A 3-DIMENSIONAL GRAVITY ANALYSIS OF THE EAST PACIFIC RISE FROM 18-DEGREES TO 21-DEGREES-30-MINUTES-S

Multibeam bathymetry and gravity coverage of the East Pacific Rise (EP R) between 18 degrees and 21 degrees 30'S is used to investigate the r elation between melt supply and tectonic segmentation at ultrafast spr eading rates. The long-wavelength features in the residual anomaly sho w a good correlation with those in the bathymetry, The highest residua l anomaly values occur over the broad discordant zone of the 20 degree s 40'S overlapping spreading center (OSC), for seafloor ages of 0 Ma t o at least 1.5 Ma. We interpret the deepening of the bathymetry and th e increase of the residual anomaly toward that discordant zone as due to a decrease of 500 +/- 200 m of the crustal thickness, Hence the 20 degrees 40'S OSC has been associated with a reduced magmatic budget fo r at least the past 1.5 m.y. and represents a persistent segmentation of the EPR. This is consistent with models in which mantle upwelling, even at the fastest spreading centers, is enhanced between large disco ntinuities rather than evenly distributed along axis. However, this de crease of the crustal thickness toward an axial discontinuity is sever al times smaller than that typically documented for slow spreading rid ges, which suggests that mantle upwelling is less focused at fast spre ading ridges, or that along-axis transport of crustal material is more efficient, or both. Across the study area, the residual anomaly decre ases toward the NW by 15-20 mGal. This regional gradient can be modele d with lateral temperature variations in the upper mantle of up to 60 degrees C, increasing toward the NW. This interpretation is consistent with the numerous seamounts present to the NW and the robust magmatic budget of the ridge between 17 degrees and 18 degrees S, and it could also explain why the ridge segments defined by the smaller OSCs betwe en 18 degrees and 19 degrees S propagate very rapidly away from the ro bust area. Similar patterns of ridge propagation away from the shallow est section of a ridge have been documented near the Galapagos and Eas ter Island hot spots. Hence these shorter ridge segments may not be as sociated with significant individual melt sources. Rather, they may re present a superficial segmentation due to the interaction between the EPR and a mantle heterogeneity located between 17 degrees and 18 degre es S.