HORIZONTAL THERMAL CONTRACTIONAL STRAIN OF OCEANIC LITHOSPHERE: THE ULTIMATE LIMIT TO THE RIGID PLATE HYPOTHESIS

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 and depth- and age-averages of widely accepted thermal models. We find that newly created 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 Myr^-1. The bias in plate displacement rates estimated from marine magnetic anomalies, expressed as a percentage of the full spreading rate, is smaller because the magnetic anomalies used to estimate rates span a region of high thermal contraction. These percentages are 0.60% of the full spreading rate when the spreading rate is based on the middle of anomaly 2A, as in the NUVEL-1A set of relative plate velocities, and is 0.85% if the spreading rate is based on the Central anomaly. For the global range of full spreading rates of 10 km Myr^-1 to 160 km Myr^-1, these contractional strain rates correspond to a range of displacement rates of 0.06 km Myr^-1 to 1.0 km Myr^-1 for rates determined from anomaly 2A to a range of displacement rates of 0.09 km Myr^-1 to 1.36 km Myr^-1 for rates determined from the Central anomaly. Displacement rates parallel to mid-ocean ridges depend strongly on lithospheric age and are proportional to the distance along which contractional strain rates are integrated. The displacement rates are negligible for old oceanic lithosphere but can be substantial for young lithosphere. In particular the displacement rates of lithosphere adjacent to southern Baja California relative to lithosphere near the Pacific-Antarctic Rise have indicated north-south displacement rates near 10 km Myr^-1. This suggests that global plate motion circuits based on the assumption of plate rigidity are biased by a substantial velocity. Expected horizontal thermal contractional strain of oceanic lithosphere appears to be the ultimate limit to the rigid plate hypothesis.