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The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 56 (1972)

Issue: 11. (November)

First Page: 2127

Last Page: 2149

Title: Mantle Creep: Elasticoviscous Versus Modified Lomnitz Law, and Problems of "The New Global Tectonics"

Author(s): Paul S. Wesson (2)


Because of the two opposing schools of thought concerning the anelasticity of the mantle--the larger of which maintains that convection is possible, and the other that it is not--data relevant to creep in the earth's interior have been examined with the object of reconciling these views. The data presented include (1) anelasticity in the laboratory; (2) anelasticity of rocks; (3) the Lomnitz and modified Lomnitz laws; and (4) Q-values. These sections are mainly a synthesis of the expected behavior and state of the mantle as judged by theory and experiments on the surface. Pertinent geophysical observations are considered at every stage, however, in stating the two sides of the controversy as typified by elasticoviscosity and the two Lomnitz laws. The conclusion reached is that although laboratory tests and sea-floor spreading hypotheses support convection, at least six global facts unequivocably oppose it. The problem is clarified by considering evidence of a more geologic type: glacial uplift data, creep processes in the earth, phase changes, the low-velocity layer and phase changes, and convection currents.

To put the plan into effect without bias entails harboring the temporary opinion that plate tectonics and convection may be implausible; this may indeed be so, and an earth model is proposed in which the lower mantle is nonconvective. This region is separated, at a depth of 700 km, by an impassable boundary that is probably a phase change connected with the earth's present depth of cooling, from a layer that is irregularly convective. In the latter region the elasticoviscous law is dominant, probably by virtue of Herring-Nabarro creep. The lower mantle (below 700 km) is responsible for the support of the nonhydrostatic equatorial bulge; it obeys the modified Lomnitz law in which the rate of creep decreases as time goes on because of grain growth in a material already having a large gr in size. The upper mantle, if this picture is correct, must be responsible for the damping of the Chandler wobble of the axis. Lack of data on the grain size of mantle material and ignorance of values of the diffusion coefficient at high pressure are the two greatest hindrances to further progress in research of this kind.

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