Near-axis seamount distribution and its relationship with the segmentation of the East Paci¢c Rise and northern Paci¢c^Antarctic Ridge, 17³N^56³S Olga Gomez *, Anne Briais LEGOS-GRGS, Observatoire Midi-Pyre ¨ne ¨es, 14 Avenue Edouard Belin, 31400 Toulouse, France Received 15 April 1999; received in revised form 20 October 1999; accepted 2 December 1999 Abstract 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 17³N to 56³S 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 14^19³S 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 (V150 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. ß 2000 Elsevier Science B.V. All rights reserved. Keywords: East Paci¢c Rise; submarine volcanoes; satellite methods; altimetry; spreading centers 1. Introduction Many volcanoes have been observed o¡-axis near the fast-spreading East Paci¢c Rise (EPR), either isolated [1^7] or organized in chains and ridges [8^13]. These seamounts do not form at the ridge axis, but on a crust at least 0.1 million years old [6,9,10,12^15]. Most of their growth oc- curs on crust 0.1^0.2 Ma [4,7,10,14], with signi¢- cant growth in a larger zone, up to 1.5 Ma for large seamounts or seamount chains in most of the EPR, and up to 3.5 Ma in the Rano Rahi Seamount Field (15^19³S) [12,13]. Afterwards, growth is signi¢cantly slower, although volcanism can still be active on crust as old as 6 Ma [13]. 0012-821X / 00 / $ ^ see front matter ß 2000 Elsevier Science B.V. All rights reserved. PII:S0012-821X(99)00305-2 * Corresponding author. Fax: +33-5-61-25-32-05; E-mail: olga.gomez@cnes.fr Earth and Planetary Science Letters 175 (2000) 233^246 www.elsevier.com/locate/epsl