Daniel S. Scheirer, ^* Donald W. Forsyth, Marie-Hélène Cormier, Ken C. Macdonald

Near the Mantle Electromagnetic and Tomography (MELT) Experiment, seamounts form and off-axis lava flows occur in a zone that extends farther to the west of the East Pacific Rise than to the east, indicating a broad, asymmetric region of melt production. More seamounts, slower subsidence, and less dense mantle on the western flank suggest transport of hotter mantle toward the axis from the west. Variations in axial ridge shape, axial magma chamber continuity, off-axis volcanism, and apparent mantle density indicate that upwelling is probably faster and more melt is produced beneath 17°15'S than beneath 15°55'S. Recent volcanism occurs above mantle with the lowest seismic velocities.

D. S. Scheirer and D. W. Forsyth, Department of Geological Sciences, Box 1846, Brown University, Providence, RI 02912, USA.
M.-H. Cormier, Lamont-Doherty Earth Observatory of Columbia University, Palisades, NY 10964, USA.
K. C. Macdonald, Department of Geological Sciences, University of California, Santa Barbara, CA 93106, USA.
^*   To whom correspondence should be addressed. E-mail: scheirer@emma.geo.brown.edu