Mantle skewness and ridge segmentation.

Mantle upwelling is generally assumed to be symmetric. Toomey et al. observe low seismic-wave velocity in off-axis mantle, and suggest that mantle upwelling is skewed, which has important implications for asthenospheric flow, ridge segmentation, crustal accretion, and volcanic, tectonic and hydrothermal vent activities along the ridge axis. However, we point out here that the mantle low-velocity zone (MLVZ) presented by Toomey et al. is not constrained by their data. We conclude that inferences pertaining to ridge segmentation and mantle flow are not reliable.