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Volume 115, Issue 5
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Articles
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Hiroyuki Hoshi, Takuya Saida2009 Volume 115 Issue 5 Pages 193-205
Published: May 15, 2009
Released on J-STAGE: October 29, 2009
JOURNAL FREE ACCESSThe Lower Miocene Hokusetsu Subgroup yields a magnetic polarity stratigraphy that is useful for inferring its depositional age. The sampled sedimentary section is about 760 m thick, consists mainly of mudstone and sandstone with numerous intercalations of pyroclastic material in upper horizons, and is lithologically divided into the following four formations (in ascending order) : the Kawakado, Shimoda, Tsubosawa, and Kuroze Formations. Paleomagnetic samples collected at 16 sites were measured for remanent magnetization and detailed stepwise demagnetization by alternating-field and thermal techniques. Demagnetization results were analyzed to determine magnetic polarity for the sampled sites. The magnetic polarity stratigraphy of the studied section consists of three newly defined polarity zones in ascending order : the Hokusetsu R1 reversed polarity zone, Hokusetsu N1 normal polarity zone, and Hokusetsu R2 reversed polarity zone. Based on constraints provided by radiolarian fossils and fission track dates, the magnetic polarity stratigraphy was correlated with the Early Miocene portion of Ogg and Smith’s (2004) geomagnetic chronostratigraphy, yielding three possible age models for the Hokusetsu Subgroup. We conclude that : (1) the sampled section is younger than the base of Chron C5Er (ca. 18.8 Ma) and older than the top of Chron C5Cr (ca. 16.7 Ma), and (2) the Kawakado Formation, for which there exists no direct age constraints, was deposited some time between the base of Chron C5Er (ca. 18.8 Ma) and the base of Chron C5Dn (ca. 17.5 Ma).View full abstractDownload PDF (1065K) -
Taiki Sawada, Ryota Shindo, Isao Motoyama, Koji Kameo2009 Volume 115 Issue 5 Pages 206-222
Published: May 15, 2009
Released on J-STAGE: October 29, 2009
JOURNAL FREE ACCESSNeogene marine sedimentary rocks exposed around the upper course of the Koitogawa River are subdivided into the Amatsu, Kiyosumi, and Anno formations. The Amatsu Formation is dominated by hemipelagic mudstone, the Kiyosumi Formation consists mainly of thick sandstone layers and thin mudstone layers, and the lower part of the Anno Formation consists of interbedded sandstone and mudstone. These formations contain many sets of key tuff layers that are useful in correlating geologic sections exposed along rivers and roads.
Radiolarian assemblages from the Amatsu and Kiyosumi formations are characterized by low-latitude species, including Diartus petterssoni, Diartus hughesi, and Didymocyrtis penultima, which are associated with mid- to high-latitude species including Lychnocanoma magnacornuta and Spongurus pylomaticus. Based on the stratigraphic distribution of radiolarian species, the sedimentary sequence extending from the Amatsu Formation to the upper part of the Kiyosumi Formation is correlated with low-latitude Zones RN5 to RN10-RN11 and the North Pacific Eucyrtidium inflatum Zone to Spongurus pylomaticus-Cycladophora sakaii Zone.
There exist some discrepancies in numerical age estimates provided by the present and previous studies of planktonic foraminiferal and radiometric ages, although the radiolarian biostratigraphy is in good agreement with that based on calcareous nannofossils. Based on these biostratigraphic and radiometric age data, we propose an age-depth model and estimated sedimentation rates. The sedimentation rate is moderate (about 20 cm/k.y. on average), but shows an abrupt decrease at the middle-upper part of the Amatsu Formation (to 2.8 cm/k.y.). This slow sedimentation occurs in an interval of relatively coarse sedimentary facies devoid of signs of tectonic movement, suggesting that fine fractions were winnowed away by water currents.View full abstractDownload PDF (2162K) -
Hiroyoshi Arai, Hideo Takagi, Ichiro Iwasaki, Yoshihiro Asahara, Masak ...2009 Volume 115 Issue 5 Pages 223-241
Published: May 15, 2009
Released on J-STAGE: October 29, 2009
JOURNAL FREE ACCESSThe exhumation of high-P/T metamorphic rocks plays a key role in the rearrangement of the geotectonic framework in arc-trench systems. The deposition of clasts of exhumed rocks constrains the timing of exhumation and the denudation history. We obtained the K-Ar ages of clasts from the Miocene Aoiwa Conglomerate, Hiki Hills, eastern Kanto Mountains. The Aoiwa Conglomerate is an unsorted breccia (maximum clast diameter up to ca. 3 m) composed of clasts of high-P/T metamorphic rocks (greenstones and crystalline schists), granitic rocks, low-P/T metamorphic rocks (mafic to felsic plutono-metamorphic rocks), and sedimentary rocks.
The clasts of high-P/T metamorphic rocks were previously considered to have been derived from the Sanbagawa metamorphic rocks, which have been exposed nearby since the Miocene. However, schist clasts yield K-Ar phengite ages of 127.2-121.0 and 98.4-80.7 Ma, significantly older than ages obtained for the Sanbagawa metamorphic rocks in the Kanto Mountains, although the mineral assemblage and metamorphic grade of clasts are similar.
Clasts of granitic rocks and low-P/T metamorphic rocks yield K-Ar hornblende ages of 131.5-114.0 Ma and phengite ages of 97.8-88.9 Ma, significantly older than ages obtained for Ryoke granites in the Kanto Mountains. In addition, a tonalite mylonite clast yields a lower 87Sr/86Sr initial ratio (0.70510-0.70520 or 0.70498-0.70505) than that of the Ryoke granites.
Based on the results of K-Ar dating and a detailed petrologic study of the clasts, we conclude that the schist clasts were derived from an older unit within the Sanbagawa metamorphic rocks and that granitic and mafic metamorphic clasts were derived from the paleo-Ryoke terrane, which previously overlaid the Sanbagawa metamorphic rocks and today occurs only as sporadic allochthonous rocks. The Aoiwa Conglomerate was probably deposited in association with denudation of the paleo-Ryoke and uppermost Sanbagawa terranes. Deposition was accelerated during exhumation of Sanbagawa metamorphic rocks during the Middle Miocene.View full abstractDownload PDF (4201K)
Report
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Takayuki Uchino, Makoto Kawamura2009 Volume 115 Issue 5 Pages 242-247
Published: May 15, 2009
Released on J-STAGE: October 29, 2009
JOURNAL FREE ACCESSThe Nedamo Complex, an Early Carboniferous accretionary complex, contains green rocks of Late Devonian age. The complex consists of two lithotectonic units: the Takinosawa Unit and Tsunatori Unit. We analyzed the chemical composition of the green rocks in both units, as this has yet to be done for the Takinosawa Unit, and has only been done for some of the green rocks of the Tsunatori Unit. The chemical composition of green rocks in the Takinosawa Unit shows features of phyric within-plate tholeiite ; that in the Tsunatori Unit shows features of phyric alkali basalt, phyric within-plate tholeiite, and aphyric MORB.View full abstractDownload PDF (1348K)
Pictorial
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Hideo Takagi, Hiroyoshi Arai, Ichiro Iwasaki, Masaki Takahashi2009 Volume 115 Issue 5 Pages IX-X
Published: May 15, 2009
Released on J-STAGE: February 23, 2013
JOURNAL FREE ACCESSDownload PDF (2133K)
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