This paper outlines the CHIME (chemical Th-U-total Pb isochron method) dating on the basis of precise electron microprobe analysis of Th, U and Pb in an area of 3-6 μm across within Th- and U-bearing accessory minerals like monazite, zircon, xenotime and polycrase. The approach consists in measuring many spots with sufficient compositional variation in domains of minerals with constant age, and in constructing a "pseudo-isochron" from which an age can be obtained by regression. This method has the potential advantage of substantial precision under the right circumstances. The regression based technique has the ability to work with minerals having substantial initial Pb, and can identify two or more homogeneous domains that separated by an age gap smaller than the analytical error on individual spot analysis of age. Many features that are insignificant in major element analysis can have major manifestation in acquired trace elements. Critical comments include detection limit on microprobe analysis, statistics in X-ray counting, characterization of spectral profiles and correction for elemental interferences. Several other considerations of analytical procedure are more briefly shown, and an age-map procedure is also described . Monazite is most suitable to the CHIME dating, since it shows a concordant Th-U-Pb relation, contains 5-20 wt.% ThO2 and 0.1-1.5 wt.% UO2 that can produce 0.01-0.06 wt.% PbO during 50 Myr, and remains immune to significant Pb-loss even under the sillimanite grade metamorphism. Occasional inconsistence between the CHIME monazite and Rb-Sr whole-rock isochron ages is due to, at least partly, a spurious isochron resulted from heterogeneity of the initial 87Sr/86Sr ratio within a single magma. The CHIME monazite dating has great chronological potential for the analysis of the detailed sequence of geologic events. The reviewed examples of the CHIME geochronology include (1) the electron microprobe observations of Pb diffusion in metamorphosed detrital monazites from high-grade Ryoke paragneisses, (2) the relationship between the Hikami Granite and Siluro-Devonian clastic rocks in the South Kitakani terrane, and (3) the denudation history of the high T/P Ryoke metamorphic belt.
The Pleistocene Ryukyu Group, composed of reef complex deposits and carbonate-siliciclastic sediments, crops out extensively on northern Motobu Peninsula, Okinawa-jima, Ryukyu Islands, southwestern Japan. We propose a major revision of the previous stratigraphic scheme for the Group and provide a formal stratigraphic description. The Group comprises the Nakoshi and Kourijima Formations and younger limestones. The Nakoshi Formation is composed of sandstone, siltstone, sandy to silty limestone, and conglomerate, passing laterally into the lowest unit (Unit 1) of the Kourijima Formation. The Kourijima Formation constitutes the main body of the Group reaching 80 m in thickness and is exposed at elevations up to 107 m. It is divisible into three units (Unit 1, 2, and 3). Unit 1 consists of deep-water deposits represented by detrital limestone associated with rhodolith and Cycloclypeus-Operculina limestones. Unit 2 begins with shallow-water coral limestone that grades upward into the deep-water deposits. Unit 3 is composed exclusively of coral limestone. Coral limestones which are exposed sporadically on the pre-Tertiary basement cannot be assigned to any certain unit due to their isolated occurrence. The younger limestones include coral, Cycloclypeus-Operculina, and well sorted detrital limestones, all of which are limited in their distribution and rest unconformably on the Kourijima Formation and pre-Tertiary basement. Calcareous nannofossil biostratigraphy indicates that deposition of the Ryukyu Group started in earliest Quaternary time (1.45-1.65 Ma) and continued for more than 0.6 million years (< 0.85 Ma).
The Sakaitoge fault is a left-lateral strike-slip active fault trending NNW-SSE to NW-SE direction in western Nagano Prefecture. Seven trenches were excavated in Nagawa and Kiso Villages; they are SA and SB trenches at Sogurazawa site, YA and YB at Yoriaido site, and HA, HB and HC at Hososhima site. In six trenches except HB trench we observed the evidence of paleoseismic activities. We observed two discrete horizons of upper terminations of the faults on the walls of YB trench. This means at least two faulting event occurred after the beginning of the deposition of the soil layers. The calibrated radiocarbon dates show that the age of the last event of the Sakaitoge fault is between 2910 BC (possibly 1670 BC) and 220 AD. The age of the penultimate event is between 5725 BC (possibly 5710 BC) and 4700 BC. The vertical slip-rate of the Sakaitoge fault is estimated to be 0.2−0.3 m/103 yrs from deformation of the terrace surface at the Sogurazawa site. The net slip-rate is larger than this considering that the fault has left-lateral strike-slip component.
The early Pleistocene Tugm Tephra Bed in the Niigata region and the Kd18 Tephra Bed in the Boso Peninsula were correlated to the Ashino Pyroclastic Flow Deposit in the Aizu region. They contain bubble-junction type glass shards and high quartz in common. Chemical composition of glass shard and orthopyroxene (Mg#=63.6-66.0) of these tephra is also coincident. These tephras are expected to be found as marker beds in Japan.
The geochronologic units of Palaeogene and Neogene have long been described in Japanese as "Kodaisanki" and "Shindaisanki", which mean old and young Tertiary, respectively. The International Commission on Stratigraphy, however, recently proposes the revised geochronologic chart, demonstrating the subdivision of the Cenozoic Era into the Palaeogene and Neogene Periods instead of the Tertiary and Quaternary. The Japanese wording "Kodaisanki" and "Shindaisanki", therefore, should be reconsidered in terms of derivatio nominis of Palaeogene and Neogene. Here we review the Japanese usages of Palaeogene and Neogene in previous textbooks back to the end of the nineteenth century. It is concluded that the words "Koseiki", "Shiseiki" or "Kyuseiki" for Palaeogene and "Shinseiki" or "Kinseiki" for Neogene have already been proposed and described by Prof. Matajiro Yokoyama. These Japanese terms for Palaeogene and Neogene would be taken into consideration to write geological reports and papers in Japanese.