Distribution of isolated volcanoes on the flanks of the East Pacific Rise, 15.3°S–20°S

Abstract

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 prominent on the 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.

Basaltic lava domes, lava lakes, and volcanic segmentation on the southern East Pacific Rise

“…This segment contains the Aldo Lake trough, which is located just to the south of the geographic center of the segment (Figure 13 [White et al, 1998]. The similarity of these two patterns is consistent with a hypothesized hierarchy of segmentation of the ridge axis magma supply [Batiza, 1996: Macdonald, 1998] and suggests that more fundamental levels of magmatic segmentation affect volcanic processes farther from the ridge axis.…”

“…First, alongstrike changes in the width of the zone over which seismic layer 2A thickens may correlate with volcanic segmentation (.wider near midsegment). Second, the pattern of axial lava dome distribution on third-order segments is similar to the pattern of isolated, off-axis volcano distribution on secondorder segments (more but generally smaller at segment ends [White et al, 1998]).…”

Iceland structure and volcanism: An alternative vision based on the model of volcanic systems

“…The extraction of magma driven by hydrostatic processes has also the advantage of explaining in a very simple manner the existence of off-axis volcanism, recognized around several mid-oceanic ridges (e.g. in the Pacific, Alexander and Macdonald, 1996;Geshi et al, 2007;White et al, 1998). Nevertheless, there is some degree of coupling between volcanic and tectonic systems that can be used to understand several features observed on the surface, and the long term evolution of a specific zone, as explained next.…”

“…East Pacific Rise, Alexander and Macdonald, 1996;Geshi et al, 2007;White et al, 1998), is expected to be much smaller in volume than the volcanism near the rift axis for two reasons. First, the amount of melt produced within the mantle will decrease as distance from the ridge increases because the upwelling is restricted to a relatively small area below the rift.…”

“…East Pacific Rise, Alexander and Macdonald, 1996;Geshi et al, 2007;White et al, 1998). Several hypotheses have been advanced to explain the occurrence of this type of volcanism, such as local stress variations and/or small-scale mantle heterogeneities (Fialko, 2001;Geshi et al, 2007;Harmon et al, 2011;White et al, 1998), but the origin of parasitic volcanoes is still controversial and not really understood. The basic hydrostatic principles behind magma migration, in the case of regular ridge jumps (north-Atlantic, Gaina et al, 2009;Gernigon et al, 2012;Talwani andEldholm, 1977, or Galapagos, e.g.…”