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Episodic swell growth inferred from variable uplift of the Cape Verde hotspot islands

Abstract

On the Beagle voyage, Charles Darwin first noted the creation and subsidence of ocean islands1, establishing in geology’s infancy that island freeboard changes with time. Hotspot ocean islands have an obvious mechanism for freeboard change through the growth of the bathymetric anomaly, or swell2, on which the islands rest. Models for swell development indicate that flexural9, thermal2,3 or dynamic pressure4,5,6,8 contributions, as well as spreading of melt residue from the hotspot7, can all contribute to island uplift. Here we test various models for swell development using the uplift histories for the islands of the Cape Verde hotspot, derived from isotopic dating of marine terraces and subaerial to submarine lava-flow morphologies. The island uplift histories, in conjunction with inter-island spacing, uplift rate and timing differences, rule out flexural, thermal or dynamic pressure contributions. We also find that uplift cannot be reconciled with models that advocate the spreading of melt residue in swell development unless swell growth is episodic. Instead, we infer from the uplift histories that two processes have acted to raise the islands during the past 6 Myr. During an initial phase, mantle processes acted to build the swell. Subsequently, magmatic intrusions at the island edifice caused 350 m of local uplift at the scale of individual islands. Finally, swell-wide uplift contributed a further 100 m of surface rise.

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Figure 1: Map of the Cape Verde islands showing the location with respect to the African coast, bathymetric swell extent and subsea volcanic edifices.
Figure 2: Sample positions and dates relative to present sea level.
Figure 3: Shape and uplift rates for a buoyantly spreading root as a function of time and distance.

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Acknowledgements

We acknowledge the Fundação para a Ciência e para a Tecnologia (FCT) for support for R.R.’s PhD scholarship SFRH/BD/24835/2005, co-financed by POPH/FSE, and the Royal Society University Research Fellowship supporting D.N.S. We thank our colleagues in Lisbon, A. Serralheiro, C. Silva, J. Madeira and A. Silveira, for their constant support, S. Kaneshima for discussions and B. Faria and E. Harms for their enthusiastic support in Cape Verde. We also thank E. Takahashi for insight into volcanic basement uplift and salute the original reviewers’ insistence on identifying a viable process for local uplift.

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R.R. undertook all field work, dated samples and interpreted results, G.H. designed the project, participated in field work and interpreted results, M.C. supervised Ar–Ar analyses and provided laboratory facilities, D.V. supervised Sr analyses and provided laboratory facilities, D.H. did U–Th analyses and D.N.S. participated in field work and provided palaeodepths from fossil assemblages.

Corresponding authors

Correspondence to G. Helffrich or D. Hoffmann.

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The authors declare no competing financial interests.

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Ramalho, R., Helffrich, G., Cosca, M. et al. Episodic swell growth inferred from variable uplift of the Cape Verde hotspot islands. Nature Geosci 3, 774–777 (2010). https://doi.org/10.1038/ngeo982

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