Lithospheric flexure and thermal anomalies

Abstract

Plots of effective elastic thickness, T e, of the oceanic lithosphere as a function of age when elastically deformed show a general increase in T e with the square root of age, although there is much scatter. Some of the scatter can be eliminated by correcting for the effect of finite yield strength in rocks, which makes T e for more sharply bent plates underestimate the true depth T m to the rheological boundary at the base of the high strength mechanical lithosphere. After converting effective values T e to true thicknesses T m, it appears that the mechanical lithosphere is always thinner beneath islands and seamounts compared to lithosphere of similar age flexed at subduction zones. From several other lines of evidence, such as depth anomalies, seamount subsidence, and heat flow data, it has been suggested that the thermal structure in plates subjected to hot spot type volcanism is reset to values appropriate for much younger lithosphere. The reduction in mechanical plate thickness beneath hot spot volcanoes is also consistent with this concept of thermal rejuvenation since the strength of rocks decreases exponentially with rising temperature. This dependence implies that variations in mechanical plate thickness can be used to map temperature anomalies in the lithosphere. Since the mechanical plate thickness depends principally upon the temperature at one depth, specifically the depth at which the temperature reaches 550–600°C (depending on strain rate), flexural data are particularly sensitive indicators of the vertical structure of lithospheric reheating, compared to quantities dependent on depth integrals of temperature, such as swell height and subsidence. The thermal rejuvenation hypothesis predicts that the flexural profile should mimic viscoelastic relaxation in the first ∼10 Ma after loading if initially reheating is confined to depths close to and below the 550°C isotherm. The flexural data combined with depth anomaly information for Hawaii point to extremely rapid reheating localized in the lower lithosphere. Rejuvenation in the Society Islands, on the other hand, is compatible with slower methods of heat injection involving the entire lithospheric column.