The Journal of the Geological Society of Japan
Online ISSN : 1349-9963
Print ISSN : 0016-7630
ISSN-L : 0016-7630
Volume 107 , Issue 7
Showing 1-5 articles out of 5 articles from the selected issue
  • Akifumi Nozawa
    2001 Volume 107 Issue 7 Pages 413-431
    Published: July 15, 2001
    Released: April 11, 2008
    JOURNALS FREE ACCESS
    Early Pleistocene Okiura caldera, 15 by 10 km in diameter, located in the Hakkoda geothermal area, Northeast Japan, is one of the examples to address caldera structure and a caldera forming process. Volcanic sequence of the caldera filling deposits, a bout 50 km3 in remnant volume, is divided into, at least, eighteen units of major and minor pyroclastic-flow deposits, debris-flow deposit, and post caldera dacite lava. In the caldera-fill deposits, several abut unconformity relations occur between upper and lower pyroclastic-flow units are observed. Those abut planes are likely considered to be fault scarps, simultaneously created by pyroclastic flow eruptions. Estimation of paleo-current directions for the subsequent major pyroclastic-flow units reveals that these eruptions took place at inner caldera fault either in the central or southern part. Gravimetric and bore hole data, and presence of the syn-eruptive inner caldera faults suggest that coherent caldera floor was separated into several large pieces. The overall caldera structure was built up by the sequence of pyroclastic flow eruptions from inner caldera vents.
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  • Shinji Nagaoka, Mitsuru Okuno, Fusao Arai
    2001 Volume 107 Issue 7 Pages 432-450
    Published: July 15, 2001
    Released: April 11, 2008
    JOURNALS FREE ACCESS
    The eruptive history of Aira caldera in southern Kyushu during 100-30 ka is revealed by tephrostratigraphical studies. In this period, seven explosive eruptions of the caldera are identified. These tephra formations are as follows : 1) Hikiyama scoria fall at 103-95 ka, 2) Kongoji pyroclastic surge at 95-86 ka, 3) Fukuyama pumice fall at 95-86 ka, 4) Iwato tephra formation at ca. 60 ka consisting of three pumice falls, five pyroclastic flows and one pyroclastic surge, 5) Otsuka pumice fall at 32.5 ka, 6) Fukaminato tephra formation at 31 ka consisting of a pumice fall and a pyroclastic flow, and 7) Kenashino tephra formation at 30 ka consisting of ash falls and pyroclastic surges. These eruptive ages are estimated by stratigraphic relation with widespread tephras, as well as 14C dating. Three lava flows also effused during this period. Among them, the Shikine andesites between the Fukuyama pumice fall and Iwato tephra formation dammed up a river and formed a local small lake. The eruptive centers during 100-30 ka are located on the eastern half of the caldera.After 0.1 Ma (=100 ka) is an active period of many explosive and effusive eruptions. The Aira pyroclastic eruption at 27 ka (more than 411 km3 in bulk volume) separates this active period. Total mass of erupted magma during 100-30 ka is 63.3×1012 kg. And the average rate is 0.9×1012 kg/ky. The Aira caldera had repeated its magmatic eruptions at intervals of ca. 7500 years. However, the interval during 32.5-30 ka just before the Aira pyroclastic eruption is 1000 years.
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  • Michinobu Kuwae, Shusaku Yoshikawa, Yasufumi Satoguchi, Hiroyuki Ogura
    2001 Volume 107 Issue 7 Pages 451-459_1
    Published: July 15, 2001
    Released: April 11, 2008
    JOURNALS FREE ACCESS
    Diatoms were examined from a 13-m-long sediment core from the Misaka peatland, Gifu Prefecture, covering the late glacial through the Holocene (ca. 0-30 ka). Diatom records produce a history of paleoenvironmental changes as follows. During most of period between ca. 30 and ca. 12 ka of the late glacial, the lake was eutrophic and neutral or alkaline with an open water area. However, during two short periods (2.5-2.4, 1.6-1.5 ka) of the late glacial, the lake became oligotrophic and acid as bog pool or low moor, probably with enhanced hygrophytes. During the open water lake intervals water temperature was relatively lower. On the contrary, during the bog pool or low moor intervals, it was relatively higher. These environmental changes depend on air temperature fluctuations for the late glacial. Between ca. 12 and ca. 4 ka, the lake became oligotrophic and acid as bog pool or low moor with enhanced hygrophytes. Diatom assemblages during this interval indicate high water temperature, probably in response to Holocene warmth. The present study shows close correlation between paleoclimate change and a environmental history of the Misaka peatland.
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  • Atsushi Yamaji
    2001 Volume 107 Issue 7 Pages 461-479
    Published: July 15, 2001
    Released: April 11, 2008
    JOURNALS FREE ACCESS
    Crustal paleo stress estimation is essential in understanding dynamic aspects of tectonics, and is also important for petroleum exploration and ground designs. Fault striation analysis is a technique to estimate the stresses from the orientations and slip directions of meso-scale faults. Variety of methods have been developed for the analysis and has been applied to many regions in the world in the past twenty years. The most extensively used method that was invented during this period is an inverse method to determine an optimal stress from fault-slip data. However, the separation of stresses from heterogeneous fault-slip data is a week point of the method, so that several attempts have been made to solve the problem. Multiple inverse method is the latest approach to this problem. Tilting of a rock mass including faults makes the analysis difficult, because the relative age of the faulting and the tiliting is difficult to determine. Methods to deal with tilted blocks will enable us to analyze more general data. The development of descriptive techniques of meso-scale faults may help the separation of plural stresses.
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  • Aiming Lin, Chyi-Tyi Lee, Ching-Fei Liau
    2001 Volume 107 Issue 7 Pages XV-XVI
    Published: 2001
    Released: November 26, 2010
    JOURNALS FREE ACCESS
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