Sea-floor spreading and plate tectonics have been widely accepted since the publications of Hess (1962), Isacks and others (1968), and many others (see Wyllie, 1971). The mechanism that causes sea-floor spreading is, however, still unknown; the flow pattern in the asthenosphere is also unknown. There may exist more than one mechanism and flow pattern in the mantle at different places and times.
Most models that have been proposed can be grouped into two classes. The first class of models is based on the hypothesis of convection currents (to depths of 600 to 700 km) in the upper mantle (Heiskanen and Vening Meinesz, 1958; Oxburgh and Turcotte, 1968; Isacks and others, 1968); currents are thought to ascend beneath mid-oceanic ridges and drag the oceanic lithosphere passively from the ridges to the subduction zones. In the second class of models (Ringwood, 1969; McKenzie, 1969), flow in the asthenosphere is assumed to be passive, caused by the movement of the lithospheric plates. The movement of a plate might be caused by a pull that the plate’s edge exerts while descending along a subduction zone, by a push from the magma that extrudes at the spreading centers, or by gravitational sliding from the topographic high of a ridge. Mantle diapirs (Maxwell, 1968) and plumes (Morgan, 1971) might cause overall flow patterns that can be included in either class of models. The flow pattern at comparable depths in the upper mantle should be quite different for these two classes (Avé Lallemant and Carter, 1970).
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