Horizontal thermal contraction of oceanic lithosphere: The ultimate limit to the rigid plate approximation

There is a contradiction between two widely accepted pillars of global tectonics, (1) the central plate tectonic assumption of plate rigidity and (2) the explanation of the relief of the seafloor as being due to lithospheric subsidence from thermal contraction. Here we quantify the rate of predictable horizontal thermal contraction of the lithosphere using depth averages of widely accepted thermal models. Depth-averaged cooling rate, and thus depth-averaged contraction rate, is proportional to t(-1), where t is the age of the lithosphere. Depth-averaged thermal contraction rate of old (i.e., 167 Ma old) lithosphere is 10(-5) Ma(-1) (3 x 10(-19) s(-1)), which is an order of magnitude greater than the average strain rate inferred from seismic moment release. Newly created (i.e., 0.1 Ma old) lithosphere is displaced by thermal contraction toward lithosphere in old ocean basins at a rate that is 1.35% of the half rate of seafloor spreading, giving displacement rates of 0.1 to 1.1 km Ma(-1) (or, equivalently, 0.1 to 1.1 mm a(-1)). Predicted displacement rates parallel to midocean ridges depend strongly on lithospheric age and are proportional to the distance along which contractional strain rates are integrated. Displacement rates can be significant (>= 1 km Ma(-1)) for young lithosphere. In particular the displacement of oceanic lithosphere adjacent to southern Baja California relative to lithosphere near the Pacific-Antarctic Rise has an indicated north-south displacement rate between approximate to 3 and approximate to 10 km Ma(-1). This suggests that global plate motion circuits based on the assumption of plate rigidity may be biased by a substantial velocity.