Abstract Variations in melt supply can shift slow and ultraslow ridges between magmatic and tectonic accretion modes, but whether this effect persists at Earth’s slowest‐spreading ridge remains unclear. The eastern Gakkel Ridge provides a critical test because it spreads at only ∼6–10 mm/yr yet shows anomalously strong magmatism. We reconstruct sediment thickness and residual basement across the Eurasian Basin using seismic constraints and time‐dependent sedimentation models for the poorly constrained eastern Nansen Basin. Most of the western Eurasian Basin shows deep residual basement and variable flank asymmetry, consistent with magma‐poor ultraslow accretion. Eastward, residual basement becomes progressively shallower and more symmetric. In the eastern Nansen Basin, shallow residual basement persists even under maximum sedimentation, indicating sustained crustal thickening on both conjugate flanks since ∼45 Ma. These results show that enhanced melt supply can shift oceanic crustal accretion toward a near‐symmetric, magma‐dominated mode even at Earth’s slowest‐spreading ridge.