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Mantle thermal pulses below the Mid-Atlantic Ridge and temporal variations in the formation of oceanic lithosphere

Abstract

A 20-Myr record of creation of oceanic lithosphere is exposed along a segment of the central Mid-Atlantic Ridge on an uplifted sliver of lithosphere. The degree of melting of the mantle that is upwelling below the ridge, estimated from the chemistry of the exposed mantle rocks, as well as crustal thickness inferred from gravity measurements, show oscillations of 3–4 Myr superimposed on a longer-term steady increase with time. The time lag between oscillations of mantle melting and crustal thickness indicates that the mantle is upwelling at an average rate of 25 mm yr-1, but this appears to vary through time. Slow-spreading lithosphere seems to form through dynamic pulses of mantle upwelling and melting, leading not only to along-axis segmentation but also to across-axis structural variability. Also, the central Mid-Atlantic Ridge appears to have become steadily hotter over the past 20 Myr, possibly owing to north–south mantle flow.

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Figure 1: The Vema transform region viewed from the northeast.
Figure 2: N–S multichannel seismic reflection profile perpendicular to the Vema transform, above the Nautile submersible profiles.
Figure 3: Temporal variations of mantle degree of melting and of crustal thickness.
Figure 4: Gravity imagery in the Vema transform region.
Figure 5: Differential upwelling of melt and of solid residual mantle beneath the ridge.
Figure 6: Model of mantle upwelling and crustal accretion consistent with the observed variations of extent of melting and crustal thickness at the EMARS.
Figure 7: Extent of melting versus ridge half segment length when a ridge intersects a long-offset slow-slip transform such as Vema.

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Acknowledgements

We thank A. Peyve and co-workers at the Geology Institute, Russian Academy of Science, and the captain and crew of the RV Akademic N. Strakhov for help with field work. We thank K. Kastens for providing cruise EW9305 gravity data; W. R. Buck and J. Karson for comments on the manuscript; and D.W. Forsyth for providing programs useful in processing gravity data. This work was supported by the Consiglio Nazionale Ricerche and the National Science Foundation.

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Bonatti, E., Ligi, M., Brunelli, D. et al. Mantle thermal pulses below the Mid-Atlantic Ridge and temporal variations in the formation of oceanic lithosphere. Nature 423, 499–505 (2003). https://doi.org/10.1038/nature01594

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