Fig. 4: Modelled layering in the lower oceanic crust.

a, 1D velocity–depth profiles at the distance of 182 km from the MAR, derived from tomography (blue) and FWI (black). Vp is the P-wave velocity of seismic waves. The thin dashed black line is the interpreted boundary of layer 2A/2B from the FWI model; the bold dashed blue and black lines indicate the boundaries of layer 2/3 from the tomographic and the FWI models, respectively. We invert the velocity model from FWI down to a maximum of 9 km depth (~5.4 km from basement at distance 182 km), because of potential Moho reflection interference for greater depths (Supplementary Fig. 22). b, A schematic diagram illustrating the oceanic crustal structure at the MAR and away from the ridge axis. The black balls in layer 2A and stripes in layer 2B refer to pillow lava and basaltic dykes, respectively. The blue ellipsoids refer to hydrothermal alteration above the roof of the AML that could contribute to the top low-velocity layer. The red vertically elongated ellipsoids suggest magma from mantle upwelling. The light brown layers in the lower crust refer to the low-velocity layers from FWI. The imaged layers in the lower crust may comprise multiple thinner layers as indicated by the darker brown patches, but are beyond data resolution.