Controls on the magmatic fraction of extension at mid-ocean ridges

•Magmatic component of seafloor spreading (M) increases non-linearly with spreading rate.•Stress interactions between fault slip events and magma intrusions provide physical basis for M.•Axial lithosphere thickness and rate of pressure build-up in sills are primary controls on M.•Transitions between modes of seafloor spreading reflect thresholds in axial lithosphere thickness. The fraction M of plate separation accommodated by magma emplacement at mid-ocean ridges has been recognized as the main control on seafloor spreading modes, yet the factors that control M itself are poorly understood. Here we put forward a simple theoretical framework explaining M in terms of short-term cycles of earthquakes and dike intrusions interacting with one another by modulating the stress state of the ridge axis. Axial lithospheric thickness and the rate of pressure build-up in shallow, replenishing magma sills are the two main parameters controlling M in our simulations. Combined with plausible scenarios for the increase of pressure build-up rate and the decrease of lithospheric thickness with increasing spreading rate, our model appropriately brackets available measurements of M from slow, intermediate, and fast-spreading ridges. Our model further suggests that the transitions between major modes of seafloor spreading (detachment faulting, symmetric faulting, and fully-magmatic) correspond to thresholds in axial lithospheric thickness, and that the great variability in M at slow and ultraslow ridges directly reflects along-axis variability in thermal structure. More generally, this implies that the balance between bottom-up magmatic heating and top-down hydrothermal cooling fully determines the time-averaged rate of magmatic intrusions in the brittle lithosphere, and thus the modes of mid-ocean ridge faulting which shape ∼2/3 of Earth's surface.