Patterns of volcanism at oceanic intraplate hotspots

One of the defining characteristics of plume-fed hotspots is the formation of a linear chain of age-progressive volcanoes [Wilson, 1963; Morgan, 1972; Courtillot et al, 2003]. However, in detail, the spatial distribution of volcanoes at oceanic hotspots is often complex and rarely takes the form of a simple linear array. Volcanoes at Hawaii, the archetype of plume-fed hotspots, have long been recognized to form two separate linear arrays, known as the Loa and Kea trends [Jackson, 1972]. Recent studies have suggested that volcanism at several additional hotspots, including the Samoa [Workman et al., 2004], Marquesas [Chauvel et al., 2009; Huang et al., 2011], and Society [Payne et al., 2012] hotspots, may also be loosely organized into sub-parallel trends. We have undertaken a systemic characterization of the spatial distribution of recent (3 Ma - present) magmatism, as reflected in bathymetry and topography, at a number of oceanic intraplate hotspots. We find that the average across-track (i.e., perpendicular to plate motion) bathymetric profile shows a distinct dual peak pattern at many hotspots. Characteristic spacing between peaks ranges from ~20 - 60 km and does not correlate with the age of the plate, as would be expected if the distribution of volcanism was being controlled by the elastic thickness of the plate [ten Brink, 1991]. Likewise, peak spacing does not appear to correlate with plate speed in the HS3 reference frame [Gripp and Gordon, 2002]. Spacing at individual hotspots does, however, correlate well with calculated plume buoyancy flux. This suggests that the time-averaged pattern, of dual-chain volcanism at the surface is reflects a bifurcated distribution of melting in the mantle rather than melt transport processes through the lithosphere. We propose that the dual-chain pattern of volcanism at hotspots results from the creation of a highly viscous plug of buoyant, dehydrated residuum that extends downwards from the base of the lithosphere, splitting upward flow within the plume conduit at shallow depths.