A consensus is emerging from studies of continental rifts and rifted margins worldwide that significant extension can be accommodated by magma intrusion prior to the development of a new ocean basin. However, the influence of loading from magma intrusion, lava extrusion, and sedimentation on plate flexure and resultant subsidence of the basin is not well understood. We address this issue by using three-dimensional flexural models constrained by geological and geophysical data from the Main Ethiopian Rift and the Afar Depres- sion in East Africa. Model results show that axial mafic intrusions in the crust are able to cause significant downward flexure of the open- ing rift and that the amount of subsidence increases with decreasing plate strength accompanying progressive plate thinning and heating during continental breakup. This process contributes to the tilting of basaltic flows toward the magma injection axis, forming the typical wedge-shaped seaward-dipping reflector sequences on either side of the eventual rupture site as the new ocean basin forms.