1. Department of Geological Sciences, University of Oregon, Eugene, Oregon 97403, USA
2. Nancy-Université, CRPG, 54501 Vandoeuvre les Nancy, France
3. Department of Geology and Geophysics, University of Hawaii-SOEST, Honolulu, Hawaii, 96822, USA
4. School of Oceanography, University of Washington, Seattle, Washington 98195, USA
5. Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543, USA

Correspondence to: Douglas R. Toomey¹ Correspondence and requests for materials should be addressed to D.R.T. (Email: drt@uoregon.edu).

Abstract

Mantle upwelling is essential to the generation of new oceanic crust at mid-ocean ridges, and it is generally assumed that such upwelling is symmetric beneath active ridges. Here, however, we use seismic imaging to show that the isotropic and anisotropic structure of the mantle is rotated beneath the East Pacific Rise. The isotropic structure defines the pattern of magma delivery from the mantle to the crust. We find that the segmentation of the rise crest between transform faults correlates well with the distribution of mantle melt. The azimuth of seismic anisotropy constrains the direction of mantle flow, which is rotated nearly 10° anticlockwise from the plate-spreading direction. The mismatch between the locus of mantle melt delivery and the morphologic ridge axis results in systematic differences between areas of on-axis and off-axis melt supply. We conclude that the skew of asthenospheric upwelling and transport governs segmentation of the East Pacific Rise and variations in the intensity of ridge crest processes.

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