2nd GRC International Frontier Seminar



"Importance of regional geology of Japan and its surrounding areas to understand the formation of supercontinent and its breakup process: Role of water "

Dr. Shigenori Maruyama
Tokyo Institute of Technology

1 July 2003 13:30-14:30
Meeting room, 6th floor, Advanced Research Building, Ehime University



  The triangular zone of Western Pacific from the Kurile through the eastern Himalaya to the Tonga islands, is characterized by (1)the double-sided subduction zones, by the Pacific from the east and by the Indo-Australian from the south, suggesting the largest amounts of water transported into mantle, (2) 600-700 m deeper ocean bottom than that of normal ocean basin at given age, suggesting lower-T than that of normal oceanic mantle, (3) common occurrence of augite phenocrysts in basalts in these oceanic basins, as reflected by higher water content and lower melting T in source mantle, and (4) abundant distribution of microplate in both eastern Asian continent and western Pacific ocean.
 Moreover, this zone faces (5) a frontier to form a future supercontinent which will be completed by collision and amalgamation firstly of Australia at 50 m.y. after the present, followed by the collisions of N. and S. America at 250 m.y. after the present. Presumably the underlying mantle at 410-660 km depth in this triangular zone is richest in water among the whole mantle of the Earth, by the water transportation of double-sided subduction zones and of the oldest oceanic plates. The fragmentation of Asian continent and even in the formation of numbers of oceanic basins in the western Pacific has been interpreted by the Indian collision and subsequent indentation into Asia. This may not be the case, instead, the water in the underlying mantle may explain all of these phenomena, i.e., water-weakening to break the continental crust, decreasing mantle viscosity, and lowering melting T of mantle to promote magmatic activity in spite of lowering mantle temperature by subduction.
 The storage of water in the mantle boundary layer (MBL) at 410-660 km with time would finally lead the continental breakup during or after the formation of supercontinent, say, 250 m.y. after the present. The sequential change of magmatic activity from kimberlite and/or carbonatite, through flood basalt to MORB, corresponding to continental breakup during the Jurassic to Cretaceous time and subsequent ocean formation, as well-documented by the Pangean breakup, may be caused by the fate of water in the MBL with time. The stored water in the hydrous wadslyite and ringwoodite underneath the center of supercontinent would become unstable by conductive heating from the underlying lower mantle with time, and finally decompose into dry phases to release free water which trigger the magmatisim and extensive mantle convection to initiate the continental breakup.




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主催:愛媛大学地球深部ダイナミクス研究センター


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