Abstract
The eastern extension of geotectonic belts and of the Median Tectonic Line (MTL) in Southwest Japan becomes ambiguous in the Kanto Plain region because of the thick cover of Neogene-Quaternary sediments. Deep well core samples provide significant clues to clarifying the location of the MTL. More than 3500 m samples of the Iwatsuki borehole drilled in 1971 by the National Research Institute for Earth Science and Disaster Prevention were re-examined petrologically and geochronologically. The deepest part of the core (Layer H: 3505.0-3510.5 m) is mainly composed of garnet-hornblende tonalitic mylonite and mylonitized epidote amphibolite. EPMA analytical data for garnet, hornblende and plagioclase all suggest that the rocks are not derived from a high P/T Sambagawa metamorphic belt as previously suggested, but from a higher T plutono-metamorphic complex. K-Ar ages of hornblende from two samples are 77.4 and 83.1 Ma, and a Rb-Sr mineral age gives 69.8 Ma. Consequently, correlation of the deepest core samples with the Paleo-Ryoke (Abukuma) Terrane (existing as nappes with the 105-112 Ma high T Yorii plutono-metamorphic complex on the Sambagawa rocks) is neglected, and the rocks are instead correlated with the Ryoke belt. The second deepest part of the core (Layer G: 2943-3327 m) is composed of tonalitic quartz porphyry. It contains biotite phenocrysts and their K-Ar age is 17.7 Ma. This age suggests that the quartz porphyry is difficult to correlate with the felsic intrusions of Setouchi volcanic rocks along the Median Tectonic Line in SW Japan. The degree of mylonitization in Layer H and of cataclasis in both Layer G and H suggests that the MTL extends within 500 m to the south of the core site, and a possible superficial extension of the estimated MTL is slightly oblique to the NW-SE trending Ayasegawa-Fukaya Fault, major active fault in the Kanto Plain. Further thought should be given to clarify the significance of the relationship between the MTL and the Ayasegawa-Fukaya Fault in the Kanto Plain.
