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 123, Issue 7
Displaying 1-6 of 6 articles from this issue
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Excursion Guidebook: 124th Annual Meeting of the Geological Society of Japan, 2017 Ehime
Regular Section
Articles
  • Deformation microstructures developed in the Iragawa mylonite zone in the western part of the Shirakami Mountains, Northeast Japan
    Shunsuke Watanuki, Takuto Kanai, Hidenori Saka, Hideo Takagi
    2017 Volume 123 Issue 7 Pages 533-549
    Published: July 15, 2017
    Released on J-STAGE: August 03, 2017
    DOIhttps://doi.org/10.5575/geosoc.2017.0006
    JOURNAL FREE ACCESS
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    The Iragawa mylonite zone is ~2.5 km long (N-S) and ~350 m wide, and occurs in the Cretaceous Shirakamidake granitic complex along the western coastline of southernmost Aomori Prefecture, Northeast Japan. As already reported and confirmed by magnetic susceptibility measurements, the Iragawa mylonite zone is not the northern extension of the Hatagawa fault zone, but exists within the Abukuma Belt. The center of the mylonite zone, which is ~200 m wide, consists of ultramylonite locally overprinted by cataclasite. The mylonitic foliation strikes N-S and dips 40°-80° to the east, while the mylonitic lineation plunges at 30°-70° to the northeast. Asymmetric deformation microstructures indicate a sinistral normal shear. The lattice preferred orientation (LPO) and grain size of recrystallized quartz across the mylonite zone, as measured using SEM-EBSD, reveal that the most fine-grained ultramylonite displays a random LPO pattern and mean grain size of recrystallized quartz of 7.8-9.2 μm. The other mylonites mostly show LPO patterns indicating activity of the rhomb <a> and/or prism <a> systems, with a mean grain size of recrystallized quartz of 13-250 μm. The former suggests grain boundary sliding as the dominant deformation mechanism, whereas the latter suggests that dislocation creep took place at 350-450℃.

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  • Depositional process in response to waveform of the 2011 Tohoku oki tsunami inundated the Atsuma gawa River in Hokkaido, Northern Japan
    Katsuichi Oota, Tsumoru Sagayama, Tetsuya Inui, Koichi Hoyanagi
    2017 Volume 123 Issue 7 Pages 551-566
    Published: July 15, 2017
    Released on J-STAGE: August 03, 2017
    DOIhttps://doi.org/10.5575/geosoc.2017.0007
    JOURNAL FREE ACCESS
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    The 2011 Tohoku-oki tsunami inundated the Atsuma-gawa River on the Pacific coast of Hokkaido, Japan, to 6.2 km inland from shore line and left tsunami deposits on the sand bars. In this study, we reconstruct the formation of the river tsunami deposits using sedimentology, and observations of the development of the tsunami waves. We divide the tsunami deposits into three main units (Unit 1 to 3, where Unit 1 is the lowermost deposit). Unit 1 comprises a mixture of shallow marine sand and fluvial deposit, involving fallen plants at the base indicating the current direction towards the sea. We interpret Unit 1 as a return flow deposit; extensive return flow occurred after the initial inundation, following a large drop in the level of the tide and rapid erosion of river mouth bar blocking the river channel. Unit 2 consists of shallow marine sand, which rapidly thins and fines inland, and cross laminae indicate the sand was transported by landward flow. We interpret it mainly as a run-up deposit, which formed from tsunami waves that encroached inland, losing energy, wave amplitude, and velocity. Unit 3 comprises alternating layers of shallow marine sand and silt (mud drapes), and represents small variations of the grain size between seaward and landward. Current directions estimated by cross laminae indicate mainly landward, and partially seaward. We interpret Unit 3 formed from repeated long-period run-up and return flow cycles in the latter stages of the tsunami, and mud drapes were deposited from fluid mud. Return flow concentrated in topographic lows and the channel. By documenting the formation processes of tsunami deposits correlate with recorded tsunami waveforms, we aim to improve our understanding of how tsunamis interact with onshore river systems, in Japan and elsewhere.

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