagating rifts (PRs) also produce V-shaped wakes, but these must be asymmetric, in contrast to Vogt-type wakes. PRs reorganize seafloor spreading geometry by breaking through existing lithosphere and transferring some from one plate to the other, resulting in “too much” lithosphere on the failed rift side and “too little” on the other, so in addition to asymmetric pseudofault/failed rift wakes, asymmetric seafloor spreading is another predictable consequence of rift propagation (Figure 1; Hey et al., 1989). Thus for the PR model to work south of Iceland, all of the conventional wisdom about the Reykjanes Ridge that was entirely consistent with pulsing plume models, that the VSRs were symmetric about the ridge axis and that seafloor spreading was also symmetric, would have to be wrong. This did not seem possible.

Figure 1: ldealised oceanic propagator system. Mid gray: crust formed at propagating rift, light gray: crust formed at dying rift, dark gray: lithosphere transferred from one plate to the other.

Everyone knew the Reykjanes Ridge was spreading symmetrically because Fred Vine (and Ellen Herron, another excellent magnetic anomaly analyst, or “wiggle picker”, as we were known) had said so, and I knew Fred was never wrong. His iconic color figure of the classic Heirtzler et al. (1966) Reykjanes Ridge aeromagnetic data correlated with the magnetic reversal timescale was one of the most important demonstrations of the magnetic symmetry predicted by seafloor spreading, at the time still a highly controversial hypothesis.

However, Fred hadn’t tested the symmetry statistically, as he admitted (to appreciative laughter) in response to a question at the influential 1966 conference on the History of the Earth’s Crust at the Goddard Institute for Space Studies in New York, where he presented his figure. "I never touch statistics. I just deal with the facts," he said. Fred was trying to have a scientific revolution, and of course on that scale pretty symmetric seafloor spreading is a reality. However, on a finer scale asymmetric spreading can be seen in his figure (Vine, 1968) now that you know it’s there.

Everyone knew the VSRs were symmetric about the Reykjanes Ridge axis because Peter Vogt (and many others) had said so, and I knew Peter is also an outstanding scientist, considerably ahead of his time in discovering the VSRs in the classic Talwani et al. (1971) single-channel data. However, these data were collected before plate tectonics defined seafloor spreading flowlines, so Talwani et al. (1971) did what any of us would have done in 1966 and ran their tracks perpendicular to structure. It later turned out that the Reykjanes Ridge was spreading obliquely. Thus the supposedly conjugate structures Vogt originally identified on these profiles as symmetric must necessarily have been formed at different times and places on the axis, and so couldn’t actually be symmetric conjugate features if his model was correct. This important point was first made by Johansen et al. (1984) in an unfortunately widely ignored paper.

Recent work

Working on the assumption that in trying to understand this type-example of hotspot/ridge interaction it couldn’t hurt to understand the ridge evolution better, Fernando Martinez, Ármann Höskuldsson, and I led a marine geophysical expedition on the R/V Knorr in the summer of 2007, with tracks along spreading flowlines to avoid the complications Vogt faced. All existing pulsing plume models (and even the innovative alternative model of Hardarson et al. (1997), which helped stimulate our thinking about the area) predicted the VSRs should be symmetric on these tracks, whereas the PR model required them to be asymmetric. Our results were recently published in G3 (Hey et al., 2010).

As it turns out, the conventional wisdom was wrong, and the pulsing plume model that most of us grew up thinking was obviously correct was based on the incorrect assumption that the VSRs are symmetric about a stable, symmetrically-spreading Reykjanes Ridge axis. We have shown that this assumption is not true. The VSRs are not symmetric about the axis, and seafloor spreading has not been symmetric about a stable axis. This asymmetry is smaller where Vine and Vogt worked, south of 62°N, but increases toward Iceland, where we had the great advantage of working. For example, where Vogt’s E scarps intersect our track 17 just south of the Iceland shelf (Figure 2), they are 20 km farther from the axis on the North American plate than on the Eurasian plate. Vogt’s A scarps are asymmetric in the opposite sense, farther from the axis on the Eurasian plate, so no single axis can make both scarps simultaneously symmetric. The conjugate features must have formed contemporaneously at a ridge axis, yet today there is ~30 km more lithosphere on the North American plate than on the Eurasian plate between the A and E troughs and scarps.

Figure 2. VSR asymmetry shown by profiles of satellite (top) and shipboard (middle) free air gravity anomalies and bathymetry (bottom). Note the asymmetry of Vogt’s E scarps (farther from the axis (red) on the North American plate) and A scarps (farther from the axis on the Eurasian plate), although on each profile equivalent scarps are equal ages because of the proposed ridge jumps (vertical lines). The gravity troughs (shaded) at the bases of the A and E scarps are always wider apart on the North American plate than the Eurasian plate, demonstrating VSR asymmetry independent of the exact location of the present ridge axis. From Hey et al. (2010).

Similarly, on track 17 anomaly 6 is ~25 km farther from the axis on North America than it is on Eurasia, and anomaly 5 asymmetry is evident near Iceland (Figure 3).

Figure 3: New marine magnetic anomaly data which demonstrate asymmetric seafloor spreading. Anomalies 5 (green dashes) and 6 (blue dashes) are always farther from the ridge axis (A, red line) on North America than Eurasia. The spacing between anomalies 5 and 6 is also consistently (~10 km) greater on the North American plate than on the Eurasian plate, demonstrating seafloor spreading asymmetry independent of the exact location of the present ridge axis. From Hey et al. (2010).

These asymmetries are inconsistent with all previous models, but rift propagation creates such asymmetries, and also produces V-shaped wakes that in other areas show crustal thickness variations similar to those proposed for these VSRs. This suggests a possible tectonic alternative to magmatic explanations such as the pulsing plume models. We suspect that many studies that have concluded they were consistent with a pulsing plume could also be consistent with our new model.

As a result, I am now agnostic on the subject of an Iceland plume. A pulsing plume (or even a plume) is not required for rift propagation, although if plume pulses do exist they would certainly provide a plausible driving mechanism for propagators. At this point I think it is fair to say that the Reykjanes VSRs can no longer be used as proof that Iceland is a pulsing plume, and that a fair question for pulsing plume proponents is, if you did not already “know” Iceland was a pulsing plume, would your own data be compelling that it is one? And probably of greatest interest to this website, if Iceland cannot be shown to be a pulsing plume, would your data still convince you it is definitely a plume, as opposed to, for example, the fertility model of Foulger, Anderson and their colleagues?

References

  • Hardarson, B.S., J.G. Fitton, R.M. Ellam, and M.S. Pringle, Rift relocation-A geochemical and geochronological investigation of a palaeo-rift in northwest Iceland, Earth Planet. Sci. Lett., 153, 181-196, doi:10.1016/S0012-821X(97)00145-3, 1997.
  • Heirtzler, J.R., X. Le Pichon, and J.G. Baron, Magnetic anomalies over the Reykjanes Ridge, Deep Sea Res., 13, 427–443, 1966.
  • Hey, R.N., J.M. Sinton, and F.K. Duennebier, Propagating rifts and spreading centers, in The Eastern Pacific Ocean and Hawaii, The Geol. of North Am., vol. N, pp. 161–176, Geol. Soc. of Am., Boulder, Colorado, 1989.
  • Johansen, B., P.R. Vogt, and O. Eldhom, Reykjanes Ridge: Further analysis of crustal subsidence and time-transgressive basement topography, Earth Planet. Sci. Lett., 68, 249-258, doi:10.1016/0012-821X(84)90157-2, 1984.
  • Talwani, M., C.W. Windisch, and M.G. Langseth Jr., Reykjanes Ridge crest: A detailed geophysical study, J. Geophys. Res., 76, 473–517, doi:10.1029/JB076i002p00473, 1971.
  • Vine, F.J., Magnetic anomalies associated with midocean ridges, in The History of the Earth’s Crust, edited by R.A. Phinney, pp. 73–89, Princeton Univ. Press, Princeton, N. J., 1968.
  • Vogt, P.R., Asthenosphere motion recorded by the ocean floor south of Iceland, Earth Planet. Sci. Lett., 13, 153–160, doi:10.1016/0012-821X(71)90118-X, 1971.
last updated 3rd June, 2010
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