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

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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.