THE WESTERN PACIFIC TRIANGULAR ZONE: FRONTIER TO FORM A FUTURE SUPERCONTINENT

A triangular zone of the Western Pacific from Kurile through eastern Himalaya to Tonga islands is characterized by the occurrence of (1) double-sided subduction zones, with the Pacific from the east and the Indo-Australian from the south, suggesting the great amount 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 normal oceanic mantle, (3) augite phenocryst in basalts of these oceanic basins, resulted from higher water content and lower melting T in source mantle, and (4) abundant microplates in eastern Asian continent and western Pacific ocean. This zone acts a frontier to form a future supercontinent which will be completed by collision and amalgamation of Australia at 50 m.y. after the present, follow by collisions of N. and S. America at 250 m.y. after the present. The mantle at 410-660 km depth of this zone contains most abundant water in the Earth mantle by water transportation through double-sided subduction zones. Fragmentation of Asian continent and formation of numerous marginal basins in the Western Pacific has been interpreted by Indian collision and subsequent indentation into Asia. However, a hydrous mantle may explain these phenomena: water-weakening to break the continental crust and water-decrease mantle viscosity, and lower melting T of mantle to promote magmatic activity. The storage of water at the mantle boundary layer (MBL) of 410-660 km with time leads to continental breakup during or after formation of supercontinent, say, 250 m.y. after the present. Sequential change of magmatic activity from kimberlite and/or carbonatite, through flood basalt to MORB, may correspond to continental breakup during Jurassic to Cretaceous time and subsequent ocean formation. This has been documented by the Pangean breakup and may be caused by the fate of water in the MBL with time. The hydrous wadslyite and ringwoodite mantle beneath the center of supercontinent would become unstable due to conductive heating from lower mantle with time. It finally dehydrates to release free water to trigger magmatism and extensive mantle convection to initiate continental breakup.