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 23 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
千秋 博紀 滝田 隼 荒井 武彦 福原 哲哉 田中 智 岡田 達明 関口 朋彦 坂谷 尚哉 はやぶさ2TIRチーム
出版者
日本惑星科学会
雑誌
日本惑星科学会誌遊星人 (ISSN:0918273X)
巻号頁・発行日
vol.24, no.2, pp.120-125, 2015

TIR(中間赤外カメラ)は,8から12ミクロンの波長帯で熱輻射の2次元イメージングを行う.ターゲット天体の1自転分の撮像から表層物質の熱履歴をもとめ,そこから熱物性を推定する.表層物質の熱物性は,ミッション遂行に必要な情報であるばかりでなく,その後の天体の運命を決める重要な情報である.
著者
M. SANTOSH 千秋 博紀
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.100-114, 2011-02-25 (Released:2011-05-20)
参考文献数
50
被引用文献数
3 3

The history of supercontinents is briefly reviewed in relation to the origin of the Japanese Islands. The Japanese Islands formed part of the S. China Block, which was a part of supercontinent Rodinia at 1.0 Ga. Rodinia was rifted at 600 Ma, separating S. China Block, N. America, Australia and other continents, to generate the Proto-Pacific Ocean in between. On the other hand, the Hida and Oki islands belong to the N. China Block, which has much longer history than the S. China Block, extending back to 1.9-2.0 Ga with minor older rocks dating back to 3.8 Ga. The 1.8-2.0 Ga high-grade gneiss in the Hida and Oki belts may be part of the 1.8-1.9 Ga Nuna/Columbia supercontinent within which N. China-Japan occurred at the NE corner, as judged from key parallel belts of 1.8-1.9 Ga in N. China. The position of Japan at 1.0 Ga within Rodinia was at the center together with S. China and western margin of N. China. The oldest fossiliferous rocks in Japan may extend back to the Early Cambrian to Ediacaran formed during the rifting of Rodinia directly after Neoproterozoic snowball Earth. Initiation of subduction began ca. 520 Ma, and evolved through five Pacific-type orogenies along the southern margin of S. China. On the other hand, the Hida and Oki belts suffered the Triassic collision orogeny at 230-240 Ma, involving platform sediments up to the Carboniferous age. The final tectonic emplacement above the Jurassic accretionary complex may be related to the extensional event during the opening of the Japan Sea in the Miocene.
著者
千秋 博紀 丸山 茂徳 李野 修士
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.1215-1227, 2010-12-25 (Released:2011-03-17)
参考文献数
57
被引用文献数
6 5

It has long been believed that granite remains on the surface of solid Earth indefinitely due to its low density, hence continents increased in volumes to cover about 30% of the Earth's surface over time. However, recent studies on the accretion history of continents reveals that at least 80% of the granite that ever formed had been subducted into the deep mantle to form “second continents” at the mantle transition zone (410-660 km). These second continents would affect mantle dynamics in two ways. First, as the second continents are gravitationally stable at the depth of the mantle transition zone, they act as a barrier to descending cold slabs. The stagnation of cold slab would be partly due to the pre-existing second continents. Another effect of the second continents on mantle dynamics relates to their chemical component. Because granite is enriched with incompatible elements, including long-lived radiogenic elements K, Th, and U, the second continents act as heat sources in the mantle. In particular, the heat generation of the second continents is a key to understanding the formation-breakup cycle of supercontinents. Granite that piled up beneath a supercontinent during continent accretion would cause thermal instability to form a superplume beneath the supercontinent. A numerical study on the thermal evolution of subducted granite gives the characteristic time scale of thermal instability which is consistent with the lives of supercontinents suggested by geological studies.
著者
丸山 茂徳 大森 聡一 千秋 博紀 河合 研志 WINDLEY B. F.
出版者
Tokyo Geographical Society
雑誌
地學雜誌 (ISSN:0022135X)
巻号頁・発行日
vol.120, no.1, pp.115-223, 2011-02-25
被引用文献数
5 23

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.<br> 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 SiO<sub>2</sub> 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.<br> 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 <i>ca.</i> 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.<br>(View PDF for the rest of the abstract.)
著者
千秋 博紀 黒澤 耕介 岡本 尚也
出版者
日本地球惑星科学連合
雑誌
日本地球惑星科学連合2016年大会
巻号頁・発行日
2016-03-10

A shooting star is caused by an entry of a cosmic dust particle into the planetary atmosphere. The light from the shooting star composed of thermal emission and emission lines from the gas in from of the dust particle and the vapor from the dust particle. It means that the physical and chemical condition of the dust particle can be estimated from a photometric and/or spectroscopic observations. However a shooting star is a sporadic and un-controled event, and thus the relation between the physical and chemical condition and the resulting spectroscopic observation is estimated by empirical equations.We are constructing a laboratory experimental system to simulate shooting stars by using a two-stage light gas gun at Planetary Exploration Research Center (PERC), Chiba Instiute of Technology, Japan. This gun shoots a projectile with size of 2 mm into a observational chamber filled with gas. The light from the projectile is observed by high-speed camera with 1 Mfps and its spectrum is taken by spectrometer simultaneously.We carried out a series of experiments using the system with a variety of projectile composition. The specific spectra relating to the projectile component were confirmed as a function of the location from the projectile (during head-neck-tail structure). We will give the experimental results and discuss the chemical and physical status of shooting star.
著者
千秋 博紀
出版者
東京工業大学
雑誌
若手研究(B)
巻号頁・発行日
2004 (Released:2004-04-01)

本研究では、天体衝突によって作られた蒸気の塊(以下衝突蒸気雲と呼ぶ)の内部で進行する化学反応系を、数値的にシミュレーションし、どのガス種が、どれだけ作られるのかを求める。衝突蒸気雲は、形成直後はきわめて高温・高圧だが、急激に膨張・冷却する。高温・高圧の条件下では化学反応の速度は速く、容易に化学平衡が達成される。これに対し、膨張・冷却が進むと、化学平衡はもはや達成されなくなる。従来の研究では、膨張過程のある瞬間で化学反応がクエンチ(停止)すると考え、単純に衝突蒸気雲の膨張のタイムスケールと、ある化学種がかかわる反応のタイムスケールとの比較から、膨張・冷却後の衝突蒸気雲の組成を見積もってきた。本研究では昨年度までに、開発した1次元球対称モデルを用いて得られる結果について、ガス成分の種類や量について議論を重ね、論文の準備を行ってきた。しかし一連の議論の中で、蒸気雲中の化学反応ネットワークは、系全体がサブシステムに細分化されてゆき、サブシステムの中で局所平衡が達成されるように進むことが明らかになってきた。この描像は従来考えられていたものとは異なっており、この性質をうまく使うことでガス種同士の相対存在度から逆に衝突条件を求めることができるようになるかも知れない。ただし、どのガス種組合せでサブシステムを構成するようになるのかは、温度圧力条件(衝突条件)だけでなく、初期組成にも依存する。本研究では今後も、衝突蒸気雲が地球システムに与える影響を議論するための、幅広いパラメタ空間でのシミュレーションなどを続けてゆく。