Archaean plate tectonics revisited 1. Heat flow, spreading rate, and the age of subducting oceanic lithosphere and their effects on the origin and evolution of continents
A simple model which relates the rate of seafloor creation and the age of the oceanic lithosphere at subduction to the rate of continental accretion can successfully explain the apparent differences between Archaean and Phanerozoic terrains in terms of plate tectonics. The model has been derived using the following parameters: (1) the spreading rate at mid-ocean ridges; (2) the age of the oceanic lithosphere at the time of subduction; (3) the area-age distribution of the seafloor; (4) the continental surface area as a fraction of the total surface area of the earth; and (5) the erosion rate of continents as a function of continental surface area and the total number of continental masses. Observations in Phanerozoic terranes suggest that there are profound differences in the nature and volume of subduction zone igneous activity depending upon the age of the oceanic lithosphere being subducted and the nature of the overriding plate (that is, either continental or oceanic). The subduction of young oceanic lithosphere (less than 50 m.y. old) which is thermally buoyant appears to result in a reduced volume of igneous activity. Most of the igneous activity caused by subduction of young oceanic lithosphere is either siliceous plutonism or bimodal tholeiitic-rhyolitic volcanism. When very young lithosphere is being subducted (<30 m.y. old), volcanism appears to cease. The subduction of old oceanic lithosphere (>50 m.y. old) appears to result in greater volumes of igneous activity, including the eruption of andesitic magmas. Thus andesites could only begin to be abundant in the rock record when older oceanic lithosphere began to be subducted. Our model predicts that as the earth aged and as heat flow from the interior of the earth diminished, the proportion of old oceanic lithosphere being subducted increased, fundamentally changing the nature of subduction zone igneous activity and the rate of continental accretion. If the subduction of old oceanic lithosphere results in an 8–10 times greater volume of subduction zone magmatism, our model predicts or explains all of the following observed features of earth history: (1) Archaean terranes appear to record two periods of rapid continental accretion, between 3.8 and 3.5 b.y. ago and between 3.1 and 2.6 b.y. ago; (2) there are very few differences and many marked similarities between rocks from Archaean terranes and equivalent rocks from Phanerozoic terranes; (3) the total continental area appears to have remained essentially constant for the past 2 b.y. (4) Archaean andesites are comparatively rare, and the relative abundances of mafic and siliceous rocks appear to change during the Archaean and the Proterozoic, with siliceous volcanics becoming proportionately more abundant in the geologic record with time; (5) plutonic tonalites and trondhjemites appear to have been relatively much more abundant during the Archaean. Plate tectonics is thus shown to have evolved over time due to a gradually decreasing rate of creation of oceanic lithosphere, meaning that Archaean tectonics and Phanerozoic tectonics are but two points on an evolutionary continuum.
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