著者

吉崎 もと子 渋谷 岳造 鈴木 勝彦 清水 健二 中村 謙太郎 大森 聡一 高井 研 丸山 茂徳

出版者

一般社団法人日本地球化学会

雑誌

日本地球化学会年会要旨集 2011年度日本地球化学会第58回年会講演要旨集

巻号頁・発行日

pp.58, 2011 (Released:2011-09-01)

---

様々な分野の研究から、初期地球の熱水系にはメタン生成菌のような水素をエネルギー源とする化学合成独立栄養細菌を一次生産者に持つ生態系が繁栄しており、水素は生命誕生・進化の重要な鍵であったと考えられる。本研究では、初期地球熱水系では当時の表層にあったとされるAl枯渇型コマチアイトが水素発生を担っていたと考え、熱水実験を通して初期地球のコマチアイト熱水系における水素発生量とその反応過程を検証した。その結果、コマチアイトの熱水変質による顕著な水素発生量を確認した。また、実験生成物の分析を行った結果、かんらん石では従来提唱されてきたように蛇紋石化反応に伴う磁鉄鉱生成によって水素が発生しているのに対し、コマチアイトでは別の水素発生プロセスが進行し、それは、粘土鉱物への?V価鉄の分配によるH2Oの還元であると推定した。コマチアイトの熱水変質による水素発生量は、現生のかんらん岩熱水系で観測される水素濃度に匹敵し、現世のかんらん岩熱水系と同様に、初期地球熱水系の生態系に豊富な量の水素を供給していたことが示唆される。

著者

沢田 輝 磯﨑 行雄 丸山 茂徳

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.127, no.5, pp.705-721, 2018-10-25 (Released:2018-11-14)

参考文献数

116

被引用文献数

2 3

---

The extensive occurrence of a felsic continental crust is one of the unique features of the Earth. The growth history of the continental crust has been a key issue in understanding the origin and evolution of the Earth. In particular, recent geological studies indicate that subduction of the continental crust into the mantle has been greater than previously imagined. The current understanding of the growth of continents and the differentiation of the crust and the mantle of the Earth is reviewed based on a detrital zircon geochronology. One of the most important achievements arises from the analysis of the age structure of individual continents and secular changes over time. The new detrital zircon geochronology suggests that the sizes of the continents have changed over time, which has been an important factor in the growth of the continents. Large continents, such as the modern examples, can preserve older crusts in their interiors, which are separated from active continental margins. Conversely, in the early Earth, continents were probably formed by the amalgamation of small fragments of crust, such as oceanic island arcs. It is speculated that the smallness of the continents was the most significant cause of the poor preservation of Hadean and Archean crusts, despite putative expected active crustal production. Consequently, the recycling of the continental crust occurred in great magnitudes during the early Earth's history. The large-scale subduction of felsic crust represents one of the most important aspects in studies of the early Earth.

著者

笠原 順三 阿部 なつ江 荒井 章司 下司 信夫 熊谷 英憲 丸山 茂徳 小原 一成 小原 泰彦

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.117, no.1, pp.1-4, 2008-02-25 (Released:2010-02-10)

参考文献数

17

著者

稲崎 富士 太田 陽子 丸山 茂徳

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.123, no.4, pp.401-433, 2014-08-25 (Released:2014-09-01)

参考文献数

52

被引用文献数

4 9

---

River improvement works in the Kanto Plain have long history of over 400 years. Rice-paddy development in the Kanto Plain was significantly delayed compared to that in other plains including the Osaka Plain and the Nobi Plain. This was because of the difficulty in constructing an irrigation and drainage network, and protecting rice paddies from inundation. Therefore, the Tokugawa government started river improvement works in the Kanto Plain just after Tokugawa settled in Edo in 1590, and the works continue now. The flow of the Tone River into Tokyo Bay was originally blocked by uplands, but was finally rerouted to Choshi, 90 km east to the original mouth. The Ara River was also rerouted to the south. The purpose of rerouting was not only to reduce the risk of flooding in Edo city, but also to develop a waterway network for rice paddies to expand in the central Plain. Moreover, rerouting and connecting the Tone River with the Edo River enhanced the inland waterway transportation network of the Kanto Plain. Accordingly, a number of riverside towns, or Kashi, grew as nodes of the network. Small sailing ships and flatboats were the major conveyors of products. During the Meiji era, which followed the Edo period, canals were constructed and steamboats were introduced to replace sailing ships, at a time when water transportation was peaking. The inland waterway soon began to be replaced by present-day economic transportation systems such as rail and road. The central Kanto Plain was featured consistently by a subsiding basin through the Quaternary. Although the entire Plain was uplifted, Tokyo Bay, at the center of the Kanto Plain, sank over 1000 m. In contrast, the outer margin of the plain was uplifted 50 to 1000 m. Choshi, at the mouth of the present Tone River, is in the uplifted area. In contrast, the lower reaches of the River are at the northern extent of the subsiding basin. This is the reason why back swamp lakes or an estuary such as Kasumiga-ura and the ancient Katori-no-umi were formed in the area. Such crustal movements in the Kanto Plain continued throughout the Quaternary. Hydration and dehydration of two plates lying beneath this region were the driving force. Hydration of mantle peridotite underneath the Kanto Plain due to dehydration of the underlying Philippine Sea Plate (PHS plate), subducting from south to north 30-60 km deep (2 cm/year), causes volumetric expansion particularly at the marginal zone of the overlying plate (North American (NA) Plate). Serpentinized peridotite, produced above the Pacific Plate (PAC plate), which subducts from east to west under the PHS and NA plates, expands like popcorn and results in uplifting of the Boso Peninsular (non-volcanic outer arc) . In contrast, the sinking of the Tokyo Bay area is explained by the overlapping of the fore arc basin towards the PHS and PAC plates. From the viewpoint of tectonics, the sinking belt including Tokyo Bay is in a physical field where a sedimentary basin formed under tensile stress in the NE-SW direction. Small mantle convection caused by serpentinization of the uppermost mantle beneath the fore arc is the key to understanding the tectonic setting of the Kanto Plain.

著者

磯崎 行雄 丸山 茂徳 中間 隆晃 山本 伸次 柳井 修一

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.120, no.1, pp.65-99, 2011-02-25 (Released:2011-05-20)

参考文献数

80

被引用文献数

32 48

---

The ca. 700 million year-long geotectonic history of the Japanese Islands comprises three distinct intervals; i.e., (1) the age of a passive continental margin off the South China continental margin (ca. 700-520 Ma), (2) the age of an active margin characterized by an arc-trench system (ca. 520-20 Ma), and (3) the age of an island arc off East Asia (20 Ma to the present). These three intervals are chronologically separated by two major boundaries with significant tectonic episodes; i.e., the ca. 520 Ma tectonic inversion from a passive to an active margin by the initiation of subduction from the Pacific side, and the ca. 20 Ma tectonic isolation of the modern island arc system from the Asian margin by the back-arc basin (Japan Sea) opening. Here, the evolutionary history of the Japanese Islands is revised significantly on the basis of new lines of information that derived from a new dating technique of detrital zircon in sandstone. Particularly noteworthy is the recognition of the Early Paleozoic to Middle Mesozoic arc batholiths that were exposed extensively in the past but not at all at present because the pre-Cretaceous granites merely occur as kilometer-size blocks in the modern Japanese Islands. As to these older granites, the remarkable disagreement between the current distribution and the predominance of their clastic grains in younger sandstones suggests the effectiveness of past tectonic erosion processes in the fore-arc domains. The newly documented historical change in sandstone provenance suggests that proto-Japan has experienced not only accretionary growth but also large-scale tectonic erosion in multiple stages. During the ca. 500 million-year history of the Japanese Islands, a large amount of juvenile arc (continental) crust was formed several times, however, most has already disappeared from the Earth's surface. In short, the orogenic growth of Japan, even in a long-lasting active continental margin setting, is explained as the intermittent repetition of ocean-ward continental growth and continent-ward contraction of an active arc-trench system. In contrast to these arc batholiths, the terrigenous flux from the neighboring two major continental blocks (South and North China) was less significant than previously imagined, except for the Jurassic to Early Cretaceous time when the collisional suture between North and South China blocks was selectively eroded to produce abundant terrigenous clastics. It is also significant that the eastern extension of this collisional suture was recognized in Japan as a chain of fragmentary remnants of the Triassic medium-pressure metamorphic belt. On the basis of these new lines of information, the South China-related origin of the main part of Japan is confirmed, whereas the Hida and Oki belts along the Japan Sea are identified as detached fragments of North China block. Summarizing all of these results, a series of revised paleogeographic maps of Japan from the Late Neoproterozoic to the Miocene is illustrated.

著者

磯崎 行雄 丸山 茂徳

出版者

Tokyo Geographical Society

雑誌

地学雑誌 (ISSN:0022135X)

巻号頁・発行日

vol.100, no.5, pp.697-761, 1991-10-25 (Released:2010-10-13)

参考文献数

195

被引用文献数

186 207

---

The Japanese Islands consist fundamentally of late Paleozoic to Cenozoic accretionary complexes that formed in situ in a subduction zone along the East Asian continental margin, i.e. 2.0 Ga Yangtze craton (South China) and 450 Ma fore-arc ophiolite. Recent research utilizing microfossil and radiometric dating has distinguished several major accretionary complexes, including high-P/T metamorphosed parts, and subordinate ophiolites. In particular, recognition of oceanic plate stratigraphy and age of subduction-related metamorphism for individual accretionary complex allows the geotectonic subdivision of the Japanese Islands be emended with a new definition of geotectonic units and their mutual boundaries. Removing the effect of arcrelated magmatism and secondary tectonic modification by microplate activities such as backarc basin opening, fore-arc sliver movement, and arc-collision, a remarkable oceanward younging polarity is recognized among the accretionary complexes. This polarity in growth is well observed in Southwest Japan where seven distinct units occur, i.e. from the Japan Sea side to the Pacific side: 400-300 Ma high-P/T schists, Permian (250 Ma) accretionary complex, 230-180 Ma high-P/T schists, Jurassic (180-140 Ma) accretionary complex, 100 Ma high-P/T schists, Late Cretaceous (80 Ma) accretionary complex, and Tertiary (50-20 Ma) accretionary complex. The sinuous surface trajectories of these geotectonic boundaries and occurrence of several tectonic outliers and windows indicate that all these complexes, including high-P/T schists, occur as subhorizontal (or gently northward dipping) thin tectonic unit, i. e. nappe. Thus the Japanese Islands form a huge pile of nappes that become younger structurally downward to the modern Nankai accretionary complex. What is remarkable in this subhorizontal orogen is that high-P/T units are tectonically intercalated between low-P units, e. g. the thin nappe of 100 Ma Sanbagawa blueschists between Jurassic and Late Cretaceous accretionary complexes of the prehnite-pumpellyite facies. Uplift of the Sanbagawa high-P/T unit appears to correlate with the arrival of the Kula/Pacific spreading ridge at the trench, suggesting that this high-P/T accretionary complex may have been extruded and uplifted into low-P domain in fore-arc by buoyant subduction of the spreading ridge at the trench. Evidence of ridge subduction at that time is supported by reconstructed paleoplate motion and the coeval climax of arc-related Ry-oke magmatism associated with low-P/T regional metamorphism. Formation of older high-P/T blueschist nappes sandwiched between low-P units can be explained likewise. Subduction of major spreading ridges seems most critical for the episodic oceanward development not only of subhorizontal high-P/T nappes but also of continent side granitic belts.

著者

丸山 茂徳 小宮 剛

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.120, no.5, pp.869-876, 2011-10-25 (Released:2012-01-17)

参考文献数

13

被引用文献数

4 12

---

グリーンランド・イスア地域は緑色片岩–角閃岩相程度の広域変成作用を受けてはいるが,見事な枕状溶岩の構造を普遍的に残している。枕の形,急冷縁,マトリックスの構造などが明瞭に残存している。枕状溶岩の存在は38億年前に海洋がすでに存在していたことを示すが,さらに付随する岩石との関係から,典型的な付加体の存在が認められる。海洋プレート層序は付加体が現在の西太平洋地域のような海洋内島弧の一部として形成されたことを示す。付加体の形成は北から南側にプレートが沈み込んだことを示す。さらにCA系列の花崗岩帯が付加体を貫く。これらは太平洋型造山運動が38-37億年前に機能していた証拠である。玄武岩の化学組成から,起源マントルの組成と温度が推定される。海洋の温度を100℃以下とみなすと,38-37億年前の海洋プレートの剛性度とプレートの厚さが求められ,プレートテクトニクスが38億年前にすでに機能していたことがわかる。

著者

磯崎 行雄 丸山 茂徳 青木 一勝 中間 隆晃 宮下 敦 大藤 茂

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.6, pp.999-1053, 2010-12-25 (Released:2011-03-17)

参考文献数

145

被引用文献数

26 74

---

The geotectonic subdivision and relevant definitions of geotectonic units in the Japanese Islands are revised on the basis of new data, particularly with detrital zircon dating of U-Pb ages and seismic profiling of the deep arc crust across the islands. In addition to the final confirmation of the subhorizontal structures of the Paleozoic to Cenozoic accretionary complexes and their high-P/T metamorphosed equivalents, several new aspects were recognized; i.e., detection of the eastern extension of the collisional suture between the Sino-Korean and Yangtze cratons in the Higo belt with medium-pressure-type metamorphism in SW Japan, and separation of the traditional Sanbagawa belt into two distinct metamorphic belts characterized by mutually different ages of protolith AC-formation and peak metamorphism. The occurrence and consumption of 4 Paleozoic to Mesozoic granite batholiths, as major provenances for the ancient Japanese Islands, are documented by detrital zircon dating of Paleozoic–Mesozoic sandstones. With respect to these new findings, the definitions of unit boundaries were thoroughly revised in terms of chronological spectrum in “ocean plate stratigraphy–metamorphism”. The geological significance of 5 major tectonic lines (faults) of the Pacific-type (or Miyashiro-type) orogen in Japan, i.e., the Nagato–Hida marginal TL, Osayama–Omi TL, Ishigaki–Kuga TL, Paleo–Median TL, and Butsuzo TL, is discussed. The current revision of the geotectonic subdivision and definitions of component units and their mutual boundaries leads to the following conclusions, which challenge the conventional understanding of the orogenic history of the Japanese Islands. (1) Proto-Japan in the Early Paleozoic was located closer to the South China (Yangtze) craton rather than the North China (Sino–Korean) craton. (2) Ever since 520 Ma, subduction of past Pacific ocean floors formed mature arc-trench systems with a full set of granite batholith, fore-arc basin, accretionary complex, and high-P/T metamorphosed equivalents at least 5 times; however, the former 4 sets were almost completely destroyed, with the exception of smaller tectonic blocks that currently occur within serpentinite mélange. (3) Tectonic erosion played a significant role in consuming ancient fore-arc crusts including 4 granite batholiths of the Paleozoic to mid-Mesozoic. (4) Serpentine mélange represents the former Wadati–Benioff plane along which tectonic erosion took place. (5) The Japanese Islands, which basically developed along the Yangtze continental margin, have experienced multiple episodes of oceanward growth and continentward retreat due to alternating subduction-accretion and tectonic erosion. (6) Net production of juvenile crust occurred on a large scale along the Japan margin during the 500 million year-long oceanic subduction regime since the Cambrian; however, intensive tectonic erosion effectively erased the older crusts from the surface and enriched the underlying sub-arc mantle with heat-generating continental material.

著者

澤木 佑介 佐藤 友彦 藤崎 渉 上田 修裕 浅沼 尚 丸山 茂徳

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.128, no.4, pp.549-569, 2019-08-25 (Released:2019-09-20)

参考文献数

61

被引用文献数

4 4

---

The evolution of eukaryotes is one of the most important issues in the history of life. Paleontological studies discovered the oldest eukaryotic fossil from the Paleoproterozoic Francevillian Group in Gabon. To clarify specific features of ancient sedimentary basins in Gabon, geologic evidence for the Francevillian Group is summarized and a new geotectonic model is proposed. The model gives much weight to the upwelling of mantle plume, which can explain why the Francevillian basin only hosted natural reactors. The Great Oxidation Event in the Paleoproterozoic played an important role in not only the evolution of eukaryotes but also in the formation of natural reactors. Reductive weathering of the continental crust, which was affected by plume-related volcanisms, transported Uranium-rich minerals into sediments of the Francevillian Group without dissolution of Uranium. The subsequent oxidation event enabled uranyl ions to accumulate within an oxic failed rift basin, and settled large quantities of organic matter on the seafloor. Hydrothermal circulation within the Francevillian Group precipitated highly Uranium-rich ores, which became natural reactors at approximately 2.0 Ga, and might have influenced the evolution of eukaryotes in this basin. In a large sense, the degree of oxidation of the ocean-atmosphere system has been linked to the amount of sedimentary rocks. In that way, mantle overturn in 2.7 Ga, which created a large continental crust, was one of the crucial events that affected the evolution of eukaryotes.

著者

武山 尚生 高橋 佑歌 永田 祥平 澤木 佑介 佐藤 友彦 丸山 茂徳 金井 昭夫

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.129, no.6, pp.899-912, 2020-12-25 (Released:2021-01-18)

参考文献数

56

被引用文献数

2

---

The origin of eukaryotic organisms is one of the most important questions in biology. So far, it has been suggested that eukaryotes are phylogenetically related to Archaea. Indeed, recent progress in archaeal genomic biology seems to have accurately determined the exact position of Archaea in the birth of the Eukaryota. In particular, identifying groups of archaeal species, such as the superphylum TACK and the Asgard archaea, has shown that primitive genes for eukaryotic signature proteins (ESP) already existed in the genomes of these archaeal species. Some ESPs are especially important, including actin and tubulin in the cytoskeleton and the ESCRT complex, which is involved in nuclear membrane formation. There have been many reports that eukaryotic intracellular organelles, such as mitochondria and chloroplasts, evolved from specific symbiotic bacteria. Moreover, eukaryotic genes are disrupted by intronic sequences, which must be removed or “spliced” and the exons connected after the primary transcript is generated, to make a mature functional mRNA. Recently, it has been suggested that the self-splicing factor in both bacterial and archaeal genomes, called “group II intron”, may cause gene disruption. In this review, the frontiers of genome biology are summarized in terms of the importance of prokaryotes (both Archaea and Bacteria) for the origin of Eukarya. From an Earth history perspective, how the increase in atmospheric oxygen concentration at 2.4-2.0 billion years ago may have contributed to the rise of the eukaryotes is discussed.

著者

鈴木 和恵 丸山 茂徳 山本 伸次 大森 聡一

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.6, pp.1173-1196, 2010-12-25 (Released:2011-03-17)

参考文献数

48

被引用文献数

20 35

---

The Japanese Islands have long been considered to be the most evolved of all the island arcs in the oceans. A simple scenario has been implicitly accepted for the growth of the Japanese Islands: since subduction started sometime around 520 Ma, the TTG crust has increased over time in association with the steady-state growth of the accretionary prism in front. Here, we show very different dynamic growths of TTG crusts over time than previously thought, i.e., four times more TTG crusts than at present must have gone into the deep mantle due to tectonic erosion, which occurred six times since subduction was initiated at 520 Ma. Tectonic erosion is a major process that has controlled the development history of the Japanese islands. It can be traced as a serpentinite mélange belt, which indicates the upper boundary of past extensive tectonic erosion.

著者

丸山 茂徳 戎崎 俊一 黒川 顕

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.128, no.4, pp.513-548, 2019-08-25 (Released:2019-09-20)

参考文献数

103

被引用文献数

3 4

---

Several models of the birthplace of life have long been proposed and discussed. However, those discussions are chaotic. To test the birthplace models, we introduce nine requirements that must be met for the emergence of life. These requirements are: (1) energy source (ionizing radiation and thermal energy), (2) supplies of nutrients such as phosphorus and potassium, (3) supplies of main life constituent elements (C, H, O, and N), (4) condensed reducing gas, (5) dry wet cycle, (6) Na poor water, (7) clean lacustrine environment, (8) diversified surface environment, and (9) cyclic nature. Based on these nine requirements, most proposed birthplaces, such as the mid-oceanic ridge hydrothermal system, do not meet the requirements for life to emerge. The only possible site is a nuclear geyser system. Under the Hadean surface environment, sites satisfying the nine requirements are extremely limited because it is significantly difficult to meet the nine requirements, including extrinsic conditions. This difficulty to fulfill all the requirements indicates that only one site is the birthplace of life on Hadean Earth.

著者

鳴海 一成 丸山 茂徳

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.128, no.4, pp.649-665, 2019-08-25 (Released:2019-09-20)

参考文献数

104

被引用文献数

2 3

---

Natural ionizing radiation, which potentially affects biota inhabiting the Earth, can be broadly divided into two types according to origin: cosmic radiation and subsurface radiation. Cosmic radiation contains galactic cosmic rays and solar energetic particles. Subsurface radiation is derived from radionuclides such as uranium, thorium, and radon. The levels of these forms of natural radiation were not constant temporally and spatially, and underwent a lot of changes in the early Earth environment. However, the ground level radiation dose rate of secondary muons derived from a supernova event that causes the most severe biological effects among forms of cosmic radiation is estimated to be 1 sievert (Sv) per year at most, which is too low to have lethal and mutagenic effects on terrestrial microbes. On the other hand, a nuclear fission chain reaction occurred in Oklo uranium ore deposit in Gabon about 2 billion years ago and continued intermittently for 105-106 years. The average total radiation dose rate of a typical natural fission reactor in Oklo is estimated to be 47.4 Sv per hour. This value is high enough to serve as a physical mutagen for subsurface microbes inhabiting areas near the reactor, and a million years is long enough to generate a new species of microbes. The observed growth-inhibitory critical dose rate for Escherichia coli is estimated to be 36 to 67 Gy per hour. On the other hand, the radioresistant bacterium Deinococcus radiodurans is shown to be cultivated without any growth delay at up to 126 to 180 Gy per hour of gamma rays. Recent EXAFS and isotopic analyses indicate that biogenic processes are more important for uranium ore genesis than previously understood. D. radiodurans and its closely related species Thermus thermophilus are shown to have the ability to reduce U(VI) to U(IV) under anaerobic conditions. These lines of evidence suggest that a common ancestor of Deinococcus and Thermus might be involved in the formation process of Oklo uranium ore deposit. Therefore, the radiation dose rate at Oklo-type natural nuclear reactors would be suitable for affecting the growth of microbes and generating genome evolution through accumulated mutations.

著者

丸山 茂徳 生駒 大洋 玄田 英典

出版者

一般社団法人 日本地質学会

雑誌

日本地質学会学術大会講演要旨 第114年学術大会(2007札幌) (ISSN:13483935)

巻号頁・発行日

pp.86, 2007 (Released:2009-01-30)

著者

丸山 茂徳 大森 聡一 千秋 博紀 河合 研志 B.F. WINDLEY

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.120, no.1, pp.115-223, 2011-02-25 (Released:2011-05-20)

参考文献数

217

被引用文献数

26 42 22

---

Pacific-type orogeny (PTO) has long been recognized as a contrasting accretionary alternative to continent-continent collisional orogeny. However, since the original concept was proposed, there have many new developments, which make it timely to produce a new re-evaluated model, in which we emphasize the following new aspects. First, substantial growth of Tonarite–Trondhjemite–Granite (TTG) crust, and second the reductive effect of tectonic erosion. The modern analog of a Pacific-type orogen developed through six stages of growth exemplified by specific regions; initial stage 1: the southern end of the Andes; stage 2: exhumation to the mid-crustal level at Indonesia outer arc; stage 3: the Barrovian hydration stage at Kii Peninsula, SW Japan; stage 4: the initial stage of surface exposure of the high-P/T regional metamorphic belt at Olympic Peninsula, south of Seattle, USA; stage 5: exposure of the orogenic core at the surface at the Shimanto metamorphic belt, SW Japan; and stage 6: post-orogenic processes including tectonic erosion at the Mariana and Japan trench and the Nankai trough. The fundamental framework of a Pacific-type orogen is an accretionary complex, which includes limited ocean floor material, much terrigenous trench sediment, plus island arc, oceanic plateau, and intra-oceanic basaltic material from the ocean. The classic concept of a PTO stresses the importance of the addition within accreted rocks of new subduction-generated arcs and TTGs, which were added along the continental margins particularly during the Cretaceous. Besides the above additional or positive aspects of a PTO, here we emphasize the negative effects of previously little-considered tectonic erosion caused by subduction over time. The evaluation of such extensive tectonic erosion leads a prospect of the presence of huge quantities of TTG material in the lower transition zone, where many subducted slabs have ponded, as illustrated by mantle tomography. This is confirmed by density profiles of the mantle, which show that TTGs are abundant only along the bottom of the upper mantle accompanied by slab peridotite, lherzolite, and MORB. The major velocity anomaly in the lower transition zone is best explained by the predominance of SiO2 phases, hence TTG, and not by MORB or ultramafic rocks. Reasonable calculations indicate that at a depth range of 520-660 km TTG material amounts to 6-7 times more than the total mass of the surface continental crust. The traditional view is that the Japanese islands evolved since 520 Ma through five Pacific-type orogenies, which grew oceanward, thus creating a continuous accretionary complex ca. 400-500 km wide, with TTG growth at the continental side of each orogen. However, the subducting oceanic lithosphere has produced five times more TTG crust compared with the present TTG crust in the Japan islands. This is explained by the fact that over time tectonic erosion has dominated the increasing arc-TTG crust. Accordingly, Japan has lost four arc-TTG crusts to tectonic erosion. TTG material, such as trench sediment, arc crust, and continental margin crust, was fragmented by tectonic erosion and transported into the bottom of the upper mantle at depths of 520-660 km. Worldwide data suggest that tectonic erosion destroyed and fragmented most of the Pacific-type orogens.(View PDF for the rest of the abstract.)

著者

丸山 茂徳

出版者

The Geological Society of Japan

雑誌

日本地質学会学術大会講演要旨 (ISSN:13483935)

巻号頁・発行日

pp.92, 2012 (Released:2013-03-26)

著者

萩原 金美 郷田 正萬 丸山 茂 中山 幸二 鶴藤 倫道 坂本 宏志 井上 匡子 栗田 陸雄

出版者

神奈川大学

雑誌

研究年報 (ISSN:03889025)

巻号頁・発行日

vol.26, pp.3-72, 2008

著者

瀬野 徹三 丸山 茂徳

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.94, no.3, pp.141-155, 1985-06-25 (Released:2010-10-13)

参考文献数

85

被引用文献数

1 3

---

The models on the tectonic evolution of the Philippine Sea can be divided mainly into two:(1) entrapment of a normal ocean at 43Ma for the West Philippine Basin formation and back-arc basin formations caused by successive landward retreats of the upper Philippine Sea plate, and (2) back-arc basin formations caused by successive seaward retreats of the trench axis. These models are discussed on the basis of the presently available data.The island arc type volcanism at the northern half of the Palau-Kyushu Ridge since 48Ma, and the age-depth relation and chemistry of basalts at Sites 291 and 292 in the West Philippine Basin favor the back-arc spreading origin for this basin. In contrast, the chemistry of basalts at Site 447 and the large distance between the Central Basin Ridge and the northern Palau-Kyushu Ridge favor the entrapment origin for this basin. It is likely that the West Philippine Basin has formed under the unique situation of the ocean basin formation, i.e., with subduction at its northern margin and a spreading center far from this subduction zone.Subduction or collision of the aseismic ridges in and around the Philippine Sea is discussed in terms of the crustal structure of the ridges, seismicity, and onland geology. It is found that all these aseismic ridges except for Izu which has a thick 20-30km crust have been subducted, although many of them have formed a cusp or discontinuity at the trench. Elucidating the times of the bending of the Cretaceous-Paleogene terranes in central Honshu and in Kyushu is critical to discriminate the two models for the Shikoku Basin formation, i. e., the fixed trench model and the trench retreating model.

著者

藤崎 渉 澤木 佑介 横山 哲也 山本 伸次 丸山 茂徳

出版者

公益社団法人 日本分析化学会

雑誌

分析化学 (ISSN:05251931)

巻号頁・発行日

vol.64, no.5, pp.341-348, 2015-05-05 (Released:2015-06-09)

参考文献数

33

被引用文献数

3

---

顕生代に起きたとされる5度の大量絶滅の原因を明らかにすることは生命と地球の共進化を明らかにする上で非常に重要な課題である.特に約2億年前のトリアス紀─ジュラ紀境界(T-J境界)で起きた大量絶滅の原因として,隕石衝突説や大規模火成活動説の相反する2説が主張されてきた.本研究ではこの大量絶滅事変の原因を特定するため,T-J境界前後の遠洋深海層状チャートの間に挟まれている頁岩を採取し,その内の28試料を用いて高精度白金族元素濃度測定を行った.Pd/Pt比とIr/Pt比の関係性,及び白金族元素濃度パターンから,T-J境界前後の白金族元素の濃集は大規模火成活動に伴う玄武岩からの混入によって説明できることが明らかになった.また化石記録と比較すると,白金族元素濃度(OsとPd)の最大濃集層はジュラ紀放散虫が出現するチャート層の1層準上位の頁岩層であることも明らかになった.これらのことは,T-J境界絶滅前後において大規模火成活動の寄与が大きかったことを示す.

著者

丸山 茂徳 宇都宮 敦 石川 晃

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.120, no.6, pp.1035-1044, 2011-12-25 (Released:2012-03-05)

参考文献数

16

被引用文献数

1

---

オントンジャワ海台は太平洋中央部の太平洋スーパープルームによって白亜紀(122 Maと90 Ma)につくられた巨大な海台である。形成後,西方に移動し,オーストラリアプレートが北上することによってプレート相対運動が変化して,20-25 Maごろから海溝に沈み込みはじめた。その結果,ソロモン諸島には,それらの海台の破片が付加体として露出している。付加しはじめた時期については異論があり,これまでの多くの研究者は2 Ma 以降であると考えてきた。 しかしながら,われわれは地質調査と文献に基づいた証拠,すなわち一連の逆断層の形成と構造浸食,および20-25 Ma以降の沈み込み帯火山活動の停止から以下の解釈を行った。海台が25-20 Maに海溝に沈み込みを開始すると,海台のその浮力のために、沈み込み角度が低角度になり島弧火山活動が停止した。そして始新世以降の付加体を構造浸食して,崩壊物をマントルへと運んだ。また中新世におけるオントンジャワ海台の衝突と沈み込みが背後のウッドラーク海盆の拡大の開始のトリガーになったと思われる。 過去25-20 Maにわたるオントンジャワ海台の沈み込みは,付加体としては海台のごく一部(< 1%)のみを上盤側のプレートに付加したのみで,主要部のほとんどはマントル深部へ沈み込んだと思われる。これは顕生代の環太平洋造山帯に普通にみられる現象である。