Near-axis seamount distribution and its relationship with the segmentationof the East Pacific Rise and northern Pacific-Antarctic Ridge, 17 degrees N-56 degrees S

We used satellite-derived gravity anomaly maps and bathymetry data to analy ze the distribution of off-axis seamounts on young crust on the flanks of t he East Pacific Rise (EPR) and northern Pacific-Antarctic Ridge (PAR), from 17 degrees N to 56 degrees S latitude. We observed large-scale variations in the distribution of the volcanoes which we attribute to variations of th e 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 discontinu ities. The second type occurs preferentially near the centers of second-ord er 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 vulne rability 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 str esses that develop in the cooling lithosphere. In the southern EPR, especia lly in the 14-19 degrees S area, the young lithosphere is probably much mor e vulnerable to off-axis volcanism, The seamount distribution seems to refl ect variations in the off-axis melt production at the scale of second-order segments (similar to 150 km). We infer that the difference in volcanic set ting (close to segment ends or close to segment centers) between the northe rn and southern EPR results from the difference in spreading rate and, cons equently, the variation in rheology of the young lithosphere. (C) 2000 Else vier Science B.V. All rights reserved.