Lithospheric Mantle Heterogeneity Drives Delayed Magmatism and Wide Continent‐Ocean Transitions in Rifted Margins

Abstract The formation of wide, magma‐starved continent–ocean transition (COT) zones remains incompletely understood. We use 2‐D thermo‐mechanical numerical experiments, coupling hydrous mantle melting with parameterized crustal accretion, to explore controls on magmatic lag: the delay between continental crustal break‐up and steady oceanic crustal accretion. Modeling results show that: (a) buoyant rise of subcontinental lithosphere up and toward the rift zone, which inhibits asthenosphere upwelling and decompression melting; (b) a weak mid‐lithospheric discontinuity lengthens magmatic lag by ∼15 Myr and widens the COT by >200 km; (c) the lateral distance over thick, depleted mantle influences rift magmatism is governed by lithospheric density and rheology; and (d) magmatic lag scales linearly with the density contrast between lithospheric mantle and asthenospheric mantle. Model outputs match observed COT widths from the central South Atlantic and conjugate Australia–Antarctica margins, linking inherited lithospheric mantle heterogeneity to melt starvation, mantle exhumation, and rifted‐margin architecture.