Douglas R. Toomey, ^* William S. D. Wilcock, Sean C. Solomon, William C. Hammond, John A. Orcutt

Relative travel time delays of teleseismic P and S waves, recorded during the Mantle Electromagnetic and Tomography (MELT) Experiment, have been inverted tomographically for upper-mantle structure beneath the southern East Pacific Rise. A broad zone of low seismic velocities extends beneath the rise to depths of about 200 kilometers and is centered to the west of the spreading center. The magnitudes of the P and S wave anomalies require the presence of retained mantle melt; the melt fraction near the rise exceeds the fraction 300 kilometers off axis by as little as 1%. Seismic anisotropy, induced by mantle flow, is evident in the P wave delays at near-vertical incidence and is consistent with a half-width of mantle upwelling of about 100 km.

D. R. Toomey and W. C. Hammond, Department of Geological Sciences, University of Oregon, Eugene, OR 97403, USA.
W. S. D. Wilcock, School of Oceanography, University of Washington, Seattle, WA 98195, USA.
S. C. Solomon, Department of Terrestrial Magnetism, Carnegie Institution of Washington, Washington, DC 20015, USA.
J. A. Orcutt, Institute of Geophysics and Planetary Physics, Scripps Institution of Oceanography, University of California at San Diego, La Jolla, CA 92093, USA.
^*   To whom correspondence should be addressed. E-mail: drt@newberry.uoregon.edu