Evidence from three-dimensional seismic reflectivity images for enhanced melt supply beneath mid-ocean-ridge discontinuities

Quantifying the melt distribution and crustal structure across ridge-axis d iscontinuities is essential for understanding the relationship between magm atic, tectonic and petrologic segmentation of mid-ocean-ridge spreading cen tres. The geometry and continuity of magma bodies beneath features such as overlapping spreading centres can strongly influence the composition of eru pted lavas(1) and may give insight into the underlying pattern of mantle fl ow. Here we present three-dimensional images of seismic reflectivity beneat h a mid-ocean ridge to investigate the nature of melt distribution across a ridge-axis discontinuity. Reflectivity slices through the 9 degrees 03' N overlapping spreading centre on East Pacific Rise suggest that it has a rob ust magma supply, with melt bodies underlying both limbs and ponding of mel t beneath large areas of the overlap basin. The geometry of melt distributi on beneath this offset is inconsistent with large-scale, crustal redistribu tion of melt away from centres of upwelling(2,3). The complex distribution of melt seems instead to be caused by a combination of vertical melt transp ort from the underlying mantle and subsequent focusing of melt beneath a ma gma freezing boundary in the mid-crust.