Marine geodynamics: Depth‐age revisited

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Abstract
More than 20 years have passed since the subsidence of the seafloor away from midocean ridges was explained by the cooling of the lithosphere (Figure 1), a thermal boundary layer at the Earth's surface [Turcotte and Oxburgh, 1967], Although the general concept has proven quite sound and found much practical application, the details of the model have never been completely agreed upon. For example, if no process limits the growth of the thermal boundary layer, the seafloor should continue to deepen as the square root of lithospheric age. In contrast, the observations indicate that little deepening occurs for lithosphere aged greater than about 100 Ma (Ma = million years before present). Early on this fact was explained by supposing that there was a limit to the depth of cooling of the boundary layer, such that the lithosphere cools as a plate of finite thickness rather than as a semi‐infinite half space [McKenzie, 1967]. According to the plate model, the depth of old lithosphere approaches a maximum as the geothermal gradient in the plate reaches thermal equilibrium. However, this theoretical model merely provided a better fit to the observations without addressing the fundamental question of what process might limit the growth of the Earth's uppermost thermal boundary layer.

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