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Volume 46 (2007)
- Issue 6 Pages 445-
- Issue 5 Pages 399-
- Issue 4 Pages 305-
- Issue 3 Pages 167-
- Issue 2 Pages 83-
- Issue 1 Pages 1-
Volume 46, Issue 2
Displaying 1-5 of 5 articles from this issue
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Articles
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Yoshio Soeda, Takahiro Miyauchi2007 Volume 46 Issue 2 Pages 83-102
Published: April 01, 2007
Released on J-STAGE: June 19, 2008
JOURNAL FREE ACCESSThe Dewa Hills in Northeast Japan are a tectonic uplifted zone parallel to the main direction of the Northeast Japan arc, bounded by the Kitayuri Thrust Faults on the western side under a compressional stress field. We examine the uplift process of the hills, related to the active tectonic structure due to crustal shortening, on the basis of the surface deformation pattern derived from the fluvial terraces and fault dislocation models. The fluvial M1 terrace is identified as the last interglacial terrace that corresponds to oxygen isotope stage 5e, depending on the stratigraphic relation between the 112-115ka Toya volcanic ash and terrace deposits. The displacement of the M1 terrace shows asymmetrical warping with uplifting at a maximum uplift rate of 0.5mm/yr. A simple dislocation model to explain the asymmetrical surface deformation requires a reverse fault with detachment at the depth of 6km under the Dewa Hills. The calculated fault geometry suggests that the uplift of the Dewa Hills is accompanied by the growth of a fault-bend fold. The horizontal strain rate under the Dewa Hills is estimated to be 6.6×10-8/yr in late Quaternary. This strain rate indicates highly horizontal crustal shortening after 3.5Ma at the Dewa Hills in the late Quaternary. The amount of slip on the modeled fault significantly decreases in the upper crust shallower than 6km, compared with that under the depth of 6km. The characteristics of slippage of the fault plane show the difference in the crustal shortening processes between deeper and shallower part of the fault.View full abstractDownload PDF (3037K) -
Masahiro Noumi, Yusuke Yokoyama, Hideki Miura, Naohiko Ohkouchi2007 Volume 46 Issue 2 Pages 103-117
Published: April 01, 2007
Released on J-STAGE: June 19, 2008
JOURNAL FREE ACCESSWe studied three sediment cores recovered from off the Antarctic Peninsula to reconstruct the melting history of the Antarctic Ice Sheet during the last 24kyrs. Ice Rafted Debris (IRD) and terriginous material from the Antarctic Continent were investigated throughout the cores to obtain the timing and magnitude of the melting events. We employed as a proxy the relative abundances of K2O and Na2O to understand the origin of the sediments. Surface sediments K2O/Na2O ratio correlated with the degree of the influence from three surface currents in the region, and hence the ratio can constrain the source area of the sediments. Chronologies of these cores were constructed based on both radiocarbon dating of sedimentary organic carbon and the relative abundance of radiolarian Cycladophora davisiana. Analyzing IRD, K2O/Na2O ratio, 14C, and radiolarian in the cores showed temporal variations of IRD flux as well as the source regions of the sediment supply from the LGM to the present. IRD maxima were observed at 25-17 and 15-12ka that show considerable expansion of the ice sheet during the LGM and rapid melting of the ice sheet at the time of the global meltwater pulse 1a event.View full abstractDownload PDF (1662K) -
Yasuhiro Takashimizu, Tsumoru Sagayama, Kenji Nishina, Takao Oka, Yugo ...2007 Volume 46 Issue 2 Pages 119-130
Published: April 01, 2007
Released on J-STAGE: June 19, 2008
JOURNAL FREE ACCESSA characteristic sandy layer probably resulting from a 17th-century tsunami was discovered in coastal sediments in the central part of the Pacific coast (eastern Iburi coast) of Hokkaido, northern Japan. The sandy layer was identified as of tsunami origin based on the following characteristics. The deposit is 1) distributed at least 20km long and 1-2km wide along the coast ; 2) widely inundated along the river toward the inland ; 3) deposited approximately eight meters at the highest ; 4) shows a fining- and thinning- landward trend, and 5) yields a marine diatom assemblage.
The sandy layer was deposited in the early to mid 1600s. The exact trigger event of this tsunami was not identified in this study. Great subduction-zone earthquake centered off eastern Hokkaido and the 1640 Hokkaido Komagatake eruption are candidate source events for this tsunami.View full abstractDownload PDF (1578K) -
Yoshihiro Takeshita, Yasuyuki Miyake, Jun'ichi Sakai2007 Volume 46 Issue 2 Pages 131-146
Published: April 01, 2007
Released on J-STAGE: June 19, 2008
JOURNAL FREE ACCESSThe Matsumoto Basin in central Japan is covered by thick gravel beds, among which the Nashinoki Gravel Formation (NGF) is the oldest. Although the age of NGF is crucial to elucidate the start age of sedimentation in the Matsumoto Basin, its age remained undetermined. In order to determine the age of the NGF, 10 tephra beds (NAT1-7, KRT1-3) intercalated in the NGF and the overlying Nashinoki Loam Formation were investigated. Based on the occurrences and petrography of these tephra beds, along with the chemical compositions of hornblende and orthopyroxene in them, these tephra beds were correlated with some of the tephra beds supplied from the Older Ontake Volcano, the age of NGF being inferred to be 0.78-0.64Ma from the stratigraphic relationships with the dated lavas on the foot of the Older Ontake Volcano.
The NGF can be correlated also with the upper part of the Kokumoto Formation-Chonan Formation or the base of the Kasamori Formation in the Kazusa Group, from the tephra correlation. This correlation suggests that the deposition of the NGF took place during the marine isotope stage (MIS) 19-17 or MIS19-16. Conclusively, the NGF is not the resulted of a single transitional stage from the interglacial stage to the glacial stage, but was formed by the actualization of the geographical feature contrast between the Matsumoto Basin and the Hida mountain range.View full abstractDownload PDF (1716K)
Review
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Ryu Uemura2007 Volume 46 Issue 2 Pages 147-164
Published: April 01, 2007
Released on J-STAGE: June 19, 2008
JOURNAL FREE ACCESSDuring the last glacial period, the climate of the Northern Hemisphere underwent abrupt warming events. This millennial-scale climate change, which was discovered from Greenland ice cores, is called the Dansgaard-Oeschger (D-O) event. Further comparison of polar ice cores suggests that warming in Antarctica preceded the onset of Greenland’s warming. These climate reconstructions rely mainly on stable isotope ratios (δD and δ18O) of water in polar ice cores because the air-temperature history has been inferred from a regional correlation between water isotope ratios and surface air-temperature. However, this reconstruction technique probably underestimates the temperature change in the Greenland summit area. This underestimation of the water isotope thermometer is caused mainly by changes in the seasonality of precipitation and changes in ocean surface conditions of the vapor source. One parameter, deuterium excess, offers the potential to reconstruct ocean surface conditions of the vapor origin ; it would improve the precision of temperature reconstruction.View full abstractDownload PDF (1574K)
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