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Volume 116, Issue 1
Displaying 1-7 of 7 articles from this issue
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Review
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Naoto Hirano, Natsue Abe, Shiki Machida, Junji Yamamoto2010 Volume 116 Issue 1 Pages 1-12
Published: 2010
Released on J-STAGE: May 29, 2010
JOURNAL FREE ACCESSOne of the mechanisms by which magma reaches the Earth’s surface is ascent along brittle fractures that develop where plates flex due to subduction and/or loading by seamounts, thereby forming a swarm of small knolls termed “petit-spots.” Such lavas are enriched in highly incompatible elements, suggesting an origin via partial melting of the asthenosphere. The widespread occurrence of this process is indicated by the discovery of small volcanoes (petit-spots) across broad regions of the NW Pacific plate and near the Tonga Trench, and is supported by a reinterpretation of the origin of alkaline basalts found within accretionary prisms. This paper provides new insights into the findings of Project Mohole based on investigations of petit-spots, as xenoliths entrained within petit-spot rocks, representing all types of geological material from the crust to the mantle, provide important information on old and subducting lithosphere of the NW Pacific plate. To complement this work, drilling undertaken as part of the proposed 21st Century Mohole project would represent a powerful tool in understanding the nature of young oceanic lithosphere. Thus, a study on petit-spot xenoliths, in conjunction with the 21st Century Mohole project (together constituting the Super Mohole Project), provides the opportunity to understand the nature of the entire oceanic lithosphere.View full abstractDownload PDF (1521K)
Articles
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Tsumoru Sagayama, Tokuji Tonosaki, Tsutomu Kondo, Satoshi Okamura, Kim ...2010 Volume 116 Issue 1 Pages 13-26
Published: 2010
Released on J-STAGE: May 29, 2010
JOURNAL FREE ACCESSRadiometric carbon dating, and analyses of diatoms and volcanic ash were carried out to clarify the stratigraphy and paleoenvironment of Late Pleistocene to Recent sediments within geologic cores obtained from five drillholes (SSC-1, 50 m depth; H16B-7, 44 m depth; H16B-3, 40 m depth; MHR-1, 18 m depth; and YUB-1, 28.5 m depth) sunk in the Ishikari Plain, northern Ishikari depression. Analyses of diatoms in cores H16B-7 and H16B-3 reveal that a large amount of seawater flowed into the brackish Paleo-Ishikari Lake during the middle Holocene. A geological horizon that contains the maximum ratio of marine to other diatom species is inferred to represent the stage with the highest sea level (ca 6,000 yBP) of the Jomon transgression. During this stage, the brackish lake (dimensions of 30 km east-west by 20 km north-south) was situated east of the Momijiyama sand-dune. An analysis of volcanic ash shows that the Toya ashfall, erupted at ca 113 ka, lies between 33.52 and 33.39 m below sea level within core SSC-1, indicating in turn that a sedimentary plain, which formed during the last interglacial stage (MIS5e) , lies beneath the Ishikari Plain.View full abstractDownload PDF (1947K) -
Hiroyuki Kawamura2010 Volume 116 Issue 1 Pages 27-44
Published: 2010
Released on J-STAGE: May 29, 2010
JOURNAL FREE ACCESSThe study area is located in the southern part of the Tabe Basin, Shimonoseki City, Japan. The recently revised stratigraphy of the Toyora and Toyonishi groups is reviewed based on detailed geological research, and new findings on the sedimentary environment and geological age are inferred from facies and paleobotanical data. The Lower to Middle Jurassic Toyora Group is divided into the following four formations: Higashinagano Formation, Nishinakayama Formation, Utano Formation, and Ohchi Formation (new name), in ascending stratigraphic order. The Higashinagano to Utano Formations contain marine faunas such as ammonites, inoceramids, and other molluscs, while the Ohchi Formation yields abundant plant fossils (i.e., the so-called Utano flora).
The Ohchi Formation has been considered non-marine or a deltaic facies, and was recently renamed as the Kiyosue Formation of the Toyonishi Group, which unconformably overlies the Utano Formation. However, the lowermost part of the Ohchi Formation has a gradational contact with the uppermost part of the Utano Formation, which is characterized by turbidite deposits laid down in a prodelta slope environment within a bay, and the Ohchi Formation contains the same Phycosiphon burrows as those found in the Utano Formation, indicating a marine sedimentary environment. In addition, muddy conglomerate at the base of the Ohchi Formation contains mud clasts deposited in an unconsolidated or semi-consolidated condition, derived from the Utano Formation. These observations suggest that the Utano and Ohchi formations represent a series of deposits laid down during a regressive phase. The flora of the Ohchi Formation has the same composition as that of Member Ut of the Utano Formation, which has been dated to Bathonian-? Callovian. Based on this correlation and other evidence, the Ohchi Formation is considered to be Bathonian-Callovian in age. Therefore, it is considered that the Ohchi Formation is unconformably overlain by the Tithonian-Hauterivian Toyonishi Group.View full abstractDownload PDF (1474K) -
Hideo Takagi, Koji Shimada, Hideki Iwano, Tohru Danhara2010 Volume 116 Issue 1 Pages 45-50
Published: 2010
Released on J-STAGE: May 29, 2010
JOURNAL FREE ACCESSFission-track (FT) dating of pseudotachylyte (PST) associated with the Median Tectonic Line (MTL) in the Taki area, Mie Prefecture, SW Japan, provides new constraints on the timing of movement upon this fault. A PST vein with a thickness of about 5 cm yields a zircon FT age of 60.0 ± 3.5 Ma (1σ). In contrast, a weighted average of zircon FT ages obtained for protolith samples (cataclastic mylonitized Hatai Tonalite) collected 10 cm and 15 m from the PST vein boundary is 69.8 ± 1.2 Ma, which is significantly older than the age of the PST. Decomposition of feldspars in the PST suggests that the temperature exceeded 1100°C, which is sufficient to completely erase previous fission tracks in zircon within several seconds. The distribution of fission-track lengths in zircon from the PST vein also supports the interpretation that the zircon FT age was completely reset during frictional fusion of the PST vein. Considering an apatite FT age of 38.0 ± 1.5 Ma for the host rock, the age of the PST indicates that the frictional fusion was occurred during cooling of the Ryoke granite at temperatures between 250°C (closure temperature of the zircon FT system) and 100°C (closure temperature of the apatite FT system). This PST age is comparable with the oldest K-Ar age obtained for fine fractions of MTL fault gouge derived from both the Sanbagawa pelitic schist and the Izumi Group in Shikoku, indicating that the initiation of brittle fault movement associated with formation of the PST and/or fault gouges along the MTL had occurred by 60 Ma.View full abstractDownload PDF (2778K)
Short Article
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Jun-ichi Tazawa, Junko Anso, Miyuki Umeda, Toshiyuki Kurihara2010 Volume 116 Issue 1 Pages 51-54
Published: 2010
Released on J-STAGE: May 29, 2010
JOURNAL FREE ACCESSA new occurrence of the Late Carboniferous productoid brachiopod species, Plicatiferina borealica Kalashnikov, is described from a sequence of alternating metamorphosed limestone and subordinate metamorphosed felsic tuff, mudstone and sandstone at Nishiamada, Fukui Prefecture, central Japan. Plicatiferina is a typical Boreal-type genus, distributed in the Upper Carboniferous and lowest Permian of Arctic Canada, Arctic Russia, and the northern and southern Urals. The fossilbearing sequence at Nishiamada is assigned to the Late Carboniferous (Kasimovian?), and is correlated with the Unazuki metamorphic rocks of the Hida Belt, central Japan. The occurrence of a Boreal-type brachiopod at Nishiamada suggests that the original calcareous and volcaniclastic sediments of the Unazuki metamorphic rocks were deposited in a shallow sea on and around the eastern North China (Sino-Korea) during the Late Carboniferous.View full abstractDownload PDF (457K)
Report
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Naoto Ishida, Fujio Toyohara, Masafumi Murata2010 Volume 116 Issue 1 Pages 55-59
Published: 2010
Released on J-STAGE: May 29, 2010
JOURNAL FREE ACCESSThis study revealed the radiolarian-based sedimentary age of the Mizukoshi Formation, a Permian siliciclastic deposit in western Kyushu, Japan. Radiolarians were recovered from four mudstone samples in this formation. The radiolarian assemblages contain abundant specimens of the genus Follicucullus. The assemblage in the upper part of the lower member includes Follicucullus scholasticus, F. porrectus, and Follicucullus sp. cf. F. ventricosus. The upper part of the upper member includes F. charveti, F. dilatatus, F. scholasticus, F. porrectus, and F. ventricosus. Based on these assemblages, the Mizukoshi Formation in the study area can be correlated with the F. scholasticus-F. ventricosus Zone and the lower part of the F. charveti-Albaillella yamakitai Zone, representing the middle to upper Guadalupian.View full abstractDownload PDF (1266K)
Pictorial
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Yukihide Matsumoto, Mototaka Saneyoshi2010 Volume 116 Issue 1 Pages I-II
Published: January 15, 2010
Released on J-STAGE: February 23, 2013
JOURNAL FREE ACCESSDownload PDF (1283K)
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