Stress Control of Processes at Extensional Plate Margins

THE concept of plate tectonics explains 1,2 a wide range of geological and geophysical observations in terms of rigid plates of lithosphere overlying an asthenosphere of no effective strength 3 . The plates are of the order of 50–150 km thick 3 except at their constructive margins where wedges of asthenosphere rise to the base of the crust 4 . These margins are coincident with the “World Rift System”. They are characterized by swarms of local earthquakes 5 which are probably a result of vulcanism 3,5 and may be explained by the hypothesis that magmatic material derived from the asthenosphere is being emplaced here 6 . Seismic activity is confined to a zone less than 20 km wide about the crest of the oceanic rift system 3 but becomes dispersed over widths of 100 km or more over the continental extensions. The plate interface here is defined by the zone where the basaltic melt separates sufficiently from the asthenosphere to increase markedly the viscosity of the latter 4 , and magma will either remain where it is or be intruded into the rigid lithosphere if this fractures appropriately. We examine here the effect of the width of stress application below a constructive plate margin. Since the mechanisms producing plate motion are uncertain we do not base our study on the size or geometry of hypothetical convection cells, but merely assume that extensional plate margins are underlain by magma chambers under hydrostatic pressure.