Subsidence of "normal" seafloor: Observations do indicate "flattening"

Seafloor topography is a key observational constraint upon the evolution of the oceanic lithosphere. Specifically, plots of oceanic depth (z) versus crustal age (t) for “normal” seafloor are well explained by depth-age predictions of thermal contraction models such as the half-space and cooling plate model. Old seafloor (t > ∼70 Ma) shallower than that predicted by half-space cooling (i.e., z ∝ √t), or “flattening,” is a key but debated discriminator between the two models. Korenaga and Korenaga (2008) in a recent paper find normal seafloor depths of all ages to be consistent with a z ∝ √t model, thus supporting a cooling half-space model for all ages of seafloor. Upon reevaluation, however, the mean depths of their “normal” seafloor flatten at ages >70 Ma, e.g., by 723.2 ± 0.5 m (1 standard error) for t > 110 Ma. This observed inconsistency with the z ∝ √t model is statistically significant (>99.9%) and remains robust (>94%) even if the number of effective independent depth observations is argued to be low (e.g., n = 10). So, if any statistically significant conclusion can be drawn from the observed depths of rare old normal seafloor, it is that old seafloor flattens, which is incompatible with the cooling half-space model applying to all ages of seafloor but does not preclude a cooling-plate style approximation to lithospheric evolution.