Magmatic accretion of new oceanic crust at intermediate- to fast-spreading mid-ocean ridges occurs along a narrow axial zone. This zone is characterized by molten sills in the crust that are emplaced within about 3 km of the ridge axis1 and overlie a zone of elevated temperatures and partial melt2,3,4. There are disparate indications of off-axis magmatism5,6,7,8 and lavas erupted in the near-axis region are more compositionally variable than in the axial zone9. Here we present three-dimensional seismic reflection images from the fast-spreading East Pacific Rise that reveal a network of sills 4 to 8 km east of the ridge axis. Our crustal model, constrained using seismic velocity and attenuation data, shows that the sills are located outside of the main axial zone of crustal accretion, and above a region containing partial melt. We infer that the sills represent sites of sustained off-axis magmatism. Pockets of melt extend from the off-axis sills to the axial zone and may represent melt migration pathways. These pathways could promote mixing between enriched off-axis melts and normal on-axis melts, contributing to the compositional variability of the near-axis lavas9. We suggest that off-axis magmatism occurs preferentially, but not exclusively, where pre-existing fractures inherited from offsets of the spreading axis promote melt transport from the mantle into the crust.
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This research was funded by the US National Science Foundation. We thank the RV M.G. Langseth’s Captain M. Landow, crew, and technical staff led by R. Steinhaus for their efforts, which made possible the success of cruise MGL0812. We thank S.A. Soule for providing digital information about the AST and 2005–2006 eruption shown in Fig. 1 and 2, and for stimulating discussions during the preparation of this manuscript. We thank D.R. Toomey for his reviews, which improved the manuscript.
The authors declare no competing financial interests.
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Canales, J., Carton, H., Carbotte, S. et al. Network of off-axis melt bodies at the East Pacific Rise. Nature Geosci 5, 279–283 (2012). https://doi.org/10.1038/ngeo1377
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