著者

川村 喜一郎 池原 研 藤岡 換太郎

出版者

一般社団法人 日本地質学会

雑誌

地質学雑誌 (ISSN:00167630)

巻号頁・発行日

vol.113, no.5, pp.184-192, 2007 (Released:2007-12-28)

参考文献数

22

被引用文献数

1

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堆積システムの急激な変化が中部沖縄トラフの3 mと5 m長の2本のコアに記録されていた.構成される石英粒子の中央粒径は,7325年前の年代のK-Ah層で下位から上位に向けて急激に減少した.その中央粒径の減少は,約20 μmの粗粒粒子の除去による結果である.K-Ah層の年代で生じたこの変化は,1)海水準上昇で沖縄トラフと堆積物供給源である中国大陸とが離れたこと,2)トラフ内で8000~7000年前に黒潮の堆積物供給バリアが形成され,大陸からの堆積物のほとんどが強い海流により掃きとばされたことによって生じたと推測される.さらに,微細組織がその下位での水平配列から上位でのランダム配列へ変化する.その水平配列は,高堆積速度条件下で生じた無生物擾乱からの結果であると判断できる.このように,中部沖縄トラフでは,今日のような堆積システムが約7300年前に形成されて,現在までにそれが持続していると考えられる.

著者

藤岡 換太郎

出版者

Tokyo Geographical Society

雑誌

地学雑誌 (ISSN:0022135X)

巻号頁・発行日

vol.97, no.1, pp.39-50, 1988-02-25 (Released:2009-11-12)

参考文献数

59

被引用文献数

1 2

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Topographic features around Izu Oshima region were summarized with special references to the characteristics relating to subduction of the Philippine Sea plate (PHS) along the Sagami Trough. Several topographic highs having circular contours around Oshima Island may be the results of parasitic eruption of Oshima Volcano. Their trend is almost parallel to the elongation of the linearity of parasitic cones distributing onshore. Along the eastern trench slope off Oshima Island, large scale collapse structures are seen in the submarine topographic map. These are the result of the falling down mostly of pyroclastic materials (tephras) during historic ages. The Oshima Island shows eastward tilting owing to the bending resulted from the subduction of the PHS. Large scale of the collapse structures may be caused by the oversteepening of the eastern trench slope of Oshima Island and soft sediments consisting mostly of pyroclastics including a large quantity of pore water slide down along the trench slope as far as the Sagami Trough.Radial distribution both of saddles and valleys observed northeastern offshore part of the Oshima Island may be lava flows and/or dike swarms derived from the old Okata Volcano whose eruption center was estimated to exist at the northeastern extension of the Oshima Island.About 4 km south off the Oshima Island, large scale of the topographic breakes consisting of caotic highs and depressions are seen in the submarine topographic map. These may be debris flow deposits along eastern slope of Ohmurodashi which consists mostly of the Quaternary rhyolites geologically and chemically similar to those exposed on the Niijima and Kozushima Islands. These are the results of catastrophic phreatomagmatic eruption of the Ohmurodashi such as that of the Myojinsho. Mixture of pyroclastics and lavas will be found from these caotic areas.The upper steam of the Sagami trough shows notable meandering whose direction is almost parallel to that of the movement of the PHS east off Oshima Island and almost perpendicular east off Ohmurodashi. At the latter case, thrust structures may trace along the subduction zone at the landward slope of the Sagami trough. Just landward side of the thrust, curious round topographic highs can be observed. They may be mud volcanoes.Topographic features observed around Oshima Island may best be understood in term of the normal and oblique subduction of the Philippine Sea plate along the Sagami Trough.

著者

葉室 和親 荒牧 重雄 藤岡 換太郎 石井 輝秋 田中 武男 宇都 浩三

出版者

東京大学地震研究所

雑誌

東京大学地震研究所彙報 (ISSN:00408972)

巻号頁・発行日

vol.58, no.2, pp.527-557, 1983-10-22

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1979年12月の淡青丸KT-79-18航海の際,東伊豆沖海底火山群,大島南方の大室出シ堆,新島東方の新島ウラノ瀬などの海底火山の岩石をドレッジにより採集した.東伊豆沖海底火山群は,岩石の鉱物・化学的特徴により北半部,中部,南部に細分することができる.北半部の岩石はすべて高アルカリソレアイト系列の玄武岩・安山岩溶岩であり,伊豆半島中部東部に分布する東伊豆単成火山群がそのまま海底に延長して孤立した海底火山として分布しているものと考えて差支えない.中部では,低アルカリ(低Na2O)ソレアイト系列の玄武岩が5点のドレッジから発見された.そのうち伊豆大島に近い2点は,伊豆大島火山の玄武岩類に似た鉱物・化学組成をもち,新鮮である.西側の3点の岩石はいずれも風化変質作用を受けており,小角礫の集合として産する.これらは伊豆大島火山や東伊豆単成火山群よりも古い海底火山に属するものと判断される.南部のドレッジ2点からは東伊豆単成火山群南西部のグループの岩石に似た玄武岩が得られた。大室出シは,ガラス質多孔質の新鮮な流紋岩溶岩流から成る平坦な頂部をもつ海底火山と考えられる.山体の中央部に深さ100m,長さ1.5km,幅0.5kmの凹陥地(大室海穴)があるが,その壁からは流紋岩溶岩が採集された.新島ウラノ瀬の南東麓からは,流紋岩溶岩と変質した玄武岩礫,石英閃緑岩礫などが採集された.後者はこの地域を構成する基盤岩類と考えられる.

著者

葉室 和親 荒牧 重雄 加賀美 英雄 藤岡 換太郎

出版者

東京大学地震研究所

雑誌

東京大學地震研究所彙報 = Bulletin of the Earthquake Research Institute, University of Tokyo (ISSN:00408972)

巻号頁・発行日

vol.55, no.1, pp.259-297, 1980-08-25

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More than 40 topographic highs resembling conical submarine volcanoes in the area between the eastern coast of the Izu Peninsula and the Izu-Oshima island, are identified by the detailed submarine topographic maps recently published. 17 dredge hauls were recovered from 26 dredge stations occupied during cruise KT78-10 of S.S. Tansei Maru of the Ocean Research Institute, University of Tokyo. Most of the rock samples are fresh basalts from the top part of the isolated peaks indicating that many of these topographic highs are young submarine volcanoes. They are very likely to be the submarine counterparts of the subaerial Higashi-Izu Monogenetic Volcano Group distributed on land just west of the dredge area. Most of them are high-alkali tholeiitic basalts with phenocrysts of olivine and plagiocalse. Augite phenocrysts may be present and some specimens contain abundant quartz and plagioclase xenocrysts derived from felsic plutonic rocks, a feature very similar to that found of the Higashi-Izu Monogenetic Volcano Group. Many are nearly aphyric and high in Al2O3 (19-17%) which is in strong contrast with the low alumina, low alkali tholeiites of Izu-Oshima island. In the Harker variation diagrasm, the highalumina, high-alkali tholeiites, both on land and under the sea, have distinctly high Na2O as compared with the basalts of Izu-Oshima. There seems no compositional gradation between the two although they are contemporaneous and occurring in adjacent areas.

著者

葉室 和親 荒牧 重雄 藤岡 換太郎 石井 輝秋 田中 武男 宇都 浩三

出版者

東京大学地震研究所

雑誌

東京大学地震研究所彙報 (ISSN:00408972)

巻号頁・発行日

vol.58, no.2, pp.p527-557, 1983

被引用文献数

3

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1979年12月の淡青丸KT-79-18航海の際,東伊豆沖海底火山群,大島南方の大室出シ堆,新島東方の新島ウラノ瀬などの海底火山の岩石をドレッジにより採集した.東伊豆沖海底火山群は,岩石の鉱物・化学的特徴により北半部,中部,南部に細分することができる.北半部の岩石はすべて高アルカリソレアイト系列の玄武岩・安山岩溶岩であり,伊豆半島中部東部に分布する東伊豆単成火山群がそのまま海底に延長して孤立した海底火山として分布しているものと考えて差支えない.中部では,低アルカリ(低Na2O)ソレアイト系列の玄武岩が5点のドレッジから発見された.そのうち伊豆大島に近い2点は,伊豆大島火山の玄武岩類に似た鉱物・化学組成をもち,新鮮である.西側の3点の岩石はいずれも風化変質作用を受けており,小角礫の集合として産する.これらは伊豆大島火山や東伊豆単成火山群よりも古い海底火山に属するものと判断される.南部のドレッジ2点からは東伊豆単成火山群南西部のグループの岩石に似た玄武岩が得られた。大室出シは,ガラス質多孔質の新鮮な流紋岩溶岩流から成る平坦な頂部をもつ海底火山と考えられる.山体の中央部に深さ100m,長さ1.5km,幅0.5kmの凹陥地(大室海穴)があるが,その壁からは流紋岩溶岩が採集された.新島ウラノ瀬の南東麓からは,流紋岩溶岩と変質した玄武岩礫,石英閃緑岩礫などが採集された.後者はこの地域を構成する基盤岩類と考えられる.

著者

葉室 和親 荒牧 重雄 藤岡 換太郎 石井 輝秋 田中 武男 宇都 浩三

出版者

東京大学地震研究所

雑誌

東京大學地震研究所彙報 = Bulletin of the Earthquake Research Institute, University of Tokyo (ISSN:00408972)

巻号頁・発行日

vol.58, no.2, pp.527-557, 1983-10-22

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1979年12月の淡青丸KT-79-18航海の際,東伊豆沖海底火山群,大島南方の大室出シ堆,新島東方の新島ウラノ瀬などの海底火山の岩石をドレッジにより採集した.東伊豆沖海底火山群は,岩石の鉱物・化学的特徴により北半部,中部,南部に細分することができる.北半部の岩石はすべて高アルカリソレアイト系列の玄武岩・安山岩溶岩であり,伊豆半島中部東部に分布する東伊豆単成火山群がそのまま海底に延長して孤立した海底火山として分布しているものと考えて差支えない.中部では,低アルカリ(低Na2O)ソレアイト系列の玄武岩が5点のドレッジから発見された.そのうち伊豆大島に近い2点は,伊豆大島火山の玄武岩類に似た鉱物・化学組成をもち,新鮮である.西側の3点の岩石はいずれも風化変質作用を受けており,小角礫の集合として産する.これらは伊豆大島火山や東伊豆単成火山群よりも古い海底火山に属するものと判断される.南部のドレッジ2点からは東伊豆単成火山群南西部のグループの岩石に似た玄武岩が得られた。大室出シは,ガラス質多孔質の新鮮な流紋岩溶岩流から成る平坦な頂部をもつ海底火山と考えられる.山体の中央部に深さ100m,長さ1.5km,幅0.5kmの凹陥地(大室海穴)があるが,その壁からは流紋岩溶岩が採集された.新島ウラノ瀬の南東麓からは,流紋岩溶岩と変質した玄武岩礫,石英閃緑岩礫などが採集された.後者はこの地域を構成する基盤岩類と考えられる.

著者

岡村 聡 坂本 泉 金 容義 石塚 治 湯浅 真人 冨士原 敏也 藤岡 換太郎 倉本 能行 前田 仁一郎

出版者

一般社団法人日本鉱物科学会

雑誌

日本鉱物科学会年会講演要旨集

巻号頁・発行日

vol.2007, pp.7, 2007

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孀婦岩構造線は,伊豆・小笠原弧を北部と南部に二分する大構造線であり,北北東―南南西方向の走向を示し,その東側斜面に沿って比高最大1500mの急崖が発達し,地殻の深部断面を観察することができる.<Br> 孀婦岩構造線の南東に位置する沢海山は,鮮新世の活動年代を示す火山フロント帯火山であり,島弧玄武岩の化学組成を示す.<Br> 孀婦岩構造線沿いに観察される地殻断面(孀婦地塊)は,後期中新世を示す塊状の溶岩・貫入岩とハイアロクラスタイト及び,それらを供給するフィーダーダイク・溶岩が観察される.孀婦地塊を基盤とする背弧側には中新世~鮮新世に活動した小海丘群が存在する.孀婦地塊と小海丘群の火成岩類は,いずれも背弧海盆玄武岩の特徴を示す点で共通するが,後者はIndian Ocean MORBタイプアセノスフェアの寄与が大きかったことを示唆する.

著者

森 慎一 山下 浩之 有馬 眞 藤岡 換太郎

出版者

一般社団法人 日本鉱物科学会

雑誌

岩石鉱物科学 (ISSN:1345630X)

巻号頁・発行日

vol.41, no.3, pp.67-86, 2012 (Released:2012-06-28)

参考文献数

185

被引用文献数

2 3

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We determined K-Ar ages of the late Miocene to Pliocene andesitic to basaltic dykes and lavas distributed in the Tanzawa-Oiso area in the southern Fossa Magna region, central Japan. Our data indicate two distinct periods of volcanic activity in this region; one took place during the middle Miocene (about 15 Ma) and another during the late Miocene to Pliocene (7.5-4.2 Ma). Combined the present results with the age data reported previously from volcanic rocks in the southern Fossa Magna region, the present study suggests that the volcanic front in this region during 8 to 4 Ma was located at about 40 km east of the present-day volcanic front. The volcanic front began to migrate westward to the present-day location at about 4 Ma. The westward migration of volcanic front was probably associated with an abrupt change of lateral motion of the Philippine Sea Plate from north- northwestward to northwestward occurred between 4 and 2 Ma.

著者

丸山 茂徳 寺林 優 藤岡 換太郎

出版者

公益社団法人 東京地学協会

雑誌

地学雑誌 (ISSN:0022135X)

巻号頁・発行日

vol.98, no.3, pp.319-349, 1989-06-25 (Released:2011-02-17)

参考文献数

143

被引用文献数

2 4

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A brief review of the study on ophiolite is given. 165 years have passed already since a first use of the term “ophiolite” by BRONGNIART (1813), but still have not yet obtained a broadly satisfying solution on its origin and emplacement. However, the rapidly increased data set during the last 15 years on both on-land ophiolite and oceanfloors clearly indicate the strong constraints on its origin and emplacement.The period during 1813-1927 was a time of description of ophiolite. BRONGNIART (1827) classified ophiolite into a group of igneous rocks, since then began a debate whether ophiolitic peridotite is igneous or the other in origin. SUESS (1909) had noticed that ophiolites appear characteristically in orogenic belts. It was STEINMANN (1927) who had first recognized a close association of peridotite, gabbro, diabase-spilite, and radiolarian chert suggesting a deep sea origin of ophiolite. The significance of his finding has never been looked back until the revolutional period of plate tectonics in the late' 60s.The second period of 1927-1949 was the time of debate on igeneous origin. BOWEN and his coworkers insisted igneous origin based on experimental petrology for the ultramafic rocks in general. But if so, an abnormally high temperature ca. 1, 900°C was necessary to explain the occurrence of dunite. BENSON (1926) pointed out that if BOWEN'S idea is true, the country rocks of ophiolite must be subjected a high-temperature contact metamorphism, but not in the field. HESS (1939) has given a new idea of serpentinite magma to solve the problem, but its possibility had completely been disproved by the experiment of MgO-SiO2-H2O by BOWEN and TUTTLE (1949).The third period (1949-1959) began by a break-through idea of DE ROEVER (1957), who speculated that ophiolitic peridotite is a piece of mantle material, which was brought into an orogen by a tectonic process.The fourth period (1959-1973) started by BRUNN (1959) who compared ophiolite with the rocks in the Mid-Atlantic Ridge. This period (1959-1973) was the time of plate tectonics. During the early' 60s the ocean-floor spreading theory was proposed by HESS and DIETZ, and both thought that the layer 3 is composed of serpentinite oreclogite. The year 1969 was a memorial year, when both MOORES and DAVIES distinguished cumulate peridotite from the underlying residue tectonite, the latter of which is a refractory mantle after the formation of oceanic crust by partial fusion of mantle peridotite. The best example of ophiolite was the Troodos massif in Cyprus, where the extensive-scale of parallel dike swarm develops indicating ocean-floor spreading. Thereafter an ophiolite boom has come out, and flood of papers appeared to regard ophiolite to be of mid-oceanic ridge in origin. However, several geologists have doubted mid-oceanic ridge origin by the facts of much thinner crust, more silicic volcanic composition, and frequent occurrence of phenocrystic augite in ophiolites. MIYASHIRO (1973) solved such problems, and concluded that Troodos was formed in an island-arc setting. This paper was very shocking for geologists who wanted to establish the basic framework of orogeny by plate tectonics in those days, but epoch-making on the study of ophiolite, and corresponding to the time, when the method of study has changed to be modernized and more interdisciplinary.

著者

森 慎一 藤岡 換太郎 有馬 眞

出版者

公益社団法人 東京地学協会

雑誌

地学雑誌 (ISSN:0022135X)

巻号頁・発行日

vol.119, no.4, pp.585-614, 2010-08-25 (Released:2010-11-08)

参考文献数

89

被引用文献数

2 6

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This paper describes topographic and geologic features of the northern part of the Sagami Trough and surrounding areas, off central Japan, and discusses the role of the Philippine Sea plate in the development of the trough's topography and fault systems. In the study area, the Okinoyama Bank Chain, Miura Basin Chain, and several spurs are aligned NW-SE parallel to the axis of the Sagami Trough. We examine the lithological compositions of rock and piston-core samples collected from the R/V Hyper Dolphin during Japan Agency for Marine-Earth Science and Technology dives 906 and 907, and rock samples dredged during Ocean Research Institute of the University of Tokyo cruise KT88. These samples represent mainly volcaniclastic rocks derived from the volcanoes of the Izu arc and subordinate amounts of terrigenous sediment interpreted as derived from the Sagami and Sakawa Rivers. The topographic and geologic data define five fault systems in the eastern part of the northern Sagami Trough. We interpret the Okinoyama Bank Chain as an elevated outer-ridge belt developed at the landward slope of the plate boundary, which is represented by NW-SE trending right-lateral reverse faults (F1). At the northeast side of the Okinoyama Bank Chain, NW-SE trending normal faults (F2) formed and the Miura Basin Chain developed. The cutting relationship between these fault sets suggests that the collision of the Izu arc with the Honshu arc imposed a right-lateral shear stress field on the Okinoyama Bank and Miura Basin Chains. This shear stress may have led to clockwise rotation of these tectonic blocks formation of a set of NE-SW trending left-lateral reverse faults (F3). The initiation of the Izu block collision is interpreted as a turning point after which the northward motion of the Philippine Sea plate abruptly changed to NW and the Sagami Bay area came under a NW-SE compressional stress field, which resulted in the development of E-W trending right-lateral faults (F4) in the narrow shelf off the Miura Peninsula. A number of spurs subsequently developed along these faults. Finally, NNE-SSW trending right-lateral normal faults (F5) developed under an E-W extensional stress field, which caused the formation of N-S trending topographic depressions such as Tokyo Canyon and submarine highs such as Okinoyama and Oiso Spur.