Some Comments on Possible Processes in the Earth's Mantle

Abstract

Many participants of the Symposium in their papers have discussed the possibilities of movements within the material of the Earth’s mantle which have the character of convection currents. As is well known, a number of researchers consider these movements to be the cause of horizontal displacements of continents. Unfortunately, the theoretical models of this type seldom take into account the limitations set by geological data. For instance, several models, rather popular at present, suggest that the distribution and size of the convective cells in the mantle are determined by the distribution on the Earth’s surface of mid-oceanic ridges and rift valleys connected Qith them. It is usually assumed that under the mid-oceanic ridges the rising currents are active, while the descending currents are in-between. The latter usually correlate with geosynclinal folded zones. The spaces between the mid-oceanic ridges and folded zones are considered as whole blocks experiencing only horizontal shifts. On continents the platforms are located outside the folded zones. The relative areas of both types of structures do not remain one and the same, but change with time: in more ancient geological epochs the area of folded zones was greater and the platforms covered correspondingly less space than in later epochs. However, we shall not dwell on this and refer to that pattern of folded zones and platforms which characterise the youngest tectonic cycle, the Alpine, because the constructions of the ‘ convectionists ’ always use this particular latest distribution of folded zones. The geological data provide undoubted indications that the platforms lying outside the limits of folded zones of the Alpine age cannot be considered as monolithic rigid blocks. Uneven distribution of the sedimentary cover on platforms testify to the division of the latter into areas of prolonged sinking and of prolonged rising. These areas have different sizes and we can distinguish among them structures of different order. The largest areas of subsidence and uplifting, correspondingly called syneclises and anteclises, are several hundreds of kilometres across. Moreover, the syneclises usually have irregular rounded contours, while the anteclises fill in the areas between them. Separate syneclises and anteclises exist for extremely long periods in the geological time scale. For instance, the Moscow syneclise as the subsidence region lasts at least from the end of the Proterozoic, i.e. more than 600 million years. It does not mean that the form of the syneclises and anteclises and their distribution remain unchanged during such long periods of time: separate syneclises extend and reduce with time and also move slightly in this or that direction on the surface. From time to time new syneclises and anteclises originate, and some of the old ones cease to exist. But all these changes are of a secondary character and on the whole the complex of syneclises and anteclises on the platforms is rather stable. The rates of vertical movements within both these structures are very small. If we average for hundreds of millions of years these rates will be of the order of mm /year, while the average From the geological point of’ view the scheme is very far-fetched.