Underplating of melange evidenced by the depositional ages: U-Pb dating of zircons from the Shimanto accretionary complex, southwest Japan

Growth of an accretionary prism is effected by frontal accretion and deep subsurface underplating at the base of the prism. A systematic oceanward and structurally downward younging of underplated sequences is expected as the prism thickens and grows. To test this hypothesis and explore the processes of underplating, the U-Pb ages of zircon grains contained in underplated melange sequences or packages of the Late Cretaceous and early Paleogene accretionary complex of the Shimanto Belt, southwest Japan, were determined using LA-ICP-MS laser technology. The results document systematic but intermittent younging ages within a single underplated melange package. This finding suggests that underplating took place episodically during a period of several million years and that between episodes of underplating, a large amount of sediment was subducted to depths much greater than where underplating was occurring.