The Implications of Terrestrial Heat Flow Observations on Current Tectonic and Geochemical Models of the Crust and Upper Mantle of the Earth

Abstract

Summary The average heat flow through continental orogenic belts decreases with the age of the orogeny to an approximately constant value for the Precambrian shields and platforms. The average heat flow for provinces of the North Pacific decreases with the age of the province. The mean heat flow through the province younger than 10 million years is 2.82 p cal cm-2 -1 whereas the mean heat flow through provinces older than the middle Cretaceous is 1.15~ cal cm-2 s-'. The contrast in the chemical composition of continental and oceanic crustal rocks and the disparity in time scale for the decay of heat flow suggest a different explanation for the heat flow through oceans and continents. The observed equality of the mean oceanic and continental heat flows may not be relevant to evaluating the thermal contribution of the upper mantle. Two geophysical and geochemical models of the oceanic and continental crust and upper mantle are presented. Both can explain the near equality of heat flow through the Precambrian shields and the old ocean basins when plates of continental and oceanic lithosphere are allowed to move. The oceanic lithosphere is approximately 100 km thick and the mantle is assumed to be the same under both the continents and oceans. The models differ principally in the condition assigned to their lower boundary. These conditions result in two different geochemical compositions for the oceanic lithosphere. Both oceanic models can explain the flow of heat through the North Pacific and the topography of the East Pacific Rise. If the effects of water in the mantle and subsolidus phase changes are considered, the lithosphere could be as thin as 75 km and still consistent with the heat flow and topographic anomalies. The models also account for the elevation of the mid-Atlantic ridge and the gross structure of the heat flow through the South Atlantic. The loss of heat in creating oceanic lithosphere may be as much as 45 per cent of the total average heat flow of the Earth. Heat lost by this process can no longer be ignored in models of the thermal history of the Earth. s