The Journal of the Geological Society of Japan
Online ISSN : 1349-9963
Print ISSN : 0016-7630
ISSN-L : 0016-7630
Volume 120, Issue 1
Displaying 1-4 of 4 articles from this issue
Articles
  • Takahiro Yamamoto, Tadao Hasebe
    2014 Volume 120 Issue 1 Pages 1-9
    Published: January 15, 2014
    Released on J-STAGE: June 24, 2014
    JOURNAL FREE ACCESS
    A newly discovered sandy lacustrine deposit containing a fossil forest has been found within the valley floor of a branch of the Tadami River, at an altitude of 367 m above sea level (a.s.l.). Fossil trunks in this area yield a 14C age of 48,180 ± 580 yBP, suggesting that this area hosted a dammed lake generated by the Mizunuma eruption of Numazawa volcano, located some 35 km downriver of the fossil forest. Although a 14C age of 45,000 yBP has been reported for the Mizunuma eruption, this age should be revised to >43,500 yBP to correct for errors in the original analysis. This indicates a calendar age for the Mizunuma eruption of ca. 53 ka, consistent with the stratigraphic age of the Daisen-Kurayoshi tephra that immediately overlies the Mizunuma pyroclastic deposit.
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  • Manami Kitamura, Hideki Mukoyoshi, Takehiro Hirose
    2014 Volume 120 Issue 1 Pages 11-21
    Published: January 15, 2014
    Released on J-STAGE: June 24, 2014
    JOURNAL FREE ACCESS
    The correlation between the displacement and thickness of faults is an important parameter that enables an increased understanding of the processes involved in fault growth and earthquake generation. We determined this correlation by measuring the thicknesses of outcropping faults and using vitrinite reflectance (Ro) to determine paleotemperature estimates for the hangingwall and footwall units of 18 faults exposed in the Upper Cretaceous Shimanto accretionary complex of southeast Kochi prefecture, Japan. Fault displacement was estimated from the temperature difference across the faults, assuming a paleothermal gradient and the dip of the fault plane. The ratio of fault thickness to displacement ranges from ~10−5 to ~10−3, far lower than compared with previously identified thickness-displacement scaling relationships (10−3 to 10−1; e.g., Shipton et al., 2006). Permeability measurements within fault zones in the study area indicate that both hangingwall and footwall units have low permeability (less than 10−21 m2 at an effective pressure of 100 MPa), although fault zones have permeabilities that are four orders of magnitude higher than these units. This type of fault zone permeability structure means that similar fault zones in accretionary wedges could sustain high fluid pressures, promoting fault slips without significant generation of fault gouge and resulting in the formation of fault zones with lower ratios of thickness to displacement, as observed in the study area.
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  • Takuto Kanai, Atsushi Yamaji, Hideo Takagi
    2014 Volume 120 Issue 1 Pages 23-35
    Published: January 15, 2014
    Released on J-STAGE: June 24, 2014
    JOURNAL FREE ACCESS
    This study reports the results of paleostress analysis using the three-dimensional orientation and distribution of healed microcracks (HCs) in quartz within granitic bodies. Although the mechanical constraints of a universal stage together with the Terzaghi factor introduce sampling bias to the orientation and distribution of microstructures, few previous studies have taken this bias into consideration. We devised a series of corrections that allow the investigation of the orientation and distribution of HCs within the Ryoke granites, located along the Median Tectonic Line (MTL) in the Chubu region of Japan. Paleostress values were determined by fitting mixed Bingham distributions to the corrected distribution of each sample, resulting in the clustering of HC orientations in each data set into a few groups, which were then inverted to yield a paleostress state. The state corresponding to the group with the maximum number of HCs was considered to be the most significant stress for each individual sample, and the orientation of σ3 of this most significant stress in most samples is roughly perpendicular to the trend of the MTL. However, significant variations in the distribution of σ1 and σ2 were also identified, suggesting that the orientation of σ3 probably reflects the regional stress field, whereas the orientations of σ1 and σ2 are more dependent on the local stress state within the granitic body.
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