On the interpretation of linear relationships between seafloor subsidence rate and the height of the ridge

Several studies have found that the 'subsidence rate' of the oceanic lithosphere measured along flow lines is a linear function of the height of the ridge. This might have very important implications concerning our understanding of the cooling of the lithosphere. Here we study the relationship between the subsidence rate and the height of the ridge in the South Atlantic and East Indian Oceans. From the regression slopes of this relationship, it has been inferred that lithosphere created at shallower ridges subsides faster and to deeper levels than that generated at deeper ridges. We show however, that the topography at young ages for various seafloor segments stays positively correlated with the topography at older ages. Synthetic data sets are obtained by adding random synthetic signals with the same spectrum as the residual topography to the average depth versus age relationship. They are also characterized by the same linear relationship between 'subsidence rates' and the height of the ridge. This relationship is therefore not proof that lithosphere formed at shallow ridges cools faster than lithosphere formed at deeper ridges. Random signals linked with crustal thickness variations, hotspot bulges and perhaps large wavelength 'dynamical topography' would also yield such a relationship.