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

研究对象是从琉球群岛到四国东部的上白垩统四万多超群。主要研究成果如下：1.玄武质岩石侵入松散的陆源沉积物，导致沉积物蚀变部分SiO2和CaO富集，K2 O减少.本文用镜质体折射率法研究了东九州幕岭组的有机变质作用。这项研究揭示了玄武质岩石的侵入影响了海沟区泥质沉积物的热效应。四国东部砂岩化学成分的构造-地层变化定义了从北到南排列的三种岩相（I、II和III）。四国东部白垩纪四万十砂岩随时间的推移而变酸。这一趋势与中国西部地区白垩纪火山岩的趋势一致。在白垩纪晚期，中国西部地区广泛被流纹岩覆盖，并在洋脊碰撞前向海沟地区提供了大量粗粒碎屑沉积物，导致四国东部的构造重复厚层浊积岩序列（如Hiwasa组）的增生。Oboke砂质片岩被推断为与四国东部Campanian期间沉积于海沟区域的Hiwasa组砂岩的变质等效物。7.坎帕尼亚期海沟区大量粗粒碎屑沉积物的堆积，引起俯冲带的铲刮和底侵作用，分别形成了日和组和Oboke单元。

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.