Effective thermal expansivity and the origin of 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 Earth's mantle. To first order, the age progression of seafloor depth is well described by half-space cooling, but it is also known that seafloor older than ~80~Ma is generally shallower than this simple model predicts. The origin of this seafloor flattening has been controversial. One prevailing idea is the so-called plate model, which limits the growth of oceanic lithosphere by imposing a temperature boundary condition. Though this boundary condition is problematic lacking a clear physical explanation, the plate model has been popular because of no convincing alternatives. We show that, on the basis of residual statistics, seafloor flattening is almost entirely caused by hotspots and oceanic plateaus. Our new statistical analysis shows that the canonical seafloor unperturbed by such anomalies exists for all ages and is well explained by half-space cooling, if the effective thermal expansivity of Maxwellian oceanic lithosphere is taken into account. Surface heat flow on the canonical seafllor is also shown to be consistent with the new cooling model. The thermal evolution of oceanic lithosphere is thus finally described by a both physically and observationally consistent model.