地学雑誌
Online ISSN : 1884-0884
Print ISSN : 0022-135X
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120 巻 , 3 号
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  • 120 巻 (2011) 3 号 p. Cover03_1-Cover03_3
    公開日: 2011/09/06
    ジャーナル フリー
     (表面)James Hutton(1726-1797)は近代地質学の確立に貢献した著名な英国人地質学者である.とりわけ,彼を有名にしたのはエジンバラの東方約100kmの英国東岸に露出する不整合の発見である(表紙).表紙の写真は問題の不整合の延長を北から南側へ鳥瞰したものである.写真を撮影した地点はSiccar Point(最も代表的な不整合の露頭所在地で,本誌でも村田(1987)や白尾(2003)によって紹介されている)で,そこから南方への眺望である.遠景にみえる岬,手前右側の丘から潮間帯に至る海岸の表層に不整合面が続く(図1).不整合面の上の地層であるオルドビス紀の赤色砂岩と泥岩の互層は陸側に緩く(約20°)傾斜しているが,下位のシルル系の砂岩・泥岩の互層はほぼ垂直に起立している.不整合面の直上は凹凸がかなり激しく,そこに基底礫岩層が顕著に発達する.下位の地層が堆積したシルル紀の時代に浅海底で水平に堆積していた地層がオルドビス紀までにほぼ垂直に起立する大変動が存在したことを示している.この時期はスコットランド(ローレンシア大陸:北米大陸)が英国(ゴンドワナ大陸:西アフリカ)に衝突合体してアパラチア山脈とその東方延長(アイルランド+連合王国)の造山帯が誕生した古生代半ばの衝突型造山運動の時代に対応する(丸山ほか,2011).
     (裏面)表紙を撮影した丘地点の背面にある斜面(Siccar Point)で観察される基底礫岩層と上位の赤色砂岩・泥岩互層の境界.基底礫岩層の内部には上位の有律互層の層理面に大きく斜交する葉理が発達し,礫岩,砂岩,少量の泥岩が発達する.ハットンの不整合面は,この礫岩層のさらに下位に位置する.
    (写真・解説:笠原順三 2011年3月27日撮影)
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総説
  • 小畑 正明, 上田 匡将
    120 巻 (2011) 3 号 p. 439-451
    公開日: 2011/09/06
    ジャーナル フリー
     This paper reviews the occurrence of pseudotachylyte having a mylonitic texture of a ultramafic composition, originally reported by Ueda et al. (2008a), as a new type of pseudotachylyte—mylonitic pseudotachylyte—and discusses its seismogenic significance in the context of deep to intermediate-depth earthquakes in the mantle. Referring to widely accepted definitions of pseudotachylyte and mylonite, it is emphasized that mylonitic fault-vein pseudotachylytes represent frozen melts generated by frictional melting of host rocks during earthquakes. It is also emphasized that the mylonitic pseudotachylyte is a complex, integrated product of a whole sequence of seismogenic processes including pre-seismic (ductile) shear localization and recrystallization, followed by a (brittle) seismic rupture and comminution and frictional heating-and-melting, and further, by rapid cooling and crystallization, with or without co-seismic or post-seismic shear deformation. Although unraveling and accurate analyses of the whole processes is a difficult task because of the extremely fine-grained nature of the pseudotachylytes and the recrystallization overprints therein, careful petrographic analyses of such rocks should provide useful insights into the seismogenic processes bridging the gap between conventional seismological, geological and petrological methods. A need for a multidisciplinary approach is emphasized for a better understanding of seismogenic processes in the mantle.
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  • 奥平 敬元, 隅田 祥光
    120 巻 (2011) 3 号 p. 452-465
    公開日: 2011/09/06
    ジャーナル フリー
     In the Ryoke belt formed at the eastern margin of East Asia during mid-Creataceous time, the most important geological record is that of felsic igneous activity, which may have been caused by the subduction of a young (hot) oceanic lithosphere. The mantle-derived arc magmas formed due to subduction of the hot lithosphere underplated the lower crust. Continuous underplating of the mantle-derived magmas resulted in partial melting of the lower crustal mafic to intermediate rocks to produce granite magmas. The granite magmas rose and were emplaced at a mid-crustal depth. They caused thermal perturbation around rocks of the Jurassic accretionary complex (Mino–Tamba belt) and formed the resultant low-pressure/high-temperature regional contact metamorphism (Ryoke metamorphism). At the time of Ryoke metamorphism, during subduction of a hot oceanic lithosphere with high obliquity, a low-angle detachment fault formed at the forearc region of the mid-Cretaceous SW Japan arc. Following this event, subduction of an inactive oceanic ridge or a very hot oceanic lithosphere with low obliquity occurred; a compressional stress regime occurred at the forearc region, forming granite mylonites and large-scale upright folds associated with crustal displacement along the detachment fault. After this event, the angle of subduction decreased gradually; the location of the felsic igneous activity moved toward the continent.
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論説
  • 鈴木 富之
    120 巻 (2011) 3 号 p. 466-485
    公開日: 2011/09/06
    ジャーナル フリー
     Many business travelers often stay in large cities because of the concentration of various urban functions. The number of foreign tourists visiting Japan has increased in recent years. The main destinations are metropolitan areas such as Tokyo and Osaka. Given this situation, it is necessary to develop cheaper accommodations for foreign and business travellers who want to stay in those areas for long periods.
     The purpose of this study is to examine the characteristics and factors behind the evolution of accommodations in the Sanya district, which is located in an “inner-city” in Tokyo.
     Cheap lodging houses in Sanya have been in decline since the collapse of Japan's asset inflation-led economic bubble in the early 1990s. Managers of accommodations started to accept foreign and business guests. These accommodations have attempted to differentiate themselves from hotels located in central Tokyo in terms of service and lower tariffs.
     The following three reasons, which this study clarifies, help explain why accommodations in the Sanya district have evolved. First, Sanya was traditionally developed as a “Yoseba”, a place in which day laborers lived. In addition, Sanya is located close to central Tokyo. Secondly, due to a decreasing number of day laborers, cheap lodging houses had empty rooms. Therefore, their managers started to accept foreign tourists and business travellers. In addition, various mass media started to report favorably on Sanya during the World Cup Football games in 2002, although the Sanya district previously had a negative image. Lastly, many foreign tourists began to stay in the area after the World Cup.
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  • 野上 道男
    120 巻 (2011) 3 号 p. 486-501
    公開日: 2011/09/06
    ジャーナル フリー
     The purpose of this paper is to describe the scheme of a landform development simulation, to explain the fundamental equations used for the main geomorphic processes in slope, river, and shoreline, and to present some results from an example of a run under virtual conditions. It is not possible to assess the results against real landform changes over geologic time. But, the simulated landforms at 110 Kyr after Present take on credibility as illustrated by the example in this paper. Geomorphologically irrational or extraordinary forms do not appear in the simulation. The author argues that the scheme of the simulation and the algorithms used for the geomorphic processes are appropriate.
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  • 松岡 憲知, 池田 敦
    120 巻 (2011) 3 号 p. 502-535
    公開日: 2011/09/06
    ジャーナル フリー
     Spacio-temporal variability of contemporary periglacial environments in the Swiss Alps is summarized on the basis of 15 years of field observations of rock weathering, permafrost creep, and soil movements, as well as other recent studies. Diurnal and annual freeze-thaw cycles loosen wet rock joints, which subsequently produce rockfalls. A large episodic rockfall can temporarily raise the rate of rockwall retreat. Rock debris derived from different parent rocks controls the types of rock glacier having different compositions, thermal characteristics, and dynamics. Some rock glaciers at the lower limit of permafrost are accelerating due to intensified mobility, but they may eventually become inactive because of permafrost thawing and the lack of debris supply. On slopes mantled with fine debris, small-scale stripes and lobes tend to develop on the upper part due to thin debris and good drainage, whereas larger scale lobes increase downslope as a result of thicker debris, poor drainage, and gentler slopes. The former mainly responds to shallow diurnal freeze-thaw cycles, whereas the latter reflects frost heave and gelifluction during deeper annual freezing-thawing. A prolonged supply of meltwater further triggers rapid mudflows superimposed on slow solifluction. Climate warming may decrease periglacial activity in seasonal frost areas, whereas in marginal permafrost areas it promotes permafrost warming that temporarily accelerates permafrost creep and/or permafrost thawing that possibly triggers large rockfalls and debris flows.
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短報
  • 早川 由紀夫, 藤根 久, 伊藤 茂, Lomtatize ZAUR, 尾嵜 大真, 小林 紘一, 中村 賢太郎, 黒沼 保子, 宮島 宏, ...
    120 巻 (2011) 3 号 p. 536-546
    公開日: 2011/09/06
    ジャーナル フリー
     Two wood trunks, one charred and 75 years old and the other not charred and more than 199 years old, were collected from Hayakawa ignimbrite of Niigata Yakeyama Volcano. They were investigated using the radiocarbon wiggle-matching method to determine the age of the eruption. The result was 1225-1244 cal AD (95.4%), which is over 200 years younger than previous estimates. The eruption, including the Hayakawa ignimbrite, was the largest during the volcano's life period of 3000 years. Co-ignimbrite fallout KGc ash has been found at many archaeological sites spreading on the eastern flanks of Myoko Volcano and the Takada Plain. The age obtained here will provide a useful time constraint for archaeologists and volcanologists studying this area.
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平成21年度助成金・援助金使用報告
  • 近藤 玲介
    120 巻 (2011) 3 号 p. 547-556
    公開日: 2011/09/06
    ジャーナル フリー
     In northern Hokkaido, only a few absolute ages have been obtained from periglacial phenomena, which characterize the landscape of the area. The present lack of age constraints has prevented studies on precise geomorphic development and terrestrial palaeoenvironmental reconstruction in this area during the last glacial period.
     The aim of this study is to apply optically stimulated luminescence (OSL) dating and AMS14C dating methods to establish the chronology of fossil frost cracks, periglacial slope deposits, and involutions from a region in northern Hokkaido including Rishiri Island. The samples for OSL dating were taken from five localities.
     To compare OSL and AMS14C aging, a loess sample for OSL dating and a charcoal sample for AMS14C were collected at Loc. 3 from the same homogeneous silt layer. Besides, at Loc. 4, an OSL and a charcoal sample were collected from a channel deposit. These charcoal samples were subjected to AMS14C dating as independent age controls. OSL ages and calibrated 14C ages from the charcoal were in close agreement in each locality. These results suggest that OSL ages of loess and channel deposit in this study are reliable.
     From Horonobe (Loc. 1) in the southern Soya hills and from Rishiri Island (Loc. 2), two OSL ages from periglacial slope deposits indicate 21 and 22 ka. Their OSL ages, showing involution from east of Wakkanai (Loc. 3), suggest that the sand was deposited between 54 and 42 ka and that the involution was formed afterwards before 13 ka. At Loc. 4, the fossil frost crack southeast of Cape Soya was formed between 120 and 13 ka, and at Loc. 5, the fossil frost crack was formed between 24 and 15 ka. These results indicate that many periglacial phenomena were possibly formed at about 20 ka in northern Hokkaido.
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