Lithological, structural, and chronological relationships between the Sanbagawa Metamorphic Complex and the Cretaceous Shimanto Accretionary Complex on the central Kii Peninsula, SW Japan

Abstract

It is essential to clarify the lithological, structural, and chronological relationships between the Sanbagawa Metamorphic Complex (MC) and the Cretaceous Shimanto Accretionary Complex (AC) for understanding the tectonic evolution of SW Japan. To this end, we carried out a detailed field survey of the Sanbagawa MC and the Cretaceous Shimanto AC on the central Kii Peninsula, where they are in direct contact with each other. We also conducted U-Pb dating of detrital zircons from these complexes. The field survey showed that the boundary between the Iro Complex of the Sanbagawa MC and the Mugitani Complex of the Shimanto AC, Narai Fault, shows a sinistral sense of shear with a reverse dip-slip component, and there are significant differences in the strain intensity and the degree of recrystallization between the two complexes across this fault. Detrital zircon U-Pb dating indicates that the Iro Complex in the hanging wall of the Narai Fault shows a significantly younger maximum depositional age than the Mugitani Complex in the footwall of the fault, and an apparently large gap in the MDA of ca. 35 Myr exists across this fault. This large age gap across the Narai Fault suggests that this fault is an essential tectonic boundary fault within the Cretaceous accretionary-metamorphic complexes on the Kii Peninsula, and is considered to be an out-of-sequence thrust. In addition, a similar shear direction and a large age gap have been identified across the Ui Thrust, which marks the boundary between the Kouyasan and Hidakagawa belts of the Cretaceous Shimanto AC. The Cretaceous accretionary-metamorphic complexes record the large-scale tectonic juxtapositions of complexes, and these juxtaposed structures had been caused by sinistral-reverse movements on the tectonic boundary faults such as the Narai Fault and the Ui Thrust. K E Y W O R D S accretionary complex, Cretaceous, detrital zircon, Kii Peninsula, metamorphic complex,

Recognition of broad thermal anomaly around the median tectonic line in central Kii peninsula, southwest Japan: Possible heat sources

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