The Journal of the Geological Society of Japan
Online ISSN : 1349-9963
Print ISSN : 0016-7630
ISSN-L : 0016-7630
Volume 117, Issue 8
Displaying 1-6 of 6 articles from this issue
Articles
  • Takanori Bessho, Shizuo Nakaya
    2011 Volume 117 Issue 8 Pages 423-438
    Published: August 15, 2011
    Released on J-STAGE: December 10, 2011
    JOURNAL FREE ACCESS
    Multi-approach combining with modal, grain size, heavy mineral and garnet composition analyses of sandstones were carried out for the Paleogene Otonashigawa Group of the Kii Peninsula, Southwest Japan. The Otonashigawa Group consists of two formations: The Uridani Formations is overlain by the Haroku Formation. The Uridani Formation is composed mainly of mudstones. The Haroku Formation is subdivided into the Lower Member, consisting of alternating beds of sandstone and mudstone, and the Upper Member, consisting of thick sandstone beds and conglomerates. The modal composition of the sandstones in the Haroku Formation shows a change from felsic volcanic-rich lithic wacke at its base, to quartz-rich feldspathic arenite toward its top. Heavy minerals in these sandstones consist of abundant opaque minerals, zircon, garnet, and allanite along with minor titanite, epidote, and hornblende. Hornblende is found in the lower Member, whereas the sandstones from the uppermost section are characterized by abundant epidote, allanite, and titanite. From the Lower to the Upper Member, the pyrope-rich almandine (intermediate P/T type) decreases while in the spessartine-rich almandine (low P/T type) increases. A few grossular garnets were extracted from the upper most part of the Haroku Formation. In terms of provenance, our results indicate that the Otonashigawa Group was sourced chiefly from felsic volcanic rocks, pyroclastics, intermediate P/T metamorphic rocks, and granulite-facies metamorphic rocks from the Paleocene to early Eocene time. The existence of pyrope-rich almandine (Ig2 type) and purple zircon suggests that granulite-facies Precambrian units were also part of the sedimentary source. Owing to unroofing, granitic rocks, low P/T metamorphic rocks and minor calcareous metamorphic rocks cropped out in early Eocene time. Though the timing of the unroofing event differs between Kyusyu, Shikoku, Kii Peninsula, and the Akaishi Mountain areas, it took place everywhere in the provenance of the Shimanto Belt.
    Download PDF (4157K)
  • Daisuke Sato, Ichiro Matsumoto, Atsushi Kamei
    2011 Volume 117 Issue 8 Pages 439-450
    Published: August 15, 2011
    Released on J-STAGE: December 10, 2011
    JOURNAL FREE ACCESS
    Late Miocene to Pliocene volcanic rocks occur in the Wakurayama and Dakesan areas of Matsue city, San-in district, Southwest Japan. We present new petrographic and geochemical data for the volcanic rocks. Previous studies refer to the volcanic rocks as the “Wakurayama Andesite.” However, on the basis of bulk rock compositions (SiO2: 64.47–65.84 wt.%) determined in this study, we propose to rename the volcanic package the “Wakurayama Dacite.” The Wakurayama Dacite is divided petrographycally into three types: Type I, Type II, and Type III lavas, reflecting the order of eruption. The Wakurayama Dacite lavas have adakitic compositions (i.e., higher Al2O3 and Sr, and lower Y). This adakitic magma may have been generated by partial melting of the subducting Philippine Sea Plate. The Wakurayama Dacite was erupted at ∼5 Ma, the earliest Tertiary to Quaternary adakitic activity along the southwestern portion of the Japanese arc. The eruption of the Wakurayama Dacite suggests that melting of the Philippine Sea Plate occurred at ∼5 Ma.
    Download PDF (5561K)
  • Observation results from February 2008 to December 2009
    Yusuke Kashima, Yoshie Nasuhara, Takashi Nakamura, Tsuneo Yamauchi, Ke ...
    2011 Volume 117 Issue 8 Pages 451-467
    Published: August 15, 2011
    Released on J-STAGE: December 10, 2011
    JOURNAL FREE ACCESS
    In order to detect the precursors of deep groundwater changes related to the anticipated large earthquakes off Miyagi Prefecture, Japan, we established an observation network of six boreholes with a real-time automatic data-acquisition system. Durning the observation period in 2008 and 2009 groundwater temperatures and levels in three deep boreholes responded to the off-Ibaraki Prefecture earthquake (MJ7.0, Japan Meteorological Agency earthquake magnitude scale) on 8 May 2008, the Iwate-Miyagi inland earthquake (MJ7.2) on 14 June 2008, the off-Fukushima Prefecture earthquake (MJ6.9) on 19 July 2008, and the northern Iwate prefecture coast earthquake (MJ6.8) on 24 July 2008. Examining the responses of our observation system to the earth tides, volumetric strain changes equivalent to water level changes were estimated at 2.27×10−9/mm and 0.5×10−9/mm for two of three boreholes. Earthquake-related changes in borehole groundwater levels, which were free from the artifacts, are consistent with the estimates from static volumetric strain changes calculated from dislocation models. The lower detection limit of our observation system to co- and post-seismic groundwater changes is approximated by the equation MJ=2.4 log r+1.0, where r represents hypocentral distance in kilometers. Similar equations can be derived not only from the attenuation relation for static strain changes but also from those for ground motion amplitude.
    Download PDF (5160K)
  • Hiroshi Yamamoto, Keita Shimotsugi, Shinsuke Doi, Hafiz Ur Rehman
    2011 Volume 117 Issue 8 Pages 468-471
    Published: August 15, 2011
    Released on J-STAGE: December 10, 2011
    JOURNAL FREE ACCESS
    In this study, we revisited a serpentinite occurrence in the Shimanto Belt. Serpentinite is sporadically distributed along a distance of 1400 m in a north—south-striking zone, across a width of about 250 m, at Yaseo on the Satsuma Peninsula. We mapped isolated blocks, debris deposits, and massive bodies of serpentinite. Two significant serpentinite bodies are exposed over areas measuring approximately 100×50 m and 200×70 m. These bodies overlie west-dipping sandstone and mudstones layers; the boundary between serpentinite and sedimentary layers is subhorizontal. Brittle shear planes near the structural base of serpentinite indicate top-to-the-east displacement. The serpentinite bodies are interpreted as klippes that were structurally emplaced onto the sedimentary sequence of the Shimanto Belt.
    Download PDF (945K)
Note
  • Toshio Kohno, Satoshi Nakano
    2011 Volume 117 Issue 8 Pages 472-475
    Published: August 15, 2011
    Released on J-STAGE: December 10, 2011
    JOURNAL FREE ACCESS
    Many transparent calcite crystals exhibit striking fluorescence together with own birefringence. On the basis of these two characteristics (fluorescence and birefringence), suitable calcite crystals can be utilized to visualize the splitting of both monochromatic and polarized light emitted from a laser pointer, as the light passes through a crystal. This new device should serve as a learning tool in introductory microscopy courses or whenever learning the principles of polarizing microscopy.
    Download PDF (317K)
Report
feedback
Top