THE GLOBAL SEISMIC TOMOGRAPHY MODELES INTO EXPLORATIONS OF THE EARTH’S STRUCTURES

Abstract

The global tomograpics modeles of upper mantle of the Earth with modern methods are analyzed. The results of different geophysical methods are analyzed in this review to characterize structure of the upper mantle. Based on these data one may assume that corresponding low temperature anomalies also extend to these depths. Based on a joint interpretation of gravity and seismic data residual mantle gravity anomalies are determined. Positive anomalies are found near active plate boundaries. Gravity influence of the temperature induced mantle inhomogeneities determined from seismic tomography data has been removed from the total mantle anomaly field, and the compositional gravity anomalies are obtained. The use of a gravitational field to determine the properties of the distribution of the density of the upper mantle has a long history, but the vast majority of these works are regional, that is, they describe the properties of the lithosphere within specific tectonic structures. The basis of such works is geotravers of deep seismic sounding, using additional information, which limits the range of possible solutions of the inverse problem of gravimetry. Specificity of the used interpretation techniques does not allow to directly compare the results obtained for different structures. In particular, density variations are always determined in relation to some "standard" model, which can vary widely. Even a simple comparison of the average density of the continental and oceanic mantle is still a problem. Thus, only a global study by a single method allows comparative analysis of isolated structures. It is known that the use of only a gravitational field without additional information does not allow us to obtain a reliable result. Global three-dimensional images of the internal structure of the Earth, created on the basis of variations in the velocities of seismic waves, have become one of the main achievements of geophysics of the last decade.

These models made it possible to compare the evolution of geostructures, often located on opposite sides of the Earth. The bulk of the global tomographic models is represented by the distribution of the velocities of transverse waves. This is due to the fact that for a modern system of seismic stations, the more or less homogeneous structure of the upper mantle can be obtained only on the basis of the analysis of surface waves, which is mainly determined by variations of horizontally and vertically polarized transverse waves and having insignificant sensitivity to the longitudinal velocities waves At the same time, tomographic models represented by variations of longitudinal waves can only be obtained on the basis of the analysis of the run-times of bulk waves, which results in a direct connection of the horizontal resolution of the model with the density of the network of seismic stations. In addition, bulk waves do not allow sufficient vertical resolution in the upper mantle, even in the presence of a large number of stations. Thus, the analysis of global tomographic models makes it possible to conclude that the temperature distribution in the upper mantle (on the structure of the thermal roots of the continents) without the interpretation of the data on the heat flux.

The generalization of the latest results of seismic, gravitational and thermal studies of continental roots makes it possible to state that seismic tomography remains the only method that gives a spatial picture of the upper mantle. The greatest values of mantle anomalies are found near the subduction zones surrounding the Pacific Ocean, as well as in the Alpine-Mediterranean folded belt, which represents the intra-continental zone of collision of lithospheric plates.

Key words: global seismic tomography, craton, geophysical methods, gravitational anomalies, upper mantle