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

Abstract

Quantifying the melt distribution and crustal structure across ridge-axis discontinuities is essential for understanding the relationship between magmatic, tectonic and petrologic segmentation of mid-ocean-ridge spreading centres. The geometry and continuity of magma bodies beneath features such as overlapping spreading centres can strongly influence the composition of erupted lavas and may give insight into the underlying pattern of mantle flow. Here we present three-dimensional images of seismic reflectivity beneath 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 robust magma supply, with melt bodies underlying both limbs and ponding of melt beneath large areas of the overlap basin. The geometry of melt distribution beneath this offset is inconsistent with large-scale, crustal redistribution of melt away from centres of upwelling. The complex distribution of melt seems instead to be caused by a combination of vertical melt transport from the underlying mantle and subsequent focusing of melt beneath a magma freezing boundary in the mid-crust.

Seismic evidence of a two-layer lithospheric deformation in the Indian Ocean

“…The crust in the vicinity of profile WG3 was formed B56 Ma ago in a fast spreading environment 22 , therefore the mantle must have been hot and the melt mobile, and hence it would have been difficult to retain melt in a steeply dipping narrow zone from the Moho down to 45 km depth; moreover, the maximum dip of imaged crustal magma chamber is 10-15°(ref. 32).…”

Sill to surface: Linking young off-axis volcanism with subsurface melt at the overlapping spreading center at 9°03′N East Pacific Rise

“…In this study, we present new measurements of 238 U-230 Th-226 Ra-210 Pb isotope compositions for a suite of lavas from the overlapping spreading center (OSC) at 9103′N on the East Pacific Rise (EPR). These lavas span a large range of compositions, from basalt to dacite, and include both axial and off-axis samples recovered from a prominent, axis-parallel pillow ridge and a flat-topped seamount that overlie the westernmost extent of a 4-km-wide melt lens (Kent et al, 2000). We report 210 Pb excesses in axial basalts and basaltic andesites, which we suggest results from gas-magma fractionation of 222 Rn from 226 Ra beneath dacite magmas.…”

“…(White et al, 2009) of the study area showing sample locations, rock types, and model ages with uncertainties. Dashed lines encircle the off-axis pillow ridge and flat-topped volcano that overlie the western extent of seismically imaged melt (shown as gray overlay; Kent et al, 2000). The inset map locates the study area.…”

“…In addition, a 3-D seismic reflection study reveals a~4 km wide, upper-crustal melt sill beneath the eastern limb that extends westward into the northern portion of the overlap basin (Kent et al, 2000). The sill reflectors deepen to the west of the east limb axis, suggesting melt may be delivered from an off-axis asthenospheric source to the deeper, off-axis part of this melt sill.…”

“…The sill reflectors deepen to the west of the east limb axis, suggesting melt may be delivered from an off-axis asthenospheric source to the deeper, off-axis part of this melt sill. The melt sill imaged by Kent et al (2000) is shown as a shaded area in Fig. 1 for reference.…”