著者

榎本 隆充

出版者

公益社団法人 東京地学協会

雑誌

地學雜誌 (ISSN:0022135X)

巻号頁・発行日

vol.112, no.3, pp.453-457, 2003-06-25

被引用文献数

1

著者

藤井 紀之

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.104, no.5, pp.793-796, 1995-10-25 (Released:2009-11-12)

参考文献数

3

著者

伊藤 英之 鈴木 正貴 佐藤 凌太 杉本 伸一 関 博充

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.124, no.4, pp.561-574, 2015-08-25 (Released:2015-09-17)

参考文献数

13

被引用文献数

5

---

This study is based on an Internet survey of prospective tourists visiting the Sanriku Geopark, which was conducted to understand their travel habits, impressions of the Sanriku coastal area, general awareness of the Geopark, and motivations for travelling. A principal component analysis (PCA) is also performed using a multivariate analysis technique to examine the characteristics of tourists and images of travel destinations they would want to visit in the future. The results demonstrate low awareness of the Sanriku Geopark, especially in metropolitan areas. The majority of respondents do not have a definitive image of the Geopark. The results also indicate that tourists visiting the Sanriku coast are primarily from neighboring prefectures, such as Miyagi and Aomori, as well as from the Tokyo metropolitan area. This suggests that increasing informational awareness of the Sanriku Geopark in these areas would be effective way for attracting more visitors. Regarding their travel habits, the respondents' answers indicate that they mostly go on two-day family trips in a private vehicle. In the majority of cases, their purpose for travelling is “to feel refreshed” or “to eat delicious food.” Based on these data, a PCA employing variance-covariance matrices reveals that tourists are basically seeking “extraordinary” and “healing” experiences from their travels. The principal component scores from the PCA are used and the average scores of each gender are calculated. A t-test reports no significant difference between genders with regard to seeking an “extraordinary” experience, but it identifies a trend among women of seeking “healing” experiences more actively than men (p < 0.10) at a level of 0.1%. On the other hand, regarding the tourists' impressions of the Sanriku coastal area, the results of the multivariate analysis suggest that prospective visitors tend to perceive the coastal area comprehensively in terms of both “nature and scenery” and “local area and culture” to the same degree as those who have previously visited the place. For the area to be resuscitated as a tourist destination, it is important to construct a regional brand and devise strategies that will lead to the region's rebirth as a top travel destination. This can be achieved by offering higher quality experiences and services without destroying the traditional image of the Sanriku coast.

著者

小泉 尚嗣

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.122, no.1, pp.159-169, 2013-02-25 (Released:2013-03-27)

参考文献数

62

被引用文献数

7 6

---

Research on coseismic and postseismic groundwater pressure changes is important for evaluating long-term groundwater stability in seismically active regions such as Japan. It is also important for preventing and reducing earthquake-induced geological disasters such as landslides. This study reviews literature and clarifies abrupt coseismic changes and subsequent postseismic gradual changes in groundwater pressure. Coseismic changes result from ground shaking and coseismic crustal deformation. Postseismic changes are gradual changes, which can be described by a diffusion equation with new initial values and boundary conditions due to ground shaking and coseismic crustal deformation. The effects of crustal deformation on groundwater pressure appear to be limited in the vicinity of the seismic source region, although that of ground shaking can remain in distant areas. Because a variety of factors and conditions affect earthquake-related groundwater pressure changes, each should be investigated to more accurately clarify the effects on coseismic and postseismic groundwater pressure changes. This research may contribute toward clarifying seismicity mechanisms triggered by distant earthquakes.

著者

上村 彩 笠原 順三 日野 亮太 篠原 雅尚 塩原 肇 金沢 敏彦

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.109, no.4, pp.506-516, 2000-08-25 (Released:2010-11-18)

参考文献数

40

被引用文献数

4 4 3

---

To predict earthquakes, huge efforts have been devoted to monitoring earthquakes, crustal deformations and water level during past decades, however, has been found to be extremely difficult. A new approach in marine seismic studies on earthquake prediction proposes focusing on the nature of the subducting plate boundary. Some Ocean Drilling Program (ODP) drillings and seismic reflection studies show fluid flows and the existence of hydrous minerals in the decollement zone. Considering that the subducting plate might contain much water, a number of hydrous minerals might be stable down to 150-300 km, in particular, serpentines and lawsonite.In 1998, the authors carried out a seismic experiment at the Izu-Bonin trench using Ocean Bottom Seismometers (OBSs) and controlled sources. The 130 km long E-W line crosses the Torishima forearc seamount, one of the serpentine diapirs at the eastern terminus. The velocity structure obtained along the line shows a relatively high velocity at the top and alow velocity at the bottom of the serpentine diapir. The velocity of the mantle wedge is slower than that of normal mantle. The dip of the subducting slab is initially quite gentle and abruptly becomes steep around 100 km from the trench axis. Hydration of peridotite in the mantle wedge might occur close to the Izu-Bonin trench axis and serpe ntines seem to be raised upward to the ocean bottom.

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

大橋 聖和 大坪 誠 松本 聡 小林 健太 佐藤 活志 西村 卓也

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.4, pp.565-589, 2020-08-25 (Released:2020-10-02)

参考文献数

108

被引用文献数

4 5

---

The 2016 Kumamoto earthquake occurred in the tectonically complex central Kyushu area where several forcing factors such as the subducting Philippine Sea plate, the Median Tectonic Line and the Nankai forearc sliver, the spreading Okinawa trough, and the migrating volcanic front are involved. Neogene–Quaternary tectonics of central Kyushu are revisited by integrating geological, seismological, and geodetical approaches. Deformation histories of the Futagawa and Hinagu fault zones, the source faults of the Kumamoto earthquake, are also established in an attempt to explain the relationship between geologic structures and rupture processes of the earthquake. The results show that present-day tectonics surrounding central Kyushu are considered to have originated in the last 1 Ma or younger, as a transtensional tectonic zone (Central Kyushu Shear Zone) characterized by combined dextral faults and rift zones (or volcanoes). Reflecting spatiotemporal variations of the crustal stress field and rift activity, the Futagawa and Hinagu fault zones show multi-stage deformation throughout the Neogene–Quaternary periods: normal faulting to dextral faulting for the Futagawa fault zone and sinistral to dextral faulting for the Hinagu fault zone. Those diverse histories of stress and strain fields in central Kyushu possibly led to the complexities of fault geometry and rupture process of the Kumamoto earthquake.

著者

関口 武

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.73, no.1, pp.11-22, 1964-02-29 (Released:2009-11-12)

参考文献数

2

被引用文献数

1

著者

松岡 憲知 藁谷 哲也 若狭 幸

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.126, no.3, pp.369-405, 2017-06-25 (Released:2017-08-03)

参考文献数

221

被引用文献数

3 6

---

Physical rock weathering has been studied through laboratory experiments, field observations, and numerical modeling, but linking these approaches and applying the results to weathering features in the field are often problematic. We review recent progress in three weathering processes—frost shattering, thermal fracturing, and lightning strikes—and explore better approaches to linking weathering processes and products. New visual and sensor technologies have led to great advances in field monitoring of weathering of fractured bedrock and resulting rockfalls in cold mountains. Laboratory simulations successfully produce fractures resulting from segregational freezing in various intact rocks. Modelling approaches illustrate the long-term evolution of periglacial slopes well, but improvements are required to apply laboratory-derived criteria to frost weathering. The efficacy of thermal weathering, which has long been under debate, is now partly supported by laboratory and field evidence that cracking takes place when wild fires or artificial explosions lead to thermal shock. Rock fracturing due to strong radiation is also reevaluated from the presence of large cooling/warming rates and meridian cracks in rocks exposed to arid environments. Linking laboratory simulations and natural features, however, needs further field-based observations of thermal fracturing. Irregular fractures formed in boulders are often attributed to lightning strikes, despite rarely being witnessed. Artificial lightning in the laboratory produces radial cracks, marking the first step toward interpreting irregular fractures in the bedrock that are unlikely to originate from other weathering processes. Identifying the origins of fractured rocks in the field requires distinguishing between fracture patterns derived from these weathering processes.

著者

菅沼 悠介 石輪 健樹 川又 基人 奥野 淳一 香月 興太 板木 拓也 関 宰 金田 平太郎 松井 浩紀 羽田 裕貴 藤井 昌和 平野 大輔

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.5, pp.591-610, 2020-10-25 (Released:2020-11-13)

参考文献数

101

被引用文献数

1

---

The Antarctic Ice Sheet (AIS) is one of the largest potential contributors to future sea-level changes. Recently, an acceleration of AIS volume loss through basal melting and iceberg calving has been reported based on several studies using satellite observations, including radar altimetry, interferometer, and gravity measurements. A recent model that couples ice sheet and climate dynamics and incorporates hydrofracturing mechanism of buttressing ice shelves predicts a higher sea-level rise scenario for the next 500 years. However, the calibration and reproducibility of the sea-level rise projection from these models relies on geological sea-level reconstructions of past warm intervals. This suggests that a highly reliable reconstruction of the past AIS is essential for evaluating its stability and anticipating its contribution to future sea-level rise. In particular, a relative sea-level reconstruction in East Antarctica is the key to solving the problems and refining future projections. The current understanding of sea-level change along the East Antarctic margin is reviewed, including Glacial Isostatic Adjustment (GIA) effects, and a new strategy is proposed to address this topic based on seamless sediment coring from marine to lake in the East Antarctic margin. This project will provide essential data on AIS change since the last interglacial period.

著者

小元 久仁夫 大村 纂

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.124, no.1, pp.127-135, 2015-02-25 (Released:2015-03-11)

参考文献数

22

被引用文献数

2

---

The Rhone Glacier is the best-documented case in the Alps of a rapidly retreating glacier. Photographs and other documents show the history of the changing glacier since the Little Ice Age. Fluctuations of the glacier tongue reflect decadal temperature fluctuations during the observation period. The last quarter century witnessed the most spectacular retreat, resulting in the development of a glacier lake. The pro-glacial lake has increased rapidly in size to the extent that the Swiss Federal Office of Cartography and Survey has given it the official geographical name of “Rhonesee”. The major stages of the Rhone Glacier are presented in this article with selected photographs and figures.

著者

神保 小虎

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.3, no.1, pp.36-38, 1891-01-25 (Released:2010-10-13)

著者

宮縁 育夫

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.125, no.3, pp.421-429, 2016-06-25 (Released:2016-07-14)

参考文献数

21

被引用文献数

1 18

---

The Kumamoto earthquake (Mj 7.3) on April 16, 2016 triggered numerous landslides in and around Minamiaso Village, which is located at the western part of Aso caldera, southwestern Japan. The landslides were divided into two types: landslides occurring at steep caldera walls and landslides generated on the slopes of post-caldera central cones of Aso Volcano. Several landslides occurred on slopes steeper than 25° at the northwestern to western caldera walls, which comprise pre-Aso volcanic rocks (lavas and pyroclastics). The largest landslide (ca. 300 m high, 130–200 m wide) occurred on the western caldera wall, damaging National Route 57 and the Hohi line of the Japan Railway. Because a clear rupture surface could not be observed, unstable blocks which had been divided by cracks, were likely to collapse due to the intense earthquake on April 16. At the post-caldera central cones of Aso Volcano, earthquake-induced landslides were generated not only on steep slopes but also on slopes gentler than 10°. They occurred in unconsolidated superficial tephra deposits overlying lavas and agglutinates, and the thickness of the slides usually ranged from 4 to 8 m. The sliding masses traveled long distances (<600 m), compared to small differences in elevation. The deposits were composed of tephra blocks of a few meters and there was no evidence that they were transported by water. These facts suggest that some landslides mobilized rapidly into debris avalanches, traveling a few hundred meters. The associated debris avalanche resulted in five casualties and severe damage to houses at the foot of the Takanoobane lava dome. The characteristics of the April 16, 2016 earthquake-induced landslides differ from those of rainfall-induced landslides in July 2012, June 2001, and July 1990 at Aso Volcano, and provide important information for preventing or mitigating future landslide disasters in the Aso caldera region.

著者

早川 裕一 松倉 公憲

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.112, no.4, pp.Plate5-Plate6, 2003-08-25 (Released:2009-11-12)

著者

齋藤 めぐみ 山田 和芳 リチャード スタッフ 中川 毅 米延 仁志 原口 強 竹村 恵二 クリストファー ラムジー

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.122, no.3, pp.493-501, 2013-06-25 (Released:2013-07-08)

参考文献数

23

被引用文献数

2 6

---

Sediment cores from Lake Suigetsu, Japan, reveal the absence of seawater intrusion into the lake caused by the historically documented tsunami of AD1586 (Tensho Tsunami). A high-precision chronology of the cores established by Bayesian modeling radiocarbon determinations enables us to ascertain the precise position of the historical event in the sediment depth. Diatom analysis of the core shows that a diatom assemblage dominated by freshwater taxa persisted through the period. This makes a clear contrast with the core section around AD1664 when the lake was artificially connected to the sea by a channel, and the subsequent intrusion of seawater was clearly recorded in the sediment cores by the occurrence of marine diatom fossils.

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

松本 良 奥田 義久 蛭田 明宏 戸丸 仁 竹内 瑛一 山王 梨紗 鈴木 麻希 土永 和博 石田 泰士 石崎 理 武内 里香 小松原 純子 Antonio Fernando FREIRE 町山 栄章 青山 千春 上嶋 正人 弘松 峰男 Glen SNYDER 沼波 秀樹 佐藤 幹夫 的場 保望 中川 洋 角和 善隆 荻原 成騎 柳川 勝則 砂村 倫成 後藤 忠則 廬 海龍 小林 武志

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.118, no.1, pp.43-71, 2009-02-25 (Released:2010-04-05)

参考文献数

46

被引用文献数

65 57

---

A number of extensive methane plumes and active methane seeps associated with large blocks of methane hydrates exposed on the seafloor strongly indicate extremely high methane flux and large accumulations of methane hydrate in shallow sediments of the Umitaka spur and Joetsu knoll of the Joetsu basin 30 km off Joetsu city, Niigata Prefecture. Crater-like depressions, incised valleys, and large but inactive pockmarks also indicate methane activities over the spur and knoll. These features imply strong expulsions of methane gas or methane-bearing fluids, and perhaps lifting and floating-up of large volumes of methane hydrate to the sea surface. High heat flow, ∼100 mK/m, deposition of organic-rich strata, ∼1.0 to 1.5%TOC, and Pliocene-Quaternary inversion-tectonics along the eastern margin of the Japan Sea facilitate thermal maturation of organic matters, and generation and migration of light-hydrocarbons through fault conduits, and accumulation of large volumes of methane as methane hydrate in shallow sediments. Microbial methane generation has also contributed to reinforcing the methane flux of the Joetsu basin. Regional methane flux as observed by the depth of the sulfate-methane interface (SMI) is significantly high, < 1 m to 3 m, when compared to classic gas hydrate fields of Blake Ridge, 15 to 20 m, and Nankai trough, 3 to 15 m. δ13C of methane hydrate and seep gases are mostly within -30 to -50‰, the range of thermogenic methane, while dissolved methane of the interstitial waters a few kilometers away from seep sites are predominated by microbial with δ13C of -50 to -100‰. Seismic profiles have revealed fault-related, well-developed gas chimney structures, 0.2 to 3.5 km in diameter, on the spur and knoll. The structures are essential for conveying methane from deep-seated sources to shallow depths as well as for accumulating methane hydrate (gas chimney type deposits). The depth of BSR, which represents the base of gas hydrate stability (BGHS), on the spur and knoll is generally 0.20 to 0.23 seconds in two-way-travel time, whereas the BSRs in gas chimneys occur at 0.14 to 0.18 seconds, exhibiting a sharp pull-up structure. The apparent shallow BGHS is due to the accumulation of large volumes of high-velocity methane hydrate in gas chimneys. The depth to BGHS is estimated to be 115 m on an experimentally determined stability diagram, based on an observed thermal gradient of 100 mK/m. Then the velocity of the sediments on the Umitaka spur is calculated to be 1000 m/s, which is anomalously low compared to normal pelagic mud of 1600-1700 m/s. This exciting finding leads to the important implication that sediments of the Umitaka spur contain significant amounts of free gas, although the sediments are well within the stability field of methane hydrate. The reasons for the existence of free gas in the methane hydrate stability field are not fully explained, but we propose the following possible mechanisms for the unusual co-existence of methane hydrate and free-gas in clay-silt of the spur. (i) High salinity effect of residual waters, (ii) degassing from ascending fluids, (iii) bound water effect and deficiency of free-waters, and (iv) micro-pore effect of porous media. All of these processes relate to the development of gas hydrate deposits of the Umitaka spur.(View PDF for the rest of the abstract.)

著者

柵山 徹也

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.2, pp.224-234, 2010-04-25 (Released:2010-07-06)

参考文献数

57

被引用文献数

8 6

---

Cenozoic western Pacific is characterized by numerous subduction volcanisms, opening of back arc basins, and within-plate volcanisms in backarc regions. These phenomena suggest that thermal disturbances induced by mantle flow caused mantle melting and eruption of magmas in back-arc regions, although diverse modes have been proposed for the mantle condition. To constrain the origin of the magmatism, it is important to clarify both the geodynamics and physicochemical conditions of the mantle in the arc-back arc system. Cenozoic back-arc volcanisms and tectonics in northern Kyushu are key objectives. This paper focuses on the dynamics of the upper mantle in relation to the tectonics of the area. Cenozoic volcanism of the northern Kyushu district is characterized by eruptions of a number of alkaline volcanisms. This volcanic province extends some one hundred kilometers along the Japan Sea coast and includes the Chugoku area. Although the volcanism has been regarded as a large single volcanic province, the northern Kyushu activity is distinguishable from that in Chugoku by geochemical characteristics. Cenozoic basalt volcanism in northern Kyushu occurred mainly in the Tertiary sedimentary basins, which is common to Cenozoic volcanism in the Japan Sea Basin. The sedimentary basin deposits underwent intensive folding with extensive inversion tectonics occurring mainly around the end of Miocene. Studies on dykes and thrust faults also indicate a compressional stress field dominated in northwestern Kyushu during the late Miocene. Initiation of the basaltic activity took place in Kita-Matsuura area and expanded outwards. Basaltic rocks from the Kita-Matsuura are mostly hypersthene-normative and sub-alkalic, whereas rocks from peripheries, such as North Kyushu and Goto Islands, are predominantly less normative-hypersthene and thus are alkalic. From spatial and temporal variations and genetic conditions of basalt magmas, a most plausible source was a small-scale mantle diapir with a potential temperature of more than 1400°C. Combined with an inversion tectonic background after the Japan Sea opening event, the origin of the back-arc magmatism was unrelated to large-scale mantle upwelling. Fluid-fluxed melting of the mantle is not supported by the chemistry. My proposal is active upwelling of a small-scale mantle diapir, probably from the depth of stagnant slab or mantle transition zone.

著者

矢田 俊文

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.91, no.6, pp.489-495, 1982-12-25 (Released:2009-11-12)

参考文献数

6

被引用文献数

1

著者

大関 仁智 清水 紀和 上松 佐知子 指田 勝男

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.130, no.3, pp.311-329, 2021-06-25 (Released:2021-07-20)

参考文献数

69

被引用文献数

2

---

Zircons in Cretaceous sandstones have recently been the focus of a new provenance analysis technique. Microfossil-bearing clasts in conglomerate are also used in this analysis; however, there are few reports on clasts, including microfossils, which are mainly found in the vicinity of Shikoku and Hokuriku districts. Although the Ishido Formation, Sanchu Group, in the Kanto Mountains, is considered to be Barremian in age based on the occurrences of ammonoids, interbedded with conglomeratic beds, there have been no reports of microfossil-bearing clasts from this bed. To confirm whether each fore-arc basin in the whole of southwest Japan had a common hinterland during the Late Mesozoic, microfossils are extracted from gravels in the Ishido Formation to examine their ages. As a result of acidic treatment, Permian and Triassic radiolarians and Triassic conodonts are obtained from eight chert and siliceous mudstone pebbles of the Ishido Formation. On the basis of their lithofacies and ages, these pebbles of this formation are likely to derive from the Chichibu Belt, which constitutes a large part of the Kanto Mountains. These results indicate that, in the Early Cretaceous period, not only granitic rocks but also Jurassic accretionary complexes were exposed as hinterlands of each fore-arc basin in the Shikoku and Kanto districts. Moreover, based on previous reports on the zircon spectra in Cretaceous sandstones and microfossil-bearing clasts in the conglomerates, hinterlands, such as granitic rocks and Jurassic accretionary complexes, of each fore-arc basin in the whole of southwest Japan including the Kanto district, as well as Kyushu and Shikoku districts, indicate common exposure and denudation histories.