BULLETIN OF THE GEOLOGICAL SURVEY OF JAPAN
Online ISSN : 2186-490X
Print ISSN : 1346-4272
ISSN-L : 1346-4272
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Article
  • Satosh NAKAE
    Type: research-article
    2021 Volume 72 Issue 1 Pages 1-21
    Published: March 30, 2021
    Released: April 14, 2021
    JOURNALS FREE ACCESS

    LA–ICP–MS zircon U–Pb dating was performed for four felsic plutonic rocks, two of which were collected from the Miyazu Granite (medium-grained and coarse-grained granites) and the remains from the Kumohara Granite, to determine their intrusive ages. These granites are widely distributed in the Tango district, northern Kyoto Prefecture, and geotectonically belong to the San’in Belt of Southwest Japan. The obtained weighted mean values of 206Pb/238U ages and 2σ errors are 61.7 ± 1.0 Ma (mediumgrained granite) and 63.2 ± 1.0 Ma (coarse-grained granite) for the Miyazu Granite, and 65.7 ± 1.2 Ma and 65.1 ± 1.2 Ma for the Kumohara Granite. These ages are grouped into the younger and older, meaning that the Miyazu and Kumohara granites can be distinguished not only by their lithology but also by their ages. Formerly reported radiometric ages through Rb–Sr whole rock–mineral isochron and K– Ar methods indicate 61.9 Ma, 60.4 Ma (Rb–Sr age) and 64.8–58.0 Ma (K–Ar age) for the Miyazu Granite and 67.2 Ma (K–Ar age) for the Kumohara Granite. Among the above ages of the Miyazu Granite, the Rb–Sr ages are almost same as or younger than the U–Pb ages, but the K–Ar ages have a wider range, being not concordant with both of the U–Pb and Rb–Sr ages. On the Kumohara Granite, the K–Ar age is much older than the U–Pb ages. These evidences represent that there is no trend of ages becoming younger from U–Pb through Rb–Sr to K–Ar methods according to their closure temperatures.

    As shown by the above, different dating methods may show different values of age, therefore using a highly accurate and reliable method of dating is required to obtain more accurate age of igneous activity. Based on this perspective, the felsic plutonism and its correlation in the northwestern part of Kinki district were investigated, and the results are follows. One is that, in the southern margin of the San’in Belt, granodiorites of magnetite series and granites of ilmenite series activated at almost the same time after 85 Ma, which is included in the plutonism of the San’yo Belt. And the other is that batholith of biotite granite along the coast of the Japan Sea began to be active around 67 to 60 Ma, suggesting that it is chronologically consistent with the stocks of granodiorite in the southern margin of the San’in Belt.

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  • Kazutoshi IMANISHI, Takahiko UCHIDE, Takahiro SHIINA, Reiken MATSUSHIT ...
    Type: research-article
    2021 Volume 72 Issue 1 Pages 23-40
    Published: March 30, 2021
    Released: April 14, 2021
    JOURNALS FREE ACCESS

    A crustal stress map of the Chugoku region, western Japan, was constructed from earthquake focal mechanism solutions. In order to increase the spatial resolution of the stress map, we included more data than the routine catalog by determining focal mechanisms of small earthquakes down to magnitude 1.5 in approximately the past 12 years. We obtained 2988 well-constrained solutions using P-wave polarity data and body wave amplitudes. We merged our focal mechanism catalog with the Japan Meteorological Agency earthquake catalog, which have become a source of information on the stress map. For each earthquake, we determined the type of stress field using rake angles and the direction of the maximum horizontal compressive stress (SHmax) based on P-, B-, and T-axes. We then computed the mean SHmax and type of stress field on a mesh interval of 10 km. Compared with previous stress maps in the present study area, our 10-km mesh stress map provides higher spatial resolution in stress fields. Our stress map shows that the area is predominantly strike-slip stress field with E-W compression but the stress orientation rotates clockwise by about 20°in the Sea of Japan side of Shimane and Tottori prefectures. Based on our stress map, we evaluated the fault reactivation potential of 30 active faults targeted by the Headquarters for Earthquake Research Promotion Investigation Committee (2016), revealing that 28 active faults satisfy the condition for reactivation under the present-day stress field and typical friction coefficient. The remaining two active faults are unfavorably oriented to the present-day stress field, requiring external factors such as a development of anomalous high fluid pressure and a stress triggering associated with the rupture of adjacent active faults for reactivation. A crustal stress map of the Chugoku region, western Japan, was constructed from earthquake focal mechanism solutions. In order to increase the spatial resolution of the stress map, we included more data than the routine catalog by determining focal mechanisms of small earthquakes down to magnitude 1.5 in approximately the past 12 years. We obtained 2988 well-constrained solutions using P-wave polarity data and body wave amplitudes. We merged our focal mechanism catalog with the Japan Meteorological Agency earthquake catalog, which have become a source of information on the stress map. For each earthquake, we determined the type of stress field using rake angles and the direction of the maximum horizontal compressive stress (SHmax) based on P-, B-, and T-axes. We then computed the mean SHmax and type of stress field on a mesh interval of 10 km. Compared with previous stress maps in the present study area, our 10-km mesh stress map provides higher spatial resolution in stress fields. Our stress map shows that the area is predominantly strike-slip stress field with E-W compression but the stress orientation rotates clockwise by about 20°in the Sea of Japan side of Shimane and Tottori prefectures. Based on our stress map, we evaluated the fault reactivation potential of 30 active faults targeted by the Headquarters for Earthquake Research Promotion Investigation Committee (2016), revealing that 28 active faults satisfy the condition for reactivation under the present-day stress field and typical friction coefficient. The remaining two active faults are unfavorably oriented to the present-day stress field, requiring external factors such as a development of anomalous high fluid pressure and a stress triggering associated with the rupture of adjacent active faults for reactivation.

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  • Makoto TAKEUCHI, Sui JIA, Yusuke SHIMURA
    Type: research-article
    2021 Volume 72 Issue 1 Pages 41-64
    Published: March 30, 2021
    Released: April 14, 2021
    JOURNALS FREE ACCESS

    With the revision of the Geological Map of Japan 1: 200,000 “Toyama”, zircon 238U–206Pb ages of pre-Paleogene plutonic rocks in the area were measured, and the igneous history in the Hida Belt was discussed. Of the Hida Older Granites, the intrusion age of the Hayatsukigawa Granite in the western area is 224.8 ± 1.7 Ma, and that of the Unazuki Granite and Funakawa Granite in the eastern area is 236.5 ± 3.1 Ma and 240.7 ± 4.1 Ma, respectively. The Hayatsukigawa Granite is an independent body from the adjacent augen mylonite, because its intrusion age is different from that (250–240 Ma) of the protolith of the augen mylonite. On the other hand, parts of the Otodani Gabbro and Unazuki Granite in the Geological Map of Japan 1:50,000 “Tomari” area show 195.6 ± 2.0 Ma and 192.0 ± 2.4 Ma, respectively, indicating the intrusion of Early Jurassic time. It was found that these should be classified as the Hida Younger Granites.

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  • Yoshiki SATO, Kiyohide MIZUNO, Rei NAKASHIMA
    Type: research-article
    2021 Volume 72 Issue 1 Pages 65-80
    Published: March 30, 2021
    Released: April 14, 2021
    JOURNALS FREE ACCESS

    Miyagawa Plain (Miyagawa Delta), located in southwestern coast of the Ise Bay, is a deltaic alluvial lowland dominated under wave and tidal currents. A 57.6 m-long core penetrating the incised valley fill, “Chuseki-so”, was obtained from the lowermost part of the plain. Sedimentary facies, radiocarbon dating, diatom fossil and tephra analyses of the core material were conducted for revealing sedimentary environmental changes after the Last Glacial Maximum [LGM] and development of the Miyagawa Delta with the help of borehole logs. The alluvium deposit comprises six sedimentary units, Unit 1 to 6 in ascending order: Unit 1; braided river channel, Unit 2; estuary, Unit 3; inner bay, Unit4; lower shoreface, Unit5; upper shoreface to beach, Unit 6; inter-ridges marsh and artificial soil.

    After the LGM, an estuary environment was occurred until ca. 10.3 ka, and then inner bay environment was established around 9.5 ka associated with post-glacial sea-level rise. After ca. 7.5–8.0 ka, deltafront or spit was presumably occurred around the GS-ISE-1’ site.

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Report
  • Shigeo OKUMA, Hiroshi KANAYA
    Type: Report
    2021 Volume 72 Issue 1 Pages 81-94
    Published: March 30, 2021
    Released: April 14, 2021
    JOURNALS FREE ACCESS

    Petrophysical measurements such as density, porosity, magnetic susceptibility and Natural

    Remanent Magnetization (NRM) of Jurassic, Cretaceous and Paleogene granitic rocks which constitute the Japanese Islands had been conducted to clarify their physical properties. Similar measurements have been conducted for Neogene granitic rocks in Japan this time. Neogene granitic rocks in Japan are distributed widely from the backbone mountains of Hokkaido, the northernmost big island to Yakushima and Ishigaki Islands in the southern island chains. However, most plutons are relatively small except the Kofu, Tanzawa and Yakushima Granites with some outcropping areas and it is not easy to collect samples from every small pluton. Therefore, samples of Neogene granitic rocks were collected mainly from the Kofu and Kai-Komagatake Granites, and the Chichibu and Tanzawa Granites in the outer zone of Southwest Japan and measured in this study. The total number of measurements amounted to 210.

    The results of the measurements were classified and summarized as in six areas: 1) Echigo-Yuzawa, Wada Pass and Nasu-Dake Granites, 2) Chichibu and Tanzawa Granites, 3) Kofu and Kai-Komagatake Granites, 4) Cape Shiono and Cape Muroto Granites, 5) Southern Kyushu (Satsuma Peninsula and Osumi Peninsula) Granites and 6) Yaksushima Granites.

    The mean density increases from the Southern Kyushu Granites (2.62 g/cm3=103 kg/m3) to the

    Yakushima, Kofu and Kai-Komagatake, Echigo-Yuzawa, Wada Pass and Nasu-Dake, Chichibu and Tanzawa, Cape Shiono and Cape Muroto Granites (2.96 g/cm3) in this order. The mean porosity is almost inversely proportional to its mean density and ranges from 0.29 % to 1.94 %.

    The mean magnetic susceptibility indicates 2 × 10-4 and 5 × 10-4 (SI) for the Southern Kyushu and Yakushima Granites, 5 × 10-3 for the Cape Shiono Granites, 10-2 for the Cape Muroto Granites, 2 × 10-2 and 3 × 10-3 for the Kofu and Kai-Komagatake Granites, 3 × 10-2 for the Echigo-Yuzawa, Wada Pass and Nasu-Dake Granites and 4 × 10-2 for the Chichibu and Tanzawa Granites.

    On a basis of petrophysical properties and their lithology, granitic rocks of the Southern Kyushu and Yakushima Granites are defined as paramagnetic to weak magnetic. The Cape Shiono and Cape Muroto Granites are paramagnetic to weak magnetic and medium magnetic, respectively. Those of the Kofu and Kai-Komagatake Granites are high magnetic and medium magnetic, respectively. Both the Echigo-Yuzawa, Wada Pass and Nasu-Dake Granites, and the Chichibu and Tanzawa Granites indicate high magnetic. No relationship between the density and NRM is observed except the samples of the Tanzawa Granites. Königsberger ratio (Qn) of the rock samples shows less than 0.4 except the samples of the Yakushima Granites, and the Cape Shiono and Cape Muroto Granites.

    These results are summarized in Table 1 (physical properties of Neogene granitic rocks in Japan).

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