Abstract

A set of geological-geophysical data provides a good basis for the recognition of two basic types of marginal zones: Atlantic and Pacific. The analysis of the tectonics and deep structure of the crust allows us to characterize their development in time, proceeding from a basic, well-grounded conception that beginning with the Cretaceous Period processes of crustal subsidence were dominant in the marginal zones of both types. In connection with the different structure of margins on the continental side and their correspondingly different mechanical properties, i.e., strength and monolithic quality, it is evident that the effects of crustal subsidence in different zones should have different manifestations.The Atlantic type of margins is characterized on the continental side by the platform regime, and the development of the marginal zones was here accompanied by the subsidence of the marginal part of the pre-Mesozoic foundation with an increase of its tilt towards the ocean. The M boundary in this zone is observed to have a steep rise of the scarp type, approximately 15 km for every 100 km. The inhomogeneity of the foundation structure and the possible irregularity of the processes of its subsidence in certain areas resulted in the formation of small plateaus detached from the shelf. The zones of the Atlantic type were evidently unconnected with considerable reduction of land, and they are mainly located within the boundaries of the pre-Mesozoic shelf.The margins of the Pacific type are characterized by another mechanical regime, which develops on the background of largely differing geosynclinal structures of continents. Sinking of large blocks is here followed by their disintegration into smaller blocks, as well as by volcanism with which the formation of island arcs of the second type and of the accompanying troughs is associated, etc. Starting with the Neogene in the zones of this type, the areas of the ocean expanded and in place of land vast marginal seas were formed with a thinned continental and suboceanic crust.The connection between the primary mechanical properties of the earth's crust of the marginal zones and the character of structures which evolved as the result of its subsidence is only one side of the phenomena causing the difference in the types of margins. As the subsidences of the crust are succeeded by its regeneration under the influence of the processes in the mantle, it might also be supposed that in the Atlantic zones with platform conditions these phenomena developed more regularly, while in the Pacific zones in the conditions of the geosynclinal state of the interior the same process had a more complicated and diverse development.

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