堆積盆地の地史情報に基づく沈降モデルと構造形成

日本海東部と中国北部湾を例にして

抄録

Concerning the origin and structural developments of sedimentary basins, detailed case studies were made in the eastern part of the Sea of Japan and the Beibu Gulf of China where enough well data and seismic data were accumulated. Through these studies, a quantitative analysis for the subsidence of sedimentary basin was investigated. Three patterns were recognized for the subsidence of these basins such as 1) the isostatic initial subsidence, 2) the thermal subsidence, and 3) the flexure. Each of them is deeply associated with the evolution of sedimentary basins.
This analysis seems to be an effective approach for the comprehensive evaluation of the sedimentary basins, therefore a detailed procedure for the analysis will be introduced.
From this study, the developments of the basins in the Sea of Japan and the Beibu Gulf were summarized as follows:
1. Phase of the isostatic initial subsidence.
a. The eastern part of the Sea of Japan subsided with rates of 16 times larger in comparison with that of the Beibu Gulf. In the eastern part of the Sea of Japan, subsidence with such a large scale in short time could be resulted from prominent volcanic activities of the final stage of the Green Tuff movement.
b. The both basins subsided to about 3, 000m in depth. Main structural elements such as grabens were formed in this phase. It is believed that the important source rocks deposited in the end of this phase.
c. Some important reservoirs are developed in the Green-tuff Formation in Japan. In the Beibu Gulf, similar reservoirs are not developed because of weak volcanic activities there, and sandstones are important reservoirs of this phase.
2. Phase of the thermal subsidence.
a. Crustal thinning (24-32km) caused the conversion of relatively thickened asthenosphere to slightly more dense mantle lid and the basins subsided slowly in accordance with the movement.
b. The ultimate amount of thermal subsidence (Do) of the eastern part of the Sea of Japan is larger than that of the Beibu Gulf. Thus the sediments in the Sea of Japan are greater than the Beibu Gulf.
c. Remarkable structural movements in large scale during this phase did not take place in both areas. Therefore, only stratigraphic traps and anticlines which resulted from differential compaction are expected in this phase.
3. Phase of the flexure.
a. Subsidence in this phase was more vigorous because of plate deflections accelerated by thick sedimentary load in comparison with that of the preceding phase.
b. Flexural rigidity in the eastern part of the Sea of Japan is smaller than that of the Beibu Gulf, therefore, deflections of the crust of the Sea of Japan are larger.
c. Tilting of the basin led to reactivation of faults, decollement movements and gravity gliding, and structural traps were formed in this phase.