Naguri Fault Zone as the Kurosegawa Terrane in the eastern part of the Kanto Mountains, central Japan

Naguri Fault Zone is newly proposed for the fault zone which represents the eastern extension of the Kurosegawa Terrane in the Chichibu Terrane of the Kanto Mountains, central Japan. It consists of i) the Naguri West Fault formerly called the Naguri Fault, ii) the newly found Naguri East Fault and iii) the Chichibu accretionary complex bounded on both sides by these two faults. It is confirmed, at present, to extend in the NNW SSE direction through the accretionary complex of the Chichibu Terrane over ca. 10 km from the southeastern margin of the Kanto Mountains. The Naguri West Fault has been known to be associated with several lenticular bodies of serpentinite. Along the Naguri East Fault, which runs parallel to and several hundreds meters to the east of the Naguri West Fault, various kinds of igneous and metamorphic rocks were found at six localities. They include garnet-granite, plagiogranite, aplite, rhyolite, garnet-gneiss, blueschist, greenschist, amphibolite, metadolerite and metagabbro. These rocks occur as tectonic blocks in the serpentinite body or the cataclasite zones associated with the Naguri East Fault and as floatstones. Serpentinite and cataclasite containing tectonic blocks form lenticular bodies with their long axes a few hundreds meters long. The mode of occurrence as well as the lithological and petrochemical characteristics of these rocks strongly suggest that they are the constituents of the Kurosegawa Terrane occupying the medial zone of the Chichibu Terrane. The Naguri Fault Zone is, therefore, referred to the eastern extremity of the Kurosegawa Terrane, which has been known to extend from Kyushu to the eastern coast of the Kii Peninsula of southwestern Japan.