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Article

Abstract

Volcanic constructions, not associated with seamount (or volcano) chains, are abundant on the flanks of the East Pacific Rise (EPR) but are rare along the axial high. The distribution of isolated volcanoes, based on multibeam bathymetric maps, is approximately symmetric about the EPR axis. This symmetry contrasts with the asymmetries in the distribution of volcano chains (more abundant on the west flank), the seafloor subsidence rates (slower on the west flank), and the distribution of plate-motion-parallel gravity lineaments (more prominento nthe west flank). Most of the isolated volcanoes complete their growth within -14 km of the axis on crust younger than 0.2 Ma, while seamount chain volcanoes continue to be active on older crust. Volcanic edifices within 6 km of the ridge axis are primarily found adjacent to axial discontinuities, suggesting a more sporadic magma supply and stronger lithosphere able to support volcanic constructions near axial discontinuities. The volume of isolated near-axis volcanoes correlates with ridge axis cross-sectional area, suggesting a link between the magma budget of the ridge and the eruption of near-axis volcanoes. Within the study area, off-axis volcanic edifices cover at least 6% of the seafloor and contribute more than 0.2% to the volume of the crust. The inferred width of the zone where isolated volcanoes initially form increases with spreading rate for the Mid-Atlantic Ridge (<4 km), northern EPR (<20 km), and southern EPR(<28 km), so that isolated volcanoes form primarily on lithosphere younger than 0.2 Ma (< 4-6 km brittle thickness), independent of spreading rate. This suggests some form of lithospheric control on the eruption of isolated off-axis volcanoes due to brittle thickness, increased normal stresses across cracks impeding dike injection, or thermal stresses within the newly forming lithosphere.

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