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Volume 106, Issue 6
Displaying 1-6 of 6 articles from this issue
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Hidemi Tanaka, Shin-ichiro Hinoki, Shuichi Hasegawa, Tsuyoshi Haraguch ...2000 Volume 106 Issue 6 Pages 385-396
Published: June 15, 2000
Released on J-STAGE: April 11, 2008
JOURNAL FREE ACCESSThis paper describes the lithology and collapse direction analysis of landslide deposits distributed along the active trace of the Median Tectonic Line (MTL), extending ENE-WSW at the Funato area in central Shikoku, Southwest Japan. The results together with topographical characteristics around the Funato area suggests about 30 m of right lateral displacement along the active trace of the MTL.The landslide deposits are distributed along the eastern side of Setodani Valley for 200 m (E-W)×150 m (N-S), overlying the fault surface of the MTL. The landslide deposits originated from sedimentary rocks of the Izumi Group to the north across the MTL, and are divided into two types (Types I and II) based on their lithological facies. The Type I deposits are characterized by boudinaged sandstone blocks contained in sheared argillaceous matrix, which were formed at the first stage of a landslide activity overlying the fault zone of the MTL. The Type II deposits are characterized by randomly oriented sandstone blocks, and were formed by the subsequent re-collapse of Type I deposits. Results of structural analysis show that the collapse directions of both Type I and Type II deposits are from N17°-31°W to S 17°-31°E. However, evidence of landslides, such as relict concave topography, are not found to the northwest (N17°-31°W) of the landslide deposits. A small ridge of these deposits extending from ENE blocks half the mouth of the Setodani Valley. From these observations, the following possibilities are suggested. (1) The source region of the landslide, which is inferred to be present to the northwest, has been lost by erosion or slope collapse. (2) The ridge composed of landslide deposits can be regarded as a "shutter ridge" which has been displaced from the east to its present position. In this case, the right lateral displacement along the active trace of the MTL after the formation of the Type II deposits is estimated to be about 30 m maximum (E-W length of the shutter ridge). This event must have ocurred after the deposition of fluival deposits (7120±260 yrs. B.P. ), since the Type II deposits cover the active fault surface, which had previously cut the fluival deposits.View full abstractDownload PDF (3800K) -
Akihiro Kano, Hidenori Fujii2000 Volume 106 Issue 6 Pages 397-412
Published: June 15, 2000
Released on J-STAGE: April 11, 2008
JOURNAL FREE ACCESSGross morphology and internal texture of tufa deposits in Shirokawa Town, Ehime Prefecture, vary due to local environments (including flow condition, substrate topography, and associated biota) within the tufa-bearing stream. Variations in gross morphologies of the tufa deposits include 1) tufa encrusting boulders, 2) plane slope, 3) mound, 4) terraces, and 5) cascade. These types are basically related to variations in the original topography although they share similarities in a local environment of strong water flow which is an essential condition for active tufa deposition. Laminated internal texture, the most striking feature of the Shirokawa tufa, normally develops in tufas colonized by cyanobacteria inhabiting the surface. Regular lamination is most commonly developed along the narrow water passage of the lower stream where strong water flow continues throughout the year. The laminated tufas consist of repetition of dense lighter-colored laminae and porous darker-colored laminae, which correspond to the summer and winter deposits, respectively. The lighter laminae are characterized by thick calcite encrustation on upward-growing filamentous cyanobacteria. The darker laminae also mainly consist of calcite encrustation of cyanobacterial filaments, but the porosity largely remains. The annual rhythm seen in the laminated texture can be explained by seasonal variation in abiotic calcite precipitation rate (large in summer and small in winter) which increases with water temperature, Ca2+ contents, and flow strength. Because of the well-defined chronological constrains, continuous deposits of laminated tufas can be excellent material for reconstructing terrestrial climates.View full abstractDownload PDF (3885K) -
Aiming Lin, Nobuyuki Matsushima, Tadashi Maruyama2000 Volume 106 Issue 6 Pages 413-425
Published: June 15, 2000
Released on J-STAGE: April 11, 2008
JOURNAL FREE ACCESSGeological and topographical investigations show that the Iida-Matsukawa fault striking northwest and extending for about 15 km in the southern Kiso Range, central Japan, is presently active with a horizontal slip rate of 1 mm/y. It is inferred that the Kiso Range is displaced about 2 km sinistrally and 500 m vertically. Many valleys were systematically sinistrally-deflected or bent at the fault trace. There is a linear relation of D=aL between the offset(D) and the length (L) of upper valley channel from the deflected point, where coefficient a is limited in a range of 0.08∼0.3. Detailed field observations show that the terrace containing AT volcanic ash formed 20, 000 y.B.P is displaced 3∼5 m vertically. The analysis of foliated cataclastic rocks shows that the fault formed before the Quaternary and moved as a sinistral strike-slip fault with a normal displacement component since it formed.View full abstractDownload PDF (4212K) -
Hiromi Kondo, Kenji Shuto, Masayuki Fukase2000 Volume 106 Issue 6 Pages 426-441
Published: June 15, 2000
Released on J-STAGE: April 11, 2008
JOURNAL FREE ACCESSAndesitic rocks, of the Pliocene Myojin-iwa volcanic field, in the northern part of Niigata Prefecture, Northeast Japan, show transitional chemical characteristics between typical calc-alkaline and tholeiitic series. The most primitive basaltic andesite among these andesitic rocks (the Myojin-iwa Formation) has geochemical characteristics similar to those of other Pliocene-Quaternary basaltic rocks from the back-arc side of the NE Japan, and has initial Sr isotope ratio slightly higher than those of the latter basaltic rocks.Andesites from the Myojin-iwa area show SiO2 contents ranging from 53.3% to 62.8, and initial Sr and Nd isotope ratios vary from 0.70324 to 0.70378 and 0.512791 to 0.512926, respectively. The initial Sr isotope ratios display positive correlation with SiO2, and their initial Nd isotope ratios gradually decrease with increasing SiO2. This suggests that the genesis of andesites from the area can not be attributed to simple fractional crystallization of the primary basaltic magma, but to an assimilation and fractional crystallization (AFC) process. Based on petrographical evidence, major and trace element compositions and Sr-Nd isotope systematics, the Paleogene granitoid rocks constituting the upper crust beneath the Myojin-iwa and surrounding areas are the possible candidates for assimilants. An AFC model using the granitoid rocks as the assimilant can successfully reproduce the chemical variations of the andesites from the Myojin-iwa Formation by assuming the r values (ratio of the rate of assimilation to the rate of fractional crystallization) of less than 0.2.View full abstractDownload PDF (1629K) -
Hiroyuki Otsuka, Ryu Kuwayama2000 Volume 106 Issue 6 Pages 442-458
Published: June 15, 2000
Released on J-STAGE: April 11, 2008
JOURNAL FREE ACCESSA large fossil frog was discovered along with many fish and plant fossils from the Lower Pleistocene deposits of the Katanoyama Member of the Masuda Formation at Katanoyama, in the southwestern part of Nishino-omote City in Tanega-shima of Osumi Islands, South Kyushu, Japan. Comparative morphological study of the fossil frog was conducted with skeletal specimens of living frogs (73 individuals belonging to 17 species of 6 genera) from Kyushu and the Nansei Islands, in order to clarify the taxonomic position of the fossil frog. As a result of this study, the fossil frog from the Katanoyama Member was identified with Ishikawa's frog Rana ishikawae presently endemic to Okinawa and Amami Oshima Islands of the Nansei Islands, based on morphologic characters such as ilium and omosternum. It is estimated by age measurement of its pumice bed that the Katanoyama Member deposited at 1.3±0.2 F.T. Ma. It is geologically evident that there was a remarkable regression around the Nansei Islands from the end of sedimentation of the Shimajiri Group in the latest Pliocene until the beginning of the Ryukyu Group in the Early Pleistocene. These facts indicate that land animals including the ancestor of Ishikawa's frog could have migrated to Osumi Islands from Amami Oshima Island over a land bridge in the latest Pliocene.View full abstractDownload PDF (2825K) -
Ryuji Tada, Yuji Matsushita, Hassan Mohamed Baioumy2000 Volume 106 Issue 6 Pages XI-XII
Published: 2000
Released on J-STAGE: November 26, 2010
JOURNAL FREE ACCESS
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