Subsidence of normal oceanic lithosphere, apparent thermal expansivity, and seafloor flattening

Seafloor topography has been a key observational constraint on the thermal evolution of oceanic lithosphere, which is the top boundary layer of convection in Earthʹs mantle. At least for the first ~ 70 Myr, the age progression of seafloor depth is known to follow the prediction of half-space cooling, and the subsidence rate is commonly believed to be ~ 350 m Ma− 1/2. Here we show that, based on a new statistical analysis of global bathymetry, the average subsidence rate of normal oceanic lithosphere is likely to be ~ 320 m Ma− 1/2, i.e., ~ 10% lower than the conventional value. We define the ‘normal’ seafloor as regions uncorrelated with anomalous crust such as hotspots and oceanic plateaus, but the lower subsidence rate appears to be a stable estimate, not depending on how exactly we define the normal seafloor. This low subsidence rate can still be explained by half-space cooling with realistic mantle properties, if the effective thermal expansivity of a viscoelastic mantle is taken into account. In light of a revised model of half-space cooling, the normal seafloor unperturbed by the emplacement of anomalous crust exists for all ages, and the so-called seafloor flattening seems to be mostly caused by hotspots and oceanic plateaus.