Development Process of the Earth's Crust In the Light of Seismic Structure of the Crust and Upper Mantle

Abstract

Slight but definite systematic variations of the P wave velocities along the Moho and the other crustal discontinuities in the different tectonic regions were comparatively investigated and a tentative hypothesis on the development process of the Earth's crust was proposed.Continental platform has been submerged and becomes sea and then ocean owing to the movement of the deep upper mantle materials from under the platform area to the neighbouring orogenic active area (Oceanization). The separation of the crust and the upper mantle by the Moho beneath the submerged platform of sea or ocean is still as clear as beneath the crust of the continental platform.On the oceanic crust of the sea or ocean geosyncline could be developed under favourable circumstances, namely continuation of the crustal subsidence and sediment supply. During the development of the geosyncline proceeds the consolidation of the sediments by gravity contraction. In due time the geosyncline begins to receive the activation from the upheaved mantle low velocity layer and the igneous intrusion solidifies the sediments and changes them into the so-called granitic layer (Granitization). The Moho discontinuity has been destructed and becomes unclear with lower value of P. wave. At the same time the thickness of the crust increases during the orogenic granitization.After the orogenic granitization stage the typical continental crust of the Alpine fold zone is completed with the clear Conrad and Moho discontinuities. Then the P. velocity first attains normal value around 8 km/sec and increases with the age of the crust. Successive tectonic and igneous activities of post-orogenic stages within the crust solidify the crustal materials and complicate the layering. Through Hercynian fold zone to Pre-Cambrian shield or platform the solidification of the crust proceeds as indicated by the corresponding increase of the velocities and number of layers. On the other hand surface granitic layer is eroded and the basaltic layer could be outcropped in these later stages of the continental crust. Moho becomes very clear with increasing P. velocity up to 8.3-8.4 km/sec and the mantle low velocity layer, energy source of tectonic activity, sinks down deep in the mantle, giving little effect to the crust. Thinning of the crust proceeds by erosion and isostatic adjustment and thus the crust waits the next oceanization process by the eventual movement of the materials deep in the mantle.The movement of the mantle materials is considered as the energy origin of the development of the crust but no detailed inquiry is given here. It is mentioned only that the movement is closely related with the planetary tectonics of the globe.Upheaval of the mantle low velocity layer could also take place under the oceanic crust where no previous development of geosyncline exists. Mid-oceanic rift zones as East Pacific Rise and Mid-Atlantic Ridge are the examples of such activity. By the repeated volcanic and intrusive activities even these mid-oceanic ridges could eventually grow up to the island arcs of small islets (2 nd kind island arc) and finally to the island arcs of large islands (1st kind island arc), preparing the possibility to supply sediments sufficient enough to develop geosynclines there. Thus anyhow under the present ocean, which is, according to the author's opinion, considered as the submerged (oceanized) platform of the former orogenic cycle, might grow up an orogenic fold zone of the new cycle in future. (Denial of the permanence of the ocean).Finally it should be noted that the proposed cycle of the development of the crust was also suggested in the author's comparative study of seismicity of the earth.