The modern island arc displays a zonal arrangement in distribution of gravity, earthquakes, volcanoes, heat flow, as well as in topography and geologic structures. The island arcs are to be classified into following three structural provinces ; that is, N, P, and I Belts from the oceanic side. N Belt-the region of the negative gravity anomaly, high intensity of shallow earthquakes, and long-continued subsidence of the crust. Geologically the N Belt is a place of (1) the geosynclinal thick accumulation of sediments, (2) the basic-ultrabasic magmatism (“ophiolite”) and (3) the regional metamorphism of the low temperature-high pressure type. P Belt the region of positive gravity anomaly, low heat flow and the non-volcanic geanticlinal uplift without significant folding. According to the seismic wave surveys, a high velocity layer (Vp 6.0 ± km/sec) of the crust rises up to a few kilometers below the surface. I Belt-the region of the acidic-intermediate magmatism, the high heat flow and the block movement of the crust. Features of “the inner volcanic arc” and “the idio-geosyncline” by UMBGROVE are of this Belt. Intrusion of granitic batholith and metamorphism of high temperature-type seem to be significant characters of this Belt. The Northeast Honshu is made up of two belts, P and I. The N Belt is occupied by the continental slope to the Japan trench. Thus, P, I and N Belts form the East Japan arc.The I Belt of the East Japan arc made its appearance in the middle of the Cenozoic time, while P and N Belts of the same arc date from the end of the Mesozoic time. The N Belt may be older than the P Belt. The N, P and I Belts must be genetically related, to each other and these three should be considered altogether as one set of geologic development which reflects an orogenic process operating in the Earth's interior. The N, P and I Belts of the East Japan arc are designated tentatively as “the Mizuho orogeny”. Present knowledge about the Mizuho orogeny suggests that the magmatism tends to decrease in its intensity as a whole, while the tectonism has tended to increase since the Early Miocene when the I Belt appeared. The Southwest Honshu (including Shikoku), on the other hand, has no distinct features of the island arc, except for the zonal arrangement of the geological terrains. Here, an attempt is made to apply the concept of N-P-I Set of the island arc to the old orogenic belts such as Southwest Honshu. Thus, the Southwest Japan is composed of two sets of the N- (P) -I, being active in the late Paleozoic time (the Sangun-Hida Set) and active in the Mesozoic to Early Tertiary time (the Sanbagawa, Shimanto-Ryoke Set). The writer proposes to apply his concept of N-P-I Set of the modern island arc to the old orogenic belts in the peripheral zones of oceans, because the modern island arc features are considered to be essentially same as those of the old orogenic belt of the Circum-Pacific Area.
Basalt magmas in Japan range in composition from tholeiite, passing through high-alumina basalt, to alkali basalt. Picrite basalt related to either of these three basalt magmas is scarcely represented. In Hawaii, both tholeiite and alkali basalt are closely associated with picrite basalt having bulk compositions intermediate between the ordinary basalt and peridotite. The Japanese basalt magmas are supposed to be produced by sudden release of stress attending the generation of the intermediate to deep-focus earthquakes. In such a case, there may be a certain limitation of the heat energy supply for melting the mantle peridotite ; only a few per cent of the peridotite would be melted to form the basalt magmas. The Hawaiian basalt magmas are supposed to be produced by the heat transfer due to convection current within the mantle. In such a case, there would be less limitation of the heat energy supply for melting ; a greater per cent of the peridotite would be melted, resulting in the production of picrite basalt magmas.
Tectogenesis or the process of the evolution of the crust is discussed from physical and chemical view points. It is described by a coupling among tectonic deformation, vertical movement, igneous activity, and metamorphism. The upper mantle is a source of energy and material for the development of the crust. To obtain a quantitative picture of the evolution of the crust and the mantle, a method of energy analysis of geological phenomena is proposed and a few examples of results are presented. A model of heat-gravitational engine of tectogenesis is presented. Two varieties of tectogenesis, where the horizontal deformation and vertical movement of the crust are considered to have a primary significance respectively, are tested. It is shown that the mantle must behave as fluid for the former while as solid for the latter. In the latter hypothesis, a shift of center of gravity of the crust is significant. A significance of thermal convection within the upper mantle is studied by numerical analysis in relation to the process of tectogenesis. Inhomogeneity of the crust is assumed to be served as a trigger to generate activation of the upper mantle. Considering the time scale of actual tectogenesis, the effective viscosity of the upper mantle is estimated as 1021-1022 poises. A sequence of epiorogenic volcanism and the following plutonism is obtained theoretically.
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.
Mt. Fuji, the highest peak in Japan, is one of the largest and most typical strato - volcano of basaltic composition. It was shown in a previous paper (Tsuya, 1940) that this volcano consists structurally of three parts, “Komitake”, “Old Fuji”, and “New Fuji”, erupted successively in the order mentioned. This result was obtained from geological and petrological studies of the volcanic ejecta, especially of lava flows and mudflow deposits. As yet few investigations have been carried out on the stratigraphy of pyroclastic fall deposits or “tephrochronology”, although such studies help to clarify some of the remaining problems such as the chronology and the nature of volcanic activity and chronological relationships between the evolution of the landform of the adjacent areas and the development of the volcano itself. In the present paper, the writer deals first with the stratigraphic division and chronology of the pyroclastic fall deposits on the lower slopes of the volcano and its surroundings, and then applies these results to the chronology of the activities of the volcano. 1) Attention has been paid to the time gaps during deposition as indicated by soil profiles and slight unconformities. Consequently the pyroclastic fall deposits are divided into three groups ; the older tephra, the black humic ash and the younger tephra (Fig. 2). These groups represent the old activity, quiescence, and younger activity respectively. The distribution of the younger tephra is shown in Fig. 4 and certain beds are shown in more detail in Fig. 3. Their volumes are tabulated in Table 1. 2) Both human remains, such as stone instruments and pottery, and the results of carbon - 14 dating afford information regarding the absolute age of the tephra groups. From these results it is inferred that the older tephra were erupted during the upper Pleistocene or the last glacial age, and that the black humic ash was formed from approximately the beginning of Holocene to middle Holocene, and the younger tephra were erupted later. 3) The horizons of various volcanic ejecta were ascertained as follows; The mudflow deposits of the “Old Fuji” volcano (in the Tsuya sense) are found conformably within the layers of older tephra, and many of the earliest lava flows of the “New Fuji” volcano are found in a horizon in the top part of the older tephra or near the base of the black humic ash layers, and finally various new lava flows with a considerable proportion of pyroclastic flow deposits, which are exposed on the flanks, are found within the layers of the younger tephra. These results are illustrated in Table 2. 4) From these facts it is pointed out that the activity of Volcano Fuji are chronologically divided into three stages ; 1) Older Fuji I, 2) Older Fuji II, 3) Younger Fuji. The first stage is characterized by explosive activity, erupting a large amount of pyroclastic “fall” as well as “flow” deposits attended by basaltic lava flows at several altitudes.The second stage of activity comes successively from the predecessor. The activity is considered to be mostly effusive, erupting a large number of basaltic lava flows with a small amount of pyroclastic material. A long quiescence, in which only fine ash fell intermittently, was followed by the last stage of activity, which was a mixed type of eruption in which lava flows and pyroclastic materials both play significant parts. These results are summarized in the right column of Table 2.