Three-dimensional shallow crustal emplacement at the 9 degrees 03 ' N overlapping spreading center on the East Pacific Rise: Correlations between magnetization and tomographic images

We report a three-dimensional (3-D) seismic reflection and tomographic surv ey conducted at the 9 degrees 03'N overlapping spreading center (OSC) on th e East Pacific Rise to understand crustal accretion at this feature. Invers ions of travel time data from 19 ocean bottom hydrophones provide a 3-D ima ge of the shallow velocity structure beneath the nontransform offset and as sociated discordance zone. Seismic analysis indicates that layer 2A thickne ss varies between 100 and 900 in and averages 430 in throughout the study a rea. The heterogeneous upper crustal structure at the OSC region contrasts with the simpler symmetric structure flanking the midsegments of the East P acific Rise. The crust affected by the OSC migration carries evidence for t he complex accretion at the axial discontinuity where the overlap basin may act as a lava pond. An area of thick layer 2A covers the southern half of the overlap basin and the propagating ridge tip and shows good correlation with a high magnetization region. Comparison of the magnetic field anomaly derived from the seismic structure model with the observed sea surface magn etic anomaly suggests that a significant portion of the high magnetization can be related to magnetic source thickness variation rather than solely to the geochemistry of the volcanic rocks.