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Geophysical evidence from the MELT area for compositional controls on oceanic plates


Magnetotelluric and seismic data, collected during the MELT experiment at the southern East Pacific Rise1,2, constrain the distribution of melt beneath this mid-ocean-ridge spreading centre and also the evolution of the oceanic lithosphere during its early cooling history. Here we focus on structures imaged at distances 100 to 350 km east of the ridge crest, corresponding to seafloor ages of 1.3 to 4.5 million years (Myr), where the seismic and electrical conductivity structure is nearly constant and independent of age. Beginning at a depth of about 60 km, we image a large increase in electrical conductivity and a change from isotropic to transversely anisotropic electrical structure, with higher conductivity in the direction of fast propagation for seismic waves. Conductive cooling models predict structure that increases in depth with age, extending to about 30 km at 4.5 Myr ago. We infer, however, that the structure of young oceanic plates is instead controlled by a decrease in water content above a depth of 60 km induced by the melting process beneath the spreading centre3.

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Figure 1: Two model cross-sections of conductivity through the region 70–350 km east of the southern East Pacific Rise in the MELT area.
Figure 2
Figure 3: Average one-dimensional profiles through our conductivity model and theoretical curves based on laboratory data.


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We thank S. Karato for comments. We are also grateful to S. Hier-Mujander and D. Kohlstedt for discussions. US participation in the MELT experiment and subsequent analysis was funded by NSF grants through the Marine Geology and Geophysics Program, Ocean Sciences Division.

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Correspondence to Rob. L. Evans.

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Supplementary information

Supplementary Notes

Test of isotropy of Upper 60km. (DOC 30 kb)

Supplemental Figure S1

Cross sections of conductivity through the region 70–350km east of the SEPR in the MELT area. (PDF 3002 kb)

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Evans, R., Hirth, G., Baba, K. et al. Geophysical evidence from the MELT area for compositional controls on oceanic plates. Nature 437, 249–252 (2005).

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