The origin of folding in the Earth's crust

In the light of recent special study, folding phenomena represent the reaction of the layered beds of the earth's crust on the differential vertical movements of separate blocks of the crust. It is possible to distinguish the following types of folding. Block folding represents the most direct reaction of layers on the vertical movements of the crust's blocks, the most characteristic of this type of folding being boxlike folds. They occur both on platforms and in geosynclines. Injection folding is due to the horizontal flow of the more plastic rocks, which are squeezed out from some areas and accumulate in others, where injection or piercement cores are formed. The flow of plastic rocks results from the uneven load of superimposed layers and especially from the fractures that cut these layers. The main places of occurrence of injection folding are foredeeps and tectonic basins. Metamorphic or deep folding is connected with the processes of granitization and metamorphism when the density of rocks becomes lower as the result of the impregnation of rocks by volatile substances. Reduced density induces upward flow, as a result of which deep diapirlike structures are formed. The folding of general crumpling mostly represents the influence of gravitational forces provoking the sliding of layers down the slopes or the spreading out of the upper parts of the elevated blocks. Such spreading out creates horizontal compressive forces which crumple the adjacent layers into folds. Sometimes the folding of general crumpling is created by the pushing apart under the mechanical influence of injection cores or of deep metamorphic diapirlike structures. The deep folding and the folding of general crumpling occur widely in geosynclinal belts. There are no indications in the phenomena of folding of the existence of general compressional forces in the earth's crust. Horizontal compression exists only as a local phenomenon and is limited to some narrow belts within geosynclines. The causes of horizontal compression are also local. Only vertical movements of separate blocks of the crust exist as primary phenomena.