On the Character of Isostatic Equilibrium Disturbances

Abstract

Summary The data of disturbances of the isostasy of the Earth’s crust and their relation to different tectonic zones give valuable information about processes in the Earth’s crust. They may be used for the evaluation of some physical parameters, for example of the asthenosphere viscosity. The last value is usually determined on the basis of the data on postglacial uplift, but there are doubts that these values can be applied to the regions with different tectonics. For this reason we have studied as a first step the character of disturbances of the isostasy in the areas of different tectonics. It was ascertained that disturbances of the isostatic equilibrium are the result of the active geotectonic process and that isostatic forces are passive as tectonic factor. In the geological time scale the readjustment takes place instantaneously, therefore isostatic anomalies of gravity may be considered as direct indications of the modern tectonic activity. Regular relations between isostatic disturbances and the character of the latest tectonic movements can be clearly revealed by the comparison of the isostatic anomalies maps with the schemes of Cenozoic crustal movements in the different tectonic zones. In particular, the antii ostatic movements of the Earth’s crust are very common features. The relations revealed between the isostatic gi avity anomalies and tectonic movements permit us to make some conclusions about the character of internal processes in the Earth. The data of isostasy are used for the evaluation of some physical parameters, characterizing the Earth’s crust state and its upper mantle. For example, for the evaluation of the asthenosphere viscosity the data are used which gives the study of Quaternary glaciations. The variety of tectonic conditions on the Earth’s surface reflects different processes taking place in the Earth’s interior and evidently shows different physical conditions. Because of this we are not sure that the results obtained from the study of isostasy in regions of glaciations (representing, as a rule, preCambrian shields) may be extrapolated on the regions differing by their tectonic positions. The results of the study of Quaternary glaciations give quite low values of the viscosity of the asthenosphere and very small times of readjustment (103-105 years). At the same time on the Earth’s surface numerous and great disturbances of equilibrium are known, manifesting in large isostatic anomalies of gravity. This fact is of great interest for an investigator. Firstly, it is very important to clear up the nature of the connection between the disturbances of isostasy and the movements of the Earth‘s crust and the change of the character of these correlations in areas of different tectonic conditions. A number of investigations has been carried out by well-known scientists: Vening Meinesz, van Bemmelen, Bullard, Lyustikh and others. On the basis of the data of isostatic gravity anomalies collected in the Institute of Physics of the Earth in Moscow an attempt was made to find basic features of isostasy in regions of different tectonics, and also the number of more particular correlations within the boundaries tectonic zones. (Artemyev 1962, 1963, 1964.)