The Daiichi-Kashima Seamount is situated near the junction area of the Japan and Izu-Bonin Trenches and characterized by the presence of the upper, eastern flat summit (about 3, 600 to 4, 000 m in water depth) and the lower, western terrace (about 5, 100 to 5, 500 m in water depth).
From the geological, mineralogical, and geochemical points of view, the present writers have obtained the following conclusions:
1) The Daiichi-Kashima Seamount is characterized mainly by the presence of alkaline rock series with or without alkali minerals such as anorthoclase and arfvedsonite.
2) Its present geomorphological feature is likely to be secondary in origin. This means that the tectonic movement of fault-making was responsible for the formation of two flat terraces of the Daiichi-Kashima Seamount.
3) The present water depth of the Daiichi-Kashima Seamount (Guyot) was produced not by the global eustatic change of sea-level, but by the tectonic sinking of the Daiichi-Kashima Seamount.

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Four formational-geochemical types of Cenozoic basaltoids were distinguished in the Japan Sea: (1) continental tholeiites, (2) volcanic rocks of island-arc type, (3) marginal sea tholeiites and products of their differentiation, (4) alkali volcanic rocks of continental rift type. The rocks have a geochemical affinity both to oceanic tholeiites and to basalts of the calc-alkali series of island-arc type. The study of Cenozoic basaltoids has enabled us to suggest a theory of the origin of the Japan Sea and its structures.

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In the Japan Sea region, there are three types of the Earth's crust: continental, subcontinental, and suboceanic. The first type is characteristic of the shelf, the second one of the submarine elevations, the third one of the deep-sea basins. The continental structures extend to the continental and island slopes. Metamorphic, sedimentary and magmatic rocks of Archean to Cenozoic age were dredged up on the submarine elevations. These rocks are similar to the formations of the surrounding land. The original continental crust of the Japan Sea region was formed in the late Proterozoic. Then, geosynclines successively developed. The Japan Sea depression started forming since the Cretaceous. Its formation can be explained by the intrusion of a mantle diapir into the lithosphere. The diapir stretched the Earth's crust. The stretching was accompanied by the compression of the crust in the periphery areas, resulting in the formation of folds and thrusts. By the end of the Paleocene, a continental slope was developed in general outlines. The submarine elevations became islands and peninsulas at that time. The subsidence of the submarine elevations began in the middle Miocene and coincided in time with the glacio-eustatic transgression.

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Recent paleomagnetic surveys revealed that Northeast Japan was rotated counter-clockwise about 20° during the middle Miocene opening of the Japan Sea, whereas Southwest Japan clockwise about 45°. A reconstruction of the pre-Miocene Japanese Islands is attempted on the basis of the following main premises. 1) The bending of zonal geologic units in the Central Japan was caused by the collision of the Izu-Ogasawara Ridge after the opening of the Japan Sea. 2) The Yamato Bank and several plateaus in the Japan Sea are regarded as continental fragments and there were no marginal basins before the opening. 3) Northeast Japan and Central Hokkaido are geologically continuous to West Hokkaido and Sakhalin, respectively. Each pair is treated as a single crustal block. The reconstruction gives consistent explanations for such geologic features as the change of volcanic front, distributions of Paleogene coal-bearing strata and Cretaceous to Paleogene subduction complex and felsic volcanic rocks, continuity of geotectonic units mainly consisting of the Jurassic subduction complex, and tectonic lines. A most significant corollary in this reconstruction is that Central Hokkaido was adjoined to Northeast Japan.

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Some characteristic magmatism on the structural highs of continental slopes indicates initiation of subduction of a new, relatively warm slab. An instance is the plagio-granitic and tholeiitic magmatism that occurred on the Middle Miocene topographic and structural highs of the continental slope of the outer zone of Southwest Japan. It was simultaneous with the initiation of subduction of the new Shikoku Basin slab, just following the off-ridge volcanism in the basin. The magmas intruded upward through the remnant fissure of the transform fault which had separated the Southwest Japan arc from the Philippine Sea plate until the cessation of the Shikoku Basin opening.
Application of the idea of "continental slope structural high magmatism" to the igneous rocks of the Daito Ridge Group provides a new hypothetical scenario for the tectonic history of the Northern Philippine Sea. Late Cretaceous collision of the "Kuroshio Paleo-island" with the Japan arc resulted in suspension of north-ward subduction of the Paleo-Philippine Sea plate, temporal southward subduction of a part of the plate under the Paleo-Daito arc, and the structural high magmatism on the Amami PIateau, the present northernmost ridge in the Daito Ridge Group.

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The DSDP data (Legs 6, 31, 59, and 60) on the composition and structure of the layers covering the Philippine Sea floor along 18°N, and the correlation of hiatuses and unconformities in the sediments of the sea floor and near-by islands, suggest the existence of six sedimentary sequences, each representing 2 to 5 formations. These data allowed us to trace the lateral rows or arrangements of structural elements with carbonaceous, volcaniclastic and clayey covers. Sedimentary sequences different in age but similar in the composition and structure, their present-day morphology and velocity parameters are also described.

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The rocks of various metamorphic complexes from Archean to Mesozoic-Cenozoic age have been found in the Pacific marginal seas. Those formations are represented by the rocks from high temperature granulitic facies to high pressure glaucophane schist facies.
The Archean-Early Proterozoic rocks of the gneiss-migmatite complex were found in the western part of the Japan Sea. That complex has been formed under conditions of granulite and amphibolite facies (Ps: 5.5-7.5 kbar, T: 630-800°C).
There were Middle Paleozoic schists and amphibolites dredged in the northern Japan Sea. They have been a result of metamorphism of oceanic tholeiites under greenschist facies conditions. The zonal schist complex of the Philippine Sea has been formed on the oceanic tholeiites at constant temperature (500-515°C) and variable pressure (Ps: 4.0-6.5 kbar). Metamorphic rocks of the Okhotsk Sea have been formed at a temperature of 550°C and pressure 5.1 kbar. In the East-China Sea we found Late Paleozoic metamorphic rocks, represented by two complexes. The lower complex is composed of amphibolites and gneisses which had been formed under conditions of glaucophane schist facies (P more than 9.5 kbar, T= 540°C). The schists of the upper complex have been formed at T = 380°C and Ps = 4 kbar. Metamorphism in the marginal seas had been developing along with decrease of the heat flow in time. That is why the high temperature metamorphism of the Precambrian complexes was changed to low temperature greenschist metamor-phism of Phanerozoic rocks which took place in linear zones of high pressure.

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Marginal seas of Eastern Asia which form a transition zone from the continent to the Pacific are considered from new positions: as a system of gigantic morpho-structures of the central type (mega-MCT). The main features of geological and geomorphological patterns and evolution of megastructures in question have been identified. The formation of the recent geomorphological image of marginal mega-MCT occurred in Mesozoic (mainly in Late Mesozoic)-Cenozoic under the conditions of continuous extension of the earth's crust over the whole western Pacific margin.

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The subsystem "gasosphere" is distinguished in the earth's system. An analysis of the equations of state of volatile substances shows that the area of supercritical state corresponds to the P-T conditions of deep petrogenesis. Supercritical gases are dissolvents and therefore can play an important role in the formation of deep conductivity anomalies and probably of anomalies in other geophysical fields.

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