Basement structure and active fault system in the Inner zone of Southwest Japan

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1. Exploration of deep-seated graniteAnalytical technique for the volatile elements (B, F, Cl) in mudstone by XRF was developed with the purpose of verifying deep-seated granite in the accretionary complex terrane. Tamba terrane was examined using this technique, and confirmed that the high concentration of those volatile elements in northern Kameoka area showing the existence of deep-seated granite.2. Newly found large scale geologic structure in the Pre-Cretaceous Terranes(1) A large-scale high angle fracture zone was found between the Maizuru and Akiyoshi zones in Ooe-cho area, northern Kyoto Prefecture.(2) The Sangun metamorphic rocks are overlain by a low angle nappe of the Maizuru zone fomations in the Kagamino-cho area, Okayama Prefecture.(3) The Pre-Cretaceous Terranes in Kohfu-cho area where the epicentral region of the Western Tottori earthquake outbreaked in October 2000, are composed of weekly metamorphosed melange formations, and they can be correlated to the Hida-gaien zone.(4) Allochthonous Kanmon Group is intercalated in the Maizuru zone embedded in two WNW-ESE trending faults in the Hikimi-cho area, Shimane Prefecture. This suggests the existence of the early Cretaceous large-scale wrench fault along the Maizuru zone.3. SummaryGeometrically strictly to say, the active fault system is not in a block structure. Accretionary complex crust is surrounded by the granitic crust forming frame work structure in the Chubu to eastern Kinki district, at where much of active faults develop in the granitic crust or its marginal part. WNW-ESE trending zonal arrangement of granitic crust and accretionary complex crust is distinct in the western Kinki. Some active faults show parallel alignment along the above mentioned zonal arrangement. Most of active faults had re-activated by the ancient faults formed in late Cretaceous in the Chugoku to northern Kyushu. They show inversion of displacement sense caused by the conversion of the stress field.