We develop a forward model of the generation of faulted seafloor topography (visualization) to estimate the relative roles of tectonic and magmatic extension in the presence of core complexes at slow-spreading ridges. The visualization assumes flexural rotation of 60° normal faults, a constant effective elastic thickness, Te, of young lithosphere, and a continuous infill of the depressed hanging wall by lava flowing from the spreading axis. We obtain a new estimate of Te = 0.5–1 km from the shapes of the toes of 6 well-documented oceanic core complexes. We model an 80-km-long bathymetric profile in the equatorial Atlantic across a core complex and the ridge axis at 13°20′N and estimate the variation in tectonic extension, which yields the variation in the fraction of upper crust extension, M, by magmatic diking at the ridge axis. Core complex formation appears to be stable for all values of M < 0.5. The visualization shows how gabbro emplaced at the base of the lithosphere during extension by magmatic diking is partitioned to each side of the spreading axis, and predicts a high probability of finding gabbros in the domes of core complexes.