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 (EPR) between 18 degrees and 21 degrees 30'S is used to investigate the relation between melt supply and tectonic segmentation at ultrafast spreading rates. The long-wavelength features in the residual anomaly show a good correlation with those in the bathymetry, The highest residual anomaly values occur over the broad discordant zone of the 20 degrees 40'S overlapping spreading center (OSC), for seafloor ages of 0 Ma to at least 1.5 Ma. We interpret the deepening of the bathymetry and the 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 for 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 discontinuities rather than evenly distributed along axis. However, this decrease of the crustal thickness toward an axial discontinuity is several times smaller than that typically documented for slow spreading ridges, which suggests that mantle upwelling is less focused at fast spreading ridges, or that along-axis transport of crustal material is more efficient, or both. Across the study area, the residual anomaly decreases toward the NW by 15-20 mGal. This regional gradient can be modeled 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 between 18 degrees and 19 degrees S propagate very rapidly away from the robust area. Similar patterns of ridge propagation away from the shallowest section of a ridge have been documented near the Galapagos and Easter Island hot spots. Hence these shorter ridge segments may not be associated with significant individual melt sources. Rather, they may represent a superficial segmentation due to the interaction between the EPR and a mantle heterogeneity located between 17 degrees and 18 degrees S.