The Journal of the Geological Society of Japan
Online ISSN : 1349-9963
Print ISSN : 0016-7630
ISSN-L : 0016-7630
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Volume 110, Issue 5
Displaying 1-6 of 6 articles from this issue
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Articles
  • Reappraisal for recent faulting history of the Hakushu fault, the southern part of the Itoigawa
    —Shizuoka Tectonic Line, central Japan
    Daisuke Miura, Ryuta Hataya, Katsuyoshi Miyakoshi, Daiei Inoue, Masash ...
    2004 Volume 110 Issue 5 Pages 255-270
    Published: 2004
    Released on J-STAGE: January 07, 2005
    DOIhttps://doi.org/10.5575/geosoc.110.255
    JOURNAL FREE ACCESS
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    Co-seismic faulting activity of the Hakushu fault in the southern part of the Itoigawa-Shizuoka Tectonic Line active fault system, central Japan was re-appraised by using newly obtained paleoseismological data for the last several hundred thousands years. The Hakushu fault, 8 km long, consists of intermittently continuous, short fault traces from northwest to southeast direction. Two parallel fault scarps (eastern and western scarps) occur in the southern part of the Hakushu fault. We conducted a trench excavation at the Yokote site across the western scarp. These excavations revealed evidences for the latest, at least two surface-rupturing events. The events were present sometimes between 1695 and 2720 cal y BP and between 7325 and 8360 cal y BP. The timing of the penultimate event is roughly coincident with the latest event (6650 and 7000 cal y BP) at the Daibo site on the eastern fault scarp. The recurrence interval of time and slip rate between these two events is approximately estimated as 5000 years and 0.4-0.6 mm/yr in vertical-slip, respectively. A total vertical-slip rate estimated only from the trenching on both fault scarps is 0.4-0.8 mm/yr. On the other hand, another vertical-slip rate estimated from a detailed correlation of reference deposits including peripheral deformation of fault scarp is 1.0-1.8 mm/yr. These slip-rates suggest that the net-slip rate for co-seismic faulting of the Hakushu fault will be over 1.0 mm/yr.
    A set of steeply/moderately dipping fault planes on the Yokote trench walls showed a palmtree-like structure as a whole deformation. This finding indicates that the faulting includes a strike-slip component. The strike-slip occurrence is evidently distinct from a character of low-angle thrust fault as suggested by previous studies. However, it is rather consistent with a character of sinistral slip on the Houohzan fault adjacent to the southern edge of the Hakushu fault.
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  • Geology and volcanic history of Kita-Hakkoda volcanic group, Northeast Japan
    Takashi Kudo, Shinji Takarada, Minoru Sasaki
    2004 Volume 110 Issue 5 Pages 271-289
    Published: 2004
    Released on J-STAGE: January 07, 2005
    DOIhttps://doi.org/10.5575/geosoc.110.271
    JOURNAL FREE ACCESS
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    Kita-Hakkoda volcanic group, located in the Northeast Japan arc, comprises eleven small-scale stratovolcanoes. The volcanic activity iniciated around 0.40 Ma and continued until present. Each volcano had been active in a specific period. The position of the eruptive center has converged to the central part through time. Each volcano consists predominantly of lava flows and domes associated with small-scale pyroclastic fall and flow deposits by vulcanian, strombolian and phreatic eruptions. Lithology of the eruptive rocks is basalt to dacite in 0.4-0.2 Ma, and andesite to dacite in 0.2 Ma-present. There are several tephra layers probably derived from Kita-Hakkoda volcanic group that are distributed to Kamikita Plain. They indicate several large-scale pyroclastic eruptions of 0.1 km3 to several km3 in volume occurred during 0.27-0.17 Ma in the volcanic group. In total 15 km3 (DRE) of magmas has been erupted during 0.4 million years. The eruption rate for the entire activity is estimated to be 0.04 km3/1000 yr. Eruption rates are relatively high (0.054-0.040 km3/ky) in 0.4-0.1 Ma, and low (0.016 km3/ky) in 0.1 Ma-present. The temporal changes of the eruption rate and the eruption style show that the culmination of the volcanic activity was in 0.4-0.1 Ma. The intensity of the volcanic activity has declined at least after 0.1 Ma. Temporal change of eruption rate and the convergence of the vent position can be explained by the process of cooling of a mantle diapir. Based on the mantle diapir model, it seems that the current activity of this volcanic group is the state that going toward an end in the long term.
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  • Stratigraphy of the Miocene Series in the Hiki Hills, Saitama Prefecture, central Japan
    —Synthesis of the lithostratigraphy based on integrated geochronology—
    Masaki Takahashi, Yukio Yanagisawa
    2004 Volume 110 Issue 5 Pages 290-308
    Published: 2004
    Released on J-STAGE: January 07, 2005
    DOIhttps://doi.org/10.5575/geosoc.110.290
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    Marine diatoms occur in the Miocene marine sequence of the Arakawa and Ichinokawa Formations distributed along the Tokigawa River in the southern part of the Hiki Hills, central Japan. Twenty-two samples yield age diagnostic diatom fossils, which indicates the sequence belongs to the lower Denticulopsis lauta Zone (NPD 4A). Combining with lithologic characters with diatom geochronology, the lithostratigraphic boundary between the underlying siltstone of the Arakawa Formation and overlying sandy siltstone with poorly sorted breccia of the Ichinokawa Formation is dated as 15.4 Ma. Fission track age of thick felsic tuff within the Arakawa Formation is also determined. Litho- and chrono-stratigraphies of the Hiki and Iwadono Hills are well correlated with those of the Miocene marine sequences in and around the Kanto Mountains.
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  • Temporal and spatial variations of paleoenvironments of Paleo-Hamana Bay, central Japan, during the Middle Pleistocene
    —Analyses of fossil ostracode assemblages, and total organic carbon, total nitrogen and total sulfur contents—
    Toshiaki Irizuki, Koji Seto
    2004 Volume 110 Issue 5 Pages 309-324
    Published: 2004
    Released on J-STAGE: January 07, 2005
    DOIhttps://doi.org/10.5575/geosoc.110.309
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    High-resolution analyses of fossil ostracode assemblages, and total organic carbon (TOC), total nitrogen (TN) and total sulfur (TS) contents were conducted in four sites exposing the Sahama Mud Member of the Middle Pleistocene Hamamatsu Formation, central Japan, to infer paleoenvironments of Paleo-Hamana Bay. Twenty-six ostracode species are included in 84 samples. Neomonoceratina delicata is the most dominant species. It is not recognized in modern similar environments in Japanese islands except for Ryukyu islands, but is living abundantly in the South China Sea. Spinileberis quadriaculeata, Bicornucythere bisanensis, Pistocythereis bradyi and Bicornucythere sp. are also abundant, and they are common and widely distributed in Japanese inner to middle enclosed muddy bays. Relative frequency of those five species reaches 98.3%.
    On the basis of the results of fossil ostracode and TOC, TN and TS analyses, temporal and spacial variations of paleoenvironments of Paleo-Hamana Bay are reconstructed as follows. An enclosed brackish shallow bay was formed during an early transgression and oxygen-poor bottom waters developed. After that warm water widely invaded the study sites and the bay changed to slightly oxygen-rich conditions. The bay became enclosed again and water depth reached the maximum (around 10 m deep at the deepest) at the central part of the study sites during the rapid increase of the sea level. At the early time of the highstand of the sea level, the bay was extremely enclosed and relative contribution of non-marine and /or terrestrial organic materials to the bay sediments increased temporarily. As a result a parallel laminated clay suggesting anoxic environments, was widely developed. Afterward, the bay became shallower under the influence of waters warmer than today.
    Thus, Paleo-Hamana Bay in the middle Pleistocene was mostly enclosed bay and salinity during the transgression and the highstand of the sea level was higher than that in present-day Hamana-ko Bay.
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