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 Earth's mantle. At least for the first similar to 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 similar to 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 similar to 320 m Ma(-1/2), i.e., similar to 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. (C) 2008 Elsevier B.V. All rights reserved.