Upper-Mantle Seismic Reflections beneath the East Pacific Rise

Multichannel seismic reflection profiles from the East Pacific Rise show distinctive patterns of reflecting events immediately below the reflection Moho in the axial region. These events begin at the reflection Moho horizon between 2 and 12 km from the ridge axis. They dip inward, in two-way traveltime, by 0.2 to 0.4 s over horizontal distances of approximately 1 to 3 km; depth conversion indicates that their dip is between 30-degrees and 45-degrees. The events terminate or fade away down dip and are not observed beneath the ridge crest itself. They are observed individually or in pairs about the ridge axis and have only been identified in the axial region. We believe that these are the first well-documented examples of upper-mantle events in marine seismic observations from an active spreading environment. They provide much-needed observational data on the structure of the uppermost mantle beneath the mid-ocean ridge. We favor the hypothesis that the sub-Moho events are directly related to the accumulation of melt in the upper mantle. If this interpretation is correct, the distribution of events implies that melt arriving at the base of the crust is not always centered with respect to the ridge axis or any imaged crustal features.