The Journal of the Geological Society of Japan
Online ISSN : 1349-9963
Print ISSN : 0016-7630
ISSN-L : 0016-7630
Search
Advanced search
OR
Browse
Search
Volume 122, Issue 10
Displaying 1-4 of 4 articles from this issue
Show all abstracts
    • |<
    • <
    • 1
    • >
    • >|
Article
  • Geochronology of the early Paleozoic Kiroko amphibolite in the Kanto Mountains, central Japan
    Atsushi Miyashita, Yukiyasu Tsutsumi, Takashi Sano
    2016 Volume 122 Issue 10 Pages 511-522
    Published: October 15, 2016
    Released on J-STAGE: January 20, 2017
    DOIhttps://doi.org/10.5575/geosoc.2016.0023
    JOURNAL FREE ACCESS
    Show abstractHide abstract

    The Kiroko Amphibolite is located between the Cretaceous Atokura nappe and the Mikabu Greenstone complex in the Kanto Mountains of Japan, and yields early Paleozoic K-Ar ages. The amphibolite consists mainly of magnesiohornblende and albite with minor amounts of titanite, apatite, and pyrite. It contains no other high-grade metamorphic index minerals. The whole-rock composition of the amphibolite is distinct from that of the surrounding Mikabu Greenstone complex, which is geochemically similar to mid-ocean ridge basalt. In this study, the Kiroko Amphibolite yielded a U-Pb zircon age of ca. 480 Ma, which is interpreted to represent the timing of magmatism. Amphibole from the amphibolite yielded an 40Ar/39Ar plateau age of ca. 430 Ma, which is considered to be the age of metamorphism. These data suggest that the Kiroko Amphibolite forms part of the Cambro-Ordovician igneous-metamorphic complexes of Japan.

    View full abstract
    Download PDF (4586K)
  • Average recurrence interval of Ansei-type earthquakes inferred from Holocene deposits of the Yaizu Plain, Shizuoka Prefecture, Japan
    Akihisa Kitamura, Yuta Mitsui, Youki Takikawa
    2016 Volume 122 Issue 10 Pages 523-531
    Published: October 15, 2016
    Released on J-STAGE: January 20, 2017
    DOIhttps://doi.org/10.5575/geosoc.2016.0022
    JOURNAL FREE ACCESS
    Show abstractHide abstract

    The AD 1854 Ansei-Tokai earthquake had a magnitude of 8.4 and was caused by rupturing along mega-thrusts in the Suruga and Nankai troughs, which resulted in ~1.5 m of uplift along the western coast of Suruga Bay. However, no other earthquake with co-seismic uplift has been documented in the region or identified from topographic features such as raised marine terraces along the western coast of the bay. To identify evidence of co-seismic uplift from geological records, we analyzed Holocene sedimentary deposits in drillcore (40 m long and 7 cm in diameter) obtained from the coastal Yaizu Plain. Our results indicate that the upper surface of marine strata occurs at 15.19 m below mean sea level and yields an age of 7410-6890 cal BP. Based on these values and the rate of subsidence during the inter-seismic period, we estimated the average recurrence interval of co-seismic uplift from Ansei-type earthquakes using two models (Models 1 and 2). In Model 1, which assumed a constant subsidence rate, the average recurrence interval was estimated to be 143-174, 188-226, 215-261, or 261-324 yr using mean co-seismic uplifts of 1.0, 1.3, 1.5, or 1.8 m, respectively. In Model 2, which assumed an inconstant subsidence rate, the average recurrence interval was estimated to be 150-178 yr using a mean co-seismic uplift of 1.0 m.

    View full abstract
    Download PDF (3093K)
  • Petrological investigation of long-term evolution of magma chambers and preparing processes of caldera-forming eruption, Numazawa volcano, NE Japan
    Yoshiko Masubuchi, Yasuo Ishizaki, Tomohito Shirai, Akiko Matsumoto, M ...
    2016 Volume 122 Issue 10 Pages 533-550
    Published: October 15, 2016
    Released on J-STAGE: January 20, 2017
    DOIhttps://doi.org/10.5575/geosoc.2016.0027
    JOURNAL FREE ACCESS
    Supplementary material
    Show abstractHide abstract

    Numazawa volcano in northeastern Japan has produced six eruptions of silicic magma (rhyolite and dacite) since volcanic activity began at 110 ka. The volcano formed the Shirifukitoge Pyroclastic Deposit (110 ka; >72 wt.% SiO2), the Mukurezawa Lava (71 ka; 69.1-71.7 wt.% SiO2), the Mizunuma Pyroclastic Deposit (53 ka; 68.1-69.0 wt.% SiO2), the Sozan Lava (43 ka; 66.9-67.7 wt.% SiO2), the Maeyama Lava (24 ka; 62.7-65.4 wt.% SiO2), and the Numazawako Pyroclastic Deposit (5.4 ka; 63.5-67.0 wt.% SiO2). The youngest eruption (5.4 ka) was the most voluminous, erupting ~2 km3 of magma and forming the Numazawa caldera. The distinct major and trace element chemistries, phenocryst assemblages and compositions, and 87Sr/86Sr ratios of the erupted silicic magmas imply that each eruption was fed by an independent and transitory silicic magma chamber. Relatively homogeneous silicic magmas were tapped between 110 and 53 ka. In contrast, evidence of interaction between silicic (dacitic) and mafic magmas is recorded in eruptions that are 43 ka in age and younger, including mafic enclaves and banded pyroclasts. This may have resulted from an increase in the rate of mafic magma input into the roots of the Numazawa system beginning at 43 ka. The silicic and mafic magmas erupted at 43 ka and 24 ka have similar 87Sr/86Sr ratios of 0.70385-0.70386 and 0.70396-0.70398, respectively. This suggests a common source material for the silicic and mafic magmas in each eruption. In contrast, during the 5.4 ka caldera-forming eruption, concurrent eruptions of silicic magma and two mafic magmas have different 87Sr/86Sr ratios. It is therefore likely that the 5.4 ka caldera-forming eruption resulted from the nearly simultaneous generation of distinct magmas from three different sources and, consequently, the rapid formation of a large magma chamber.

    View full abstract
    Download PDF (5782K)
Lecture
    • |<
    • <
    • 1
    • >
    • >|
feedback
 Top