Study on Formation of Rift System and Spreading in the Continental Margin

该项目的主要目的是揭示白垩纪至中新世日本西南内带火成岩活动与活动大陆边缘裂陷和弧后盆地扩张有关的构造关系。研究结果如下：(1)白垩纪—中新世火成岩活动可分为4个阶段：白垩纪—早古近纪；晚古近纪；中新世早期-中中新世晚期。(2)白垩纪至早古近纪，上地幔和下地壳的化学成分比晚早古近纪更不均匀。日本西南内带发生斜脊俯冲。(3)在白垩纪至晚古近纪和晚古近纪至中新世中期，亚碱性岩石的不相容元素和Sr-Nd同位素急剧减少。(4)晚古近纪发生裂陷，岩石圈重建，日本海开始扩张。(5)火成岩活动的间歇期约为29 ~ 23ma。在此阶段之后，上地幔和下地幔的晶体物质被衰竭的地幔底辟所重建。中新世早期至中期，日本海的扩张达到顶峰。(6)西日本内带白垩系—中中新世火成岩活动与大陆裂谷的火成岩活动在岩石系上存在差异，前者以亚碱性岩系为特征，后者以碱性岩系为特征。

The main purpose of the project was to reveal the relationship between igneous activity in the Inner Zone of SW-Japan and tectonics related to a rifting in active continental margin and spreading of back-arc basin during Cretaceous to Miocene age. The results are as follows ; (1) Igneous activity of Cretaceous to Miocene is divided into the following 4 stages : Cretaceous to early Paleogene ; late Paleogene ; early-middle Miocene and post-middle Miocene. (2) In Cretaceous to early Paleogene chemical composition of upper mantle and lower crust is more heterogeneous as compared with those of post-early Paleogene. Oblique ridge subduction took place in the Inner Zone of SW-Japan. (3) Incompatible elements and Sr-Nd isotopes of sub-alkaline rocks were drastically depleted from Cretaceous to late Paleogene, and late Paleogene to middle Miocene. (4) In late Paleogene rifting took place, and reconstruction of lithosphere and spreading of Japan Sea began. (5) Hiatus of igneous activity is recognized from about 29-23Ma. After this stage upper mantle and lower crystal materials were reconstructed by depleted mantle diapir. In early to middle Miocene the spreading of Japan Sea became at its peak. (6) Cretaceous to middle Miocene igneous activity in the Inner Zone of SW-Japan is different from those in continental rift in terms of rock series, i. e., the former is characterized by sub-alkaline rock series but the latter by alkaline.