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The Pukapuka volcanic chain

Edward L. Winterer

Geosciences Research Division, Scripps Institution of Oceanography, La Jolla, CA 92092-0220, USA

jwinterer@ucsd.edu

Map of the Pacific ocean rotated so the Euler pole is due north. In this view, volcanic chains that are parallel to plate motion align E-W. Major major volcanic chains are shown. H: Hawaii, C: Carolines, S: Samoa, SO: Societies, M: Marquesas, PP: Pukapuka, A: Australs, L: Louisville.

The Pukapuka ("head of the rock") chain of volcanic ridges in the southeastern Pacific is traceable for about 3,000 km, from near the Tuamotus all the way to the East Pacific Rise (Figure 1). The ridges are mainly arrayed en echelon (Figure 2). Some of them are closely grouped (Figure 3), and show a consistent scheme of growth. They begin with small (1 – 2 km) “pancake” volcanoes distributed along a sigmoidal trace, followed by stacking on of more small volcanoes to form a narrow ridge, and finally by the building of a central volcano with radial rift zones.

Figure 1. Satellite gravity map of part of the eastern Pacific, showing the Pukapuka seamount chain (black dots, P-P’), aligned in a gravity trough within the set of parallel gravity ridges and troughs. Seamounts are about 30 Ma at the western end of the chain, while the eastern end is at the volcanically active East Pacific Rise (black line). Click on figure to enlarge.

Figure 2. Outline map of the Pukapuka chain, showing the en echelon arrangement of volcanic ridges.

Figure 3. Group of en echelon ridges. Note sigmoid trends. The ridges are composites of small volcanoes.

The strain pattern, suggesting N – S tension with a component of right-lateral shear, is expressed only in the distribution of extruded volcanic rocks, and cannot be discerned in the detailed multibeam images of the seabed between volcanic ridges (Figure 4). The dikes/pipes that fed the ridges must therefore be narrow.

Figure 4. Multibeam chart of part of the Pukapuka chain, showing pre-ridge N-trending abyssal hill topography, which is not offset by faulting. The sediment cover is less than 30 m thick.

Geochemical evidence favors a shallow origin for the lavas: “The lava compositions have weak garnet signatures and trace element evidence for equilibration of the most alkalic lavas with residual phlogopite or amphibole suggest that the Pukapuka lavas were primarily derived from the shallow upper mantle rather than a deep-seated mantle plume” (Janney et al., 2000). Because volcanism is focused in the troughs of gravity rolls, indicating tension in the lithosphere, the rolls are thus viewed as boudins (Figure 5). (Ed: For an alternative view, that suggests volcanic ridges are warps and cracks in the plate due to non-uniform thermal contraction of the lithosphere, see the Powerpoint presentation "Cracks and Warps in the Lithosphere from Thermal Contraction" by David Sandwell.)

 

Figure 5. Model for the Pukapuka seamount chain. Volcanism is focused in the trough of gravity rolls, indicating tension in the lithosphere. The rolls are thus viewed as boudins.

References

  • Winterer, E.L. & Sandwell, D.T., 1987, Evidence from en-echelon cross-grain ridges for tensional cracks in the Pacific plate, Nature, 329, 534-537.
  • Goodwillie, A.M., 1995, short-wavelength gravity lineations and unusual flexure results at the Pukapuka volcanic ridge system, Earth Planet. Sci. Lett., 136, 297-314.
  • Sandwell, D.T., Winterer, E.L., Mammerickx, J., Duncan, R.A., Lynch, M.A., Levitt, D.A., & Johnson, C.L., 1995, Evidence for diffuse extension of the Pacific plate from Pukapuka ridges and cross-grain gravity anomalies, J. Geophys. Res., 100, 15,087-15,099.
  • Lynch, M.A., 1999, Linear ridge groups: evidence for tensional cracking in the Pacific plate, J. Geophys. Res., 104, 29,321-29,333.
  • Janney, P.E., Macdougall, J.D., Natland, J.H., & Lynch, M.A., 2000, Geochemical evidence fom he Pukapuka volcanic ridge system for a shallow enriched mantle domain beneath the South Pacific Superswell, Earth Planet. Sci. Lett., 181, 47-60.

last updated 25th January, 2006

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