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| index | Vol. 48 | 1 | 2 | 3/4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | Japanese Index | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Relationship between geological occurrences and twinning laws of plagioclase in granitic and metamorphic rocks in the Sor Rondane Mountains, Antarctica
Yuhei Takahashi, Yoshiaki Tainosho, Yasuhito Osanai and Noriyoshi Tsuchiya
1997
vol. 48 (6), p. 305-312, 4 figs., 1 table.
Keywords: Twinning law, plagioclase, C-twins, pericline twin, Antarctica
Abstract: Twinning laws of plagioclase were determined for granitic and metamorphic rocks in the Sor Rondane Mountains in Antarctica. Granitic rocks in this area are subdivided on the basis of their emplacement mode into discordant and concordant types. C-twins, which are defined as all type of twins except albite and pericline twins, are more dominant in the discordant-type bodies than in the concordant-type bodies. This result agrees fairly well with the previous studies showing that the C-twins are common in typical intrusive rocks (i.e., those with discordant boundaries to surrounding rocks). However, the frequency of C-twinning in the discordant-type intrusive rocks in this area is often less than that of typical intrusive rocks in Japan. The C-twins in previous studies might have been overestimated because of ambiguity in optical methods at that time. Frequency of pericline twinning is locally variable in the high-grade metamorphic rocks in the study area. This suggests that the frequency is not only dependent on metamorphic temperature but also on local differences of shear stress.
Offscraping, underplating and out-of-sequence thrusting process of an accretionary prism: On-land example from the Mino-Tamba Belt, central Japan
Katsumi Kimura
1997
vol. 48 (6), p. 313-337, 22 figs.
Keywords: Mino-Tamba Belt, chert-clastic unit, Jurassic, accretionary prism, offscraping, underplating, imbricate structure, out-of-sequence thrust, decollemant, duplex
Abstract: The Inuyama Sequence, a coherent Jurassic chert-clastic unit in the Mino-Tamba Belt, central Japan, represents structureal styles characteristic of accretionary processes at a shallow structural level. It is characterized by a series of regional-scale thrust sheets, which form a south-verging imbricate structure asymptotically converging down into the master sole thrust of the Inuyama Sequence. Each thrust sheet, consisting of an Early Triassic to Late Jurassic oceanic plate stratigraphy, is further subdivided into an upper sheet and a lower sheet by an intrasheet detachment along the hemipelagic interval. The upper sheet is characterized by a homoclinal structure with imbricate thrusts rooted into the intrasheet detachment, while the lower sheet is characterized by the development of duplex structures and mesoscopic F1 folds. Slip vectors from all kinds of thrusts, corrected for tectonic rotation, are directed between SSE and SSW with respect to the N60E regional trend of the Inuyama Sequence. The structural process of accretion can be resolved into the following four stages; (1) Formation of a decollement zone was initiated within the hemipelagic horizon, and the stratigraphic section above the decollement was progressively offscraped by in-sequence thrusting on the toe of the accretionary prism. This offscraping process was followed by (2) underplating of hemipelagic and pelagic intervals to the base of the prism in the form of duplexes with F1 folds. This deformed section got a secondary thickening by (3) later-stage imbricate thrusting due to the reactivation of some in-sequence thrusts and their extension to the decollement, resulting in the formation of a series of regional-scale thrust sheets. Finally, (4) these imbricated thrust sheets were uplifted over a younger accreted section by out-of -sequence thrusting, leading to substantial thickening of the wedge.
Magnetostratigraphy of Neogene rocks around the Himi area in Toyama Prefecture, Japan
Yasuto Itoh and Mahito Watanabe
1997
vol. 48 (6), p. 339-346, 8 figs., 1 table.
Keywords: Himi,Toyama Prefecture, magnetostratigraphy, diatom biostratigraphy, Neogene
Abstract: Neogene rocks distributed in the Himi Area adjacent to the Japan Sea have geen subjected to a mangetostratigraphic study. The upper Neogene System is diveded into the Ao and the Yabuta Formations in ascending order. The Ao Formation, which is 200 m thick, consists entirely of siltstone. The Yabuta Formation is more than 120 m in thickness and consists of calcareous sandy siltstone and sandstone. Both of the formations are intercalated with many tuff layers. Progressive demangetization tests using thermal and alternating-field methods show that the tuff samples from 14 sites preserve reliable records of paleomagnetic directions. On the basis of magneto- and biostratigraphic data, the Ao and the Yabuta Formations are assigned to the Gilbert reversed polarity Chron and the Gauss normal polarity Chron on the standard geomagnetic polarity time scale.
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Bandai volcano debris avalanche deposit in the northern part of Lake Inawashiro, northeast Japan, revealed by sonic prospecting
Shigeru Suto and Takahiro Yamamoto
1997
vol. 48 (6), p. 347-353, 6 figs.
Keywords: Bandai Volcano, Okinajima Debris Avalanche Deposit, Lake Inawashiro, hummocky hill, sonic prospecting
Abstract: Sonic prospecting was carried out in the northern part of the Lake Inawashiro in northeast Japan, to reveal topographic and geologic features of lake deposits, including a debris avalanche deposit thought to be derived from Bandai volcano. Bathymetric sounder, side scan sonar, and Uniboom prospecting show the distribution of the Okinajima Debris Avalanche Deposit in the northwest corner of the study area. The diameter of the hummmocky hills in the Okinajima Debris Avalanche Deposit in the lake vary from 100 to 300m, and the relative heights from 6 to 14 m. The concentration of hummocky hills in the lake is lower than on land. Hummocky hills in the lake are surrounded by well stratified sandy to muddy deposits, with thicknesses up to 5m. The main part of the Okinajima Debris Avalanche Deposit flowed toward the west on land. The study area in the lake bounds the east and south of the deposit. No reflection data is available from the unconsolidated sediments in the norhternmost part of the lake, that is south of the Bandai volcano. It is considered to be composed of a coarse-grained flood deposit or a gas rich layer. A well stratified 30 m thick layer was found in the central part of the lake.
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Abstracts for Geological Survey Seminar, no. 247: Evolution of magmatic hydrothermal systems and related mineralization
1997
vol. 48 (6), p. 355-361.
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Abstracts for Geological Survey Seminar, no. 248: Recent development of secondary ion mass spectrometry on geology
1997
vol. 48 (6), p. 362-363.
