Tectonosphere of the Earth: upper mantle and crust interaction

Abstract

The endogenic geological processes, which include tectonic, magmatic and metamorphic processes, form regular combinations called endogenic regimes. These regimes are: géosynclinal, orogenic, platform, rift, tectonic-magmatic activation (diwa), taphrogenic, plateau-basalt, oceanic basins and mid-oceanic ridges. The endogenic regimes are connected with the peculiarities of the structure, composition and state of the entire tectonosphere, i.e. not only of the crust but of the upper mantle as well. Heat flow is a major factor controlling the type of the regime. The other conditions are the temperature distribution in the tectonosphere and the degree and type of penetrability of the tectonosphere to melts and fluids. There is a certain regular succession of regimes. The structural evolution of the tectonosphere and the transformation of the matter in it are in close relationship. The main trend in the development of the tectonospheric material is directed towards geochemical depletion of the upper mantle by fractioning. At the initial stages, fractioning occurred mostly by degassing, and under these conditions the continental crust was formed, rich in non-compatible elements. At that stage the calc-alkaline magmas prevailed. As the upper mantle was depleted and began to lose its volatiles, the mechanism of fractioning changed: degassing was substituted for selective melting, and in this environment most of the tholeiitic magmas were formed. This change in magma composition and in fractioning mechanism was combined with the destruction of the continental crust and the formation of the oceanic crust. The diwa regime and the rifts were the first steps in the destruction of continental crust. The stages that followed were represented by taphrogenic regimes at various levels. These kinds of regimes were manifested in deep continental and marine depressions, compensated and not compensated by sediments. Taphrogenic regimes are advancing from the east and west onto the Eurasian continent: in the east they form marginal seas and cause subsidence of the eastern parts of the Chinese platform; in the west they produce collapses of the crust in the Mediterranean area. The major crisis occurred between the Palaeozoic and Mesozoic and since that time the process of substitution of the continental crust by the oceanic crust has proceeded over increasingly large territories. The evolution of the tectonosphere, instigated by the changes in its matter, was further complicated by temporal and spatial irregularities in deep heat escape, which caused the alternation of excited and quiescent endogenic regimes (tectonomagmatic periodicity) and their co-existence. The combination of all these phenomena creates the structural inhomogeneity of the Earth's crust at any stage of its history.