Near-axis seamount distribution and its relationship with the segmentation of the East Pacific Rise and northern Pacific–Antarctic Ridge, 17°N–56°S

We used satellite-derived gravity anomaly maps and bathymetry data to analyze the distribution of off-axis seamounts on young crust on the flanks of the East Pacific Rise (EPR) and northern Pacific–Antarctic Ridge (PAR), from 17°N to 56°S latitude. We observed large-scale variations in the distribution of the volcanoes which we attribute to variations of the regional magmatic budget of the underlying mantle. Two distinct settings for off-axis volcanism were observed at the scale of second-order segments. The first one favors volcano growth near first- or second-order discontinuities. The second type occurs preferentially near the centers of second-order segments. We infer that the distribution of seamounts is controlled both by the availability of melt under the flanks of the ridge and by the vulnerability of the young lithosphere. Near the northern EPR and in some places near the PAR, the off-axis volcanism results mostly from fracturing of the crust near axial discontinuities, probably due to the thermomechanical stresses that develop in the cooling lithosphere. In the southern EPR, especially in the 14–19°S area, the young lithosphere is probably much more vulnerable to off-axis volcanism. The seamount distribution seems to reflect variations in the off-axis melt production at the scale of second-order segments (∼150 km). We infer that the difference in volcanic setting (close to segment ends or close to segment centers) between the northern and southern EPR results from the difference in spreading rate and, consequently, the variation in rheology of the young lithosphere.