RHEOLOGY OF THE EARTH: THE BASIC PROBLEM OF GEODYNAMICS

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Abstract
A discussion is given of the data which are available to provide information on the rheological behavior of the Earth's mantle and crust. It is shown that a fundamental distinction has to be made between stress durations that are "short" (up to 4 hours), "intermediate" (4 hours to 15,000 years), and "long" (longer than 15,000 years). For short durations, the behavior of the Earth is well known from the results of seismological investigations. However, these results should not be applied directly to the longer time durations. For intermediate time durations, the Earth shows an elastic aftereffect. One obtains a value for the (Kelvin) viscosity if it is assumed that the damping of the variation of latitude is caused by a Kelvin effect. The same (Kelvin) viscosity is arrived at if the strain rebound characteristics of earthquake aftershock sequences are also interpreted in terms of an elastic aftereffect. For stresses of long duration, some information may be obtained from the uplift of Fennoscandia. It appears that viscous creep is the prevailing phenomenon. If this information is combined with some further considerations, indications are that the Earth behaves like a Bingham body. Then, the fact that orogenetic phenomena are concentrated in narrow bands may be due to the existence of solutions of the Bingham equation exhibiting rheological instability.

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