Northeastern Section - 36th Annual Meeting (March 12-14, 2001)

We present the first tomographic images of upper mantle structure beneath an overlapping spreading center (OSC) on the East Pacific Rise. Our results reveal that this ridge axis discontinuity is underlain by a continuous, though variable, region of anomalously low P-wave velocities. The anomalous structure can be explained by a ~20-km-wide region of high temperatures and a few percent melt. Our results show that OSCs are not necessarily associated with a discontinuity in melt supply and that both limbs of an OSC are supplied melt from a mantle source located beneath the OSC. Our results are not compatible with models in which melt is supplied to OSCs from distal sources located beneath separate ridge segments. We conclude that tectonic segmentation of the ridge by OSCs is not the direct result of magmatic segmentation at mantle depths.