Evolution of young oceanic lithosphere and the meaning of seafloor subsidence rate

Plate tectonics, a special class of mantle convection so far observed only on the Earth, is responsible for a vast array of geological processes, from the generation of continental crust to the modulation of atmospheric composition. Whereas conditions for its operation are still debated, the minimum requirement is generally thought that surface plates become denser than the underlying asthenosphere so that they can subduct. Recent studies, however, have raised the possibility that even mature oceanic plates remain buoyant because chemical buoyancy is too high to be overcome by negative thermal buoyancy, challenging the basic tenet of plate tectonics. Here we show that on the basis of new integrated geophysical and petrological modeling, oceanic plates do become negatively buoyant after ∼30 Myr. Our modeling also indicates that the seafloor would subside at a rate of ∼500 m Myr−1∕2, which is considerably faster than the observed rate of ∼320 m Myr−1∕2. We suggest that this discrepancy in subsidence rate is best explained by the combined effect of incomplete viscous relaxation within oceanic plates, radiogenic heat production in the convecting mantle, and the secular cooling of the Earth.