Effects of ridge collision on accretionary complex - An example of eastern Shikoku

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The research was performed on the Upper Cretaceous Shimanto Supergroup from Ryukyu Islands to eastern Shikoku. The following summary of the research results is obtained.1. Instrusion of basaltic rocks into unconsolidated terrigenous sediments induceced enrichment of SiO2 and CaO,and decrease of K2O in the altered part of sediments.2. Organic metamorphism in the makimine Formation, eastern Kyushu, was studied by means of vitrinite refrectance. This study has revealed that intrusion of basaltic rock has affected thermal effects on argillaceous sediments in a trench area.3. Tectono-stratigraphic variations in chemical compositions of the sandstones in eastern Shikoku define three petrofacies (I,II and III) arranged from north to south.4. The Cretaceous Shimanto sandstones in eastern Shikoku become more acidic with time. This trend is in harmony with that of the Cretaceous volcanic rocks in the western Chugoku District.5. During the latest Cretaceous, the western Chugoku District was widely covered by rhyolitic rocks and supplied a large amount of coarse-grained clastic sediment into the trench area just before ridge collision, resulting in the accretion of tectonically-repeated thick sequences of turbidites such as Hiwasa Formation in eastern Shikoku.6. The Oboke psammitic schists are inferred to be metamorphic equivalent of sandstones in the Hiwasa Formation, deposited in a trench area during the Campanian, in eastern Shikoku.7. Accumulation of a large amount of coarse-grained clastic sediment in the trench area during the Campanian induced offscraping and underplating in the subduction zone, forming the Hiwasa Fomastion and Oboke unit, respectively.