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Volume 119, Issue 6
Displaying 1-5 of 5 articles from this issue
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Articles
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Kazuma Seike, Hiromichi Hirano2013 Volume 119 Issue 6 Pages 397-409
Published: June 15, 2013
Released on J-STAGE: October 01, 2013
JOURNAL FREE ACCESS
Supplementary materialThe thermal maturity of the Izumi Group within the Izumi Mountains and the Asan Mountains of southwestern Japan was characterized using vitrinite reflectance and Rock-Eval Tmax data. Mean random reflectance and Rock-Eval Tmax values range from 0.69 to 2.64%, and from 432 to 548°C, respectively, indicating that the thermal maturity of the entire Izumi Mountains area increases towards the north, compatible with thermal diagenesis previously reconstructed based on zeolite facies distribution.
The three-dimensional thermal structure of the study area was estimated by multiple regression analysis of the surface maturity data collected from the western and central parts of the Izumi Mountains, assuming that the altitude of each sample reflected the vertical maturity gradient. This analysis yielded a thermal maturity isograd striking aat N78°E that dips 23° to the south, and has a maximum temperature gradient of 23–26°C/km. The higher maturity of the eastern Izumi Mountains can be attributed to the influence of regional uplift in this area. Three hypothesized processes can explain the thermal structure of the Izumi sedimentary basin: tectonic block tilting of the Izumi Group around the Median Tectonic Line (MTL), influence of heat sources near the Izumi belt, and differential subsidence rates between the northern margin and the depocenter of the Izumi sedimentary basin.View full abstractDownload PDF (2937K) -
Koji Kameo, Tomoyuki Sekine2013 Volume 119 Issue 6 Pages 410-420
Published: June 15, 2013
Released on J-STAGE: October 01, 2013
JOURNAL FREE ACCESSCalcareous nannofossil assemblages were examined to determine the age of the Anno Formation, a constituent part of the Awa Group, and to estimate the gap in timing between the formation of the Awa and Kazusa groups of the central Boso Peninsula, Japan. Three nannofossil datums and biohorizons have been identified within the Anno Formation, enabling correlation between this formation and calcareous nannofossil zones CN11–CN12a (~4.5–3.0 Ma). The chronological gap between the Awa and the Kazusa groups is estimated to be ~1 m.y. during the latest Pliocene to earliest Pleistocene in the western part of the peninsula, a shorter duration than that identified in the east of the peninsula. Contemporaneous unconformities and/or sections that formed during periods with low rates of sedimentation have also been identified in forearc basins along the southwestern part of the Pacific coast of Japan. These data suggest that a major change in sedimentation occurred at this time, possibly relating to significant tectonism within these areas.View full abstractDownload PDF (3118K) -
Toru Yamasaki2013 Volume 119 Issue 6 Pages 421-431
Published: June 15, 2013
Released on J-STAGE: October 01, 2013
JOURNAL FREE ACCESSK–Ar ages of 70.7 ± 1.8 Ma (Kamihara Tonalite), 71.0 ± 1.8 Ma (Mitsuhashi Granodiorite), 70.1 ± 1.8 Ma (Inagawa Granite, massive facies), 66.7 ± 1.7 Ma (Inagawa Granite, gneissose porphyritic facies) and 71.1 ± 1.8 Ma (Busetsu Granite) were determined from the Ryoke plutonic rocks in the Asuke area, central Japan. All of these ages are interpreted to be cooling ages related to the blocking temperature of biotite. The coincident 71–70 Ma cooling ages of the Kamihara Tonalite, Mitsuhashi Granodiorite, massive facies of Inagawa Granite and Busetsu Granite in 71–70 Ma suggests that these plutonic rocks followed a common cooling path in this area after solidification of the Busetsu Granite (ca. 83–82 Ma). On the other hand, solidification age of the gneissose porphyritic facies of Inagwa Granite is ca. 76–67 Ma, and K–Ar age of 66.7 ± 1.7 Ma from the gneissose porphyritic facies of Inagawa Granite was younger than other plutonic rocks in the Asuke area. thus, the K–Ar age is concordantly interpreted as cooling age of the granite.View full abstractDownload PDF (3009K)
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Chieko Shimada, Yoshinori Hikida, Makoto Yamasaki, Yoshihiro Tanimura2013 Volume 119 Issue 6 Pages 432-436
Published: June 15, 2013
Released on J-STAGE: October 01, 2013
JOURNAL FREE ACCESSWell-preserved diatoms are reported from the Omagari Formation (Santonian–Early Campanian) of the Upper Yezo Group in the Teshio-Nakagawa area of northern Japan. The Unit has a diatom floral assemblage that is rich in diverse species of Hemiaulus spp. and Triceratium spp., in addition to a few other genera of diatoms and silicoflagellates. Although valve microstructures are generally somewhat dissolved, preservation of these diatoms is significantly better than those described from previous research on Cretaceous siliceous phytoplanktons in Japan. The diatom floral assemblage in the study area is one of the oldest recorded in Japan, and should provide important insights into the Late Cretaceous evolutionary radiation of diatoms around the northwestern Pacific margin, an area where detailed information on primary producers is significantly lacking.View full abstractDownload PDF (1518K) -
Kazumasa Aoki, Brian Windley, Keiko Sato, Yusuke Sawaki, Takahiro Kawa ...2013 Volume 119 Issue 6 Pages 437-442
Published: June 15, 2013
Released on J-STAGE: October 01, 2013
JOURNAL FREE ACCESSNew K–Ar ages of phengite concentrates from the Barrovian metapelites of the Loch Leven area, Scotland yield 398 ± 10, 406 ± 11, 405 ± 11, 400 ± 11 and 445 ± 12 Ma. All the phengites have homogeneous mineral compositions, and the metamorphic temperatures are below the closure temperature of the K–Ar phengite system; this suggests that the time of metamorphic recrystallization was between the significant age-gap of ca. 445 and ca. 400 Ma. Integration of our K–Ar ages with previous geological and geochronological data from the Loch Leven area and from the Moine Thrust, which is the major lower boundary of the Barrovian metamorphic belt, suggests that the lower boundary is present as the Fort William Slide in the Loch Leven area.View full abstractDownload PDF (2007K)
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