著者

徳仁親王

出版者

学術雑誌目次速報データベース由来

雑誌

地學雜誌 (ISSN:0022135X)

巻号頁・発行日

vol.100, no.1, pp.3-18, 1991

被引用文献数

1

著者

都司 嘉宣

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.106, no.4, pp.486-502, 1997-08-25 (Released:2009-11-12)

参考文献数

12

被引用文献数

2 7

---

A field survey of the damage of the Amami-Oshima-Kinkai earthquake (Mw 7.1) and its tsunami of October 18, 1995 was conducted for four days from the next day of the main shock. A large aftershock (Mw 6.8) occurred in the next day and was also accompanied with a small tsunami. We conducted interviews of the inhabitants, and measured heights of the both tsunamis. The maximum height of the tsunami of the main shock was 3.0 m at Urahara Port on the south coast of Kikai-jima. Earthquake damage mainly occurred on Kikai-jima, where stone walls were fallen down at 91 places, the water supply facility of the island was disordered, and several concrete slabs slid down at Wan Port. Several fishing boats were wrecked due to the both tsunamis at ports on Kikai-jima and Amami-Oshima. Imamura-Hatori's magnitudes of the main shock and the large aftershock are m=1.0 and m=0.0, respectively. Abe's magnitude of the tsunamis the main shock is estimated to be Mt=7.6 and is large for the earthquake magnitude. A gigantic earthquake (M8.0) with a larger tsunami occurred on June 15, 1911 in the sea region close to the present event, and its folktale is handed down by the inhabitants of Kikai-jima.

著者

新井 健司

出版者

Tokyo Geographical Society

雑誌

地学雑誌 (ISSN:0022135X)

巻号頁・発行日

vol.104, no.2, pp.267-283, 1995-04-25 (Released:2009-11-12)

参考文献数

17

被引用文献数

1 2

---

Plio-Pleistocene fluvial sediments are distributed in the hills along the western margin of the Kanto Plain. In this paper, the author reports its sedimentary environments at Hidaka City, Saitama Prefecture, namely, those of Yaoroshi tuff, Hanno gravel and their correlatives.The basement in this area (Chichibu Paleozoic and Mesozoic formations and Takaoka formation) is divided into three zones structurally by Koma-Hongo and Komagawa fault systems. And terrace deformation dipping toward the Kanto Plain is recognized on its surface.Yaoroshi tuff is in fault or unconformity contact with the basement in part of Koma-Hongo fault system. The vertical displacement of each fault system is over 50 m. Yaoroshi tuff is not distributed in west of Koma-Hongo fault system.Hanno gravel, conformably overlying Yaoroshi tuff, is in fault or unconformity contact with the basement. Hanno gravel and its correlatives were formed by old long rivers, of and the valley bottoms were at least 140 m above sea level in Koma River system area and at least 150 m above sea level in Shukuya River system area.Boulders of granite and diorite characterizing Hanno gravel in the south of Tenran-zan (Mt. Tenran) are not found in the investigated area. This fact suggests that the area was in different conditions of sedimentation from the southern area. The ridge directions of Koma Hills and Moroyama Hills had already settled before dissection proceeded.The gravel from Moroyama Hills and the adjacent areas is divided into the lower and the upper layers. The former is correlated with Yaoroshi tuff, while the latter with Hanno gravel.When Yaoroshi tuff deposited, forested stable land had extended widely. And short rivers flowing out from near mountains sometimes flooded and destroyed forests. After Yaoroshi tuff was formed, long rivers transported boulders with sands in large quantities from distant western mountains. These old long rivers kept their main courses to the present as Koma River system and Shukuya River system, and they mixed gravels by gathering currents and forming talus deposits.

著者

大倉 敬宏 安藤 雅孝

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.103, no.5, pp.464-470, 1994-10-25 (Released:2009-11-12)

参考文献数

15

被引用文献数

2

---

The relation between the Pinatubo eruption of 1991 and the Philippine earthquake of 1990 is studied on the basis of strain changes calculated from a fault model of the Philippine earthquake. At Pinatubo volcano, which is located 100 km away from the earthquake fault, the volumetric strain change induced by the earthquake is about 10-6. This caused gradual squeeze up of magma to the surface and the Pinatubo volcano erupted eleven months after the earthquake. The volumetric change calculated by a fault model of the 1990 earthquake is, however, several orders of magnitude smaller than the total volume observed during the eruptions. The magma squeezing model alone cannot explain the whole volume of ejected magma. A possible interpretation is that the volumetric strain change could have squeezed up magma in the magma reservoir, resulting in lowering the density of magma and enhanced the magma to rise further more. Such a positive feedback process could have occurred after the Philippine earthquake of 1990. It is possible that the Philippine earthquake triggered the activity of Pinatubo volcano.

著者

宮本 真二 安田 喜憲 北川 浩之

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.6, pp.865-873, 1995-12-10 (Released:2009-11-12)

参考文献数

13

被引用文献数

1 2

---

A continuous long core of peaty sediments bored from the Naka-Ikemi Moor Central Japan (lat. 35°39' N., long. 136°5' E.), contains records of paleoenvironmental changes during the Last Glacial and Holocene periods. The Naka-Ikemi Moor stretches about 1.3 km from the west to east. This peculiar shapes and crustal movements had been pointed out by purely from a topographical point of view. And this moor is regarded as a waste-filled valley created by sort of tectonic depression at the eastern part of the Tsuruga Plain. From the stratigraphical examination, some volcanic ashes were detected as follows : Kikai-Akahoya Tephra (K-Ah : 6.3 ka), Aira-Tn Tephra (AT : 24 ka), Daisen Kurayoshi Tephra (DKP : ca. 50 ka). Values of bulk density measurements and radiocarbon dates of core samples indicate that the Naka-Ikemi core samples contain continuous records for the past 50 ka. The average sedimentation rate of Naka-Ikemi Moor was increased after the fall of AT volcanic ash had occurred. The sedimen tation process of Naka-Ikemi Moor has also been clarified by sedimentary facies and value of loss on ignition of core samples. Value of loss on ignition began to increase since the end of Last Glacial, suggesting the increase of organic material caused by environmental changes.

著者

片山 一道

出版者

Tokyo Geographical Society

雑誌

地学雑誌 (ISSN:0022135X)

巻号頁・発行日

vol.105, no.3, pp.384-397, 1996-06-25 (Released:2009-11-12)

参考文献数

29

---

The present paper describes the physical characteristics of Ocean Mongoloid peoples who are indigenous in the Oceania proper, analyses variation patterns among them, and reconstructs the processes that they colonised and settled the oceanic world, following a brief introduction to the human population history in Oceania. Based on the analysis, their remote origins are discussed. The best example of Ocean Mongoloids is the Polynesian population. Polynesians are very homogenous in physique, language and culture. Linguistic and archaeological evidences so far gathered suggest they differentiated into island groups in the past 3, 000 years from a Lapita-associated ancestoral population. Anthropological data show their uniqueness for the large, muscular and robust body form as well as a hyperostotic growth pattern from Homo sapiens standards, and their Asiatic nature in many genetic characters. Accordingly they should be considered “the unique Asian in South Pacific”, and their origins should be sought in Asia. It is highly probable that the Polynesian phenotype was developed during their dispersals into Oceania as the result of adaptation to the specific oceanic habitats humans ever encountered. The speculation here suggests that the remote origins of Ocean Mongoloids were somewhere in circum-China Sea areas, and that some group associated with Jomon people in Japan should be a candidate for their ancestors.

著者

豊 遙秋 春名 誠

出版者

Tokyo Geographical Society

雑誌

地学雑誌 (ISSN:0022135X)

巻号頁・発行日

vol.107, no.6, pp.794-802, 1998-12-25 (Released:2010-11-18)

参考文献数

66

著者

安藤 寿男 勢司 理生 大島 光春 松丸 哲也

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.104, no.2, pp.284-303, 1995-04-25 (Released:2009-11-12)

参考文献数

38

被引用文献数

14 22

---

The Futaba Group is the Lower Coniacian to Lower Santonian, fluvial to shallow-marine sedimentary succession distributed in the southern Abukuma Belt in north Honshu. Fifteen depositional facies were discriminated. They include alluvial fan, meandering river, braided river, transgressive lag and upper shoreface to inner shelf sediments. It is divided into three formations, the Ashizawa (the Asamigawa and Obisagawa Members), Kasamatsu and Tamayama Formations in upward sequence. Each formation consists of a third-order depositional sequence (DS).The first depositional sequence (DS 1) has a lowstand systems tract (LST) as the lower main part of the Asamigawa Member distributed in the northern half of the area. It is lithologically characterized by upper alluvial fan conglomerate and mid to lower fan sandstone and siltstone nonconformably overlying lower Cretaceous granite (sequence boundary 1 : SB 1). In the southern periphery of the distributed area the member contains talus or upper fan angular conglomerate. The upper part of the member distributed over the area is interpreted to be transgressive conglomerate with a sharp base called a ravinement surface (RS 1) which was formed by transgressive shoreface erosion. It may represent transgressive systems tract (TST). The following highstand systems tract (HST) is represented by the upper shoreface to inner shelf, medium to fine sandstone of the Obisagawa Member.The Kasamatsu Formation (DS 2) unconformably (SB 2) overlying the preceding depositional sequence (DS 1), is substantially characterized by fluvial upward-fining successions composed of channel sandstone and flood plain siltstone. The third sequence (DS 3) is characterized by thick fluvial (braided river) sandstone of the lower to middle part of the Tamayama Formation interpreted as LST and the upper shoreface to inner shelf sandstone of the upper Tamayama interpreted as HST. The sudden lithofacies change between the Kasamatsu and Tamayama Formations implies some large-scale changes in fluvial depositional systems by sea-level fall (SB 3). The transgressive conglomerate just above RS 2 in the basal part of the upper Tamayama Formation represents a thin TST. The upper sequence boundary (SB 4) of DS 3 shows the conspicuous clinounconformity between the Futaba Group and the Paleogene Shiramizu Group.

著者

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.126, no.3, pp.271-296, 2017-06-25 (Released:2017-08-03)

参考文献数

131

被引用文献数

3 6

---

A brief review of our weathering studies is presented, specifically from a geomorphic perspective. The review suggests that experimental work and microscale investigations using new analytical methods have advanced studies on weathering processes, while steady advances have been made in research on changes in properties and rates of weathering. Assignments and priorities are presented arising from our studies, including the need to: (1) accumulate data on rates of change of rock properties due to weathering (especially strength reduction); and, (2) construct a geomorphological equation of weathering rates based on the relationship between weathering processes and rock properties.

著者

阿部 隆 磯田 弦 山科 絢香

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.2, pp.213-238, 2021-04-25 (Released:2021-05-19)

参考文献数

18

被引用文献数

7

---

In this study, evacuees' rosters and other relevant disaster records are used to geographically clarify the relationship between people's evacuation behavior following the Great East Japan Earthquake, social and demographic characteristics of evacuees, and housing damage caused by tsunami following the earthquake in Yamada Town, Iwate Prefecture. The trend of shelter entry and exit is analyzed on the basis of three scales. The first scale is the municipality unit; trends in the numbers of people entering shelters in Yamada Town are compared to those in other affected municipalities on the coast in Iwate Prefecture and regional differences in these trends are clarified. The second scale is the district or small area unit; regional differences in rates of entry to shelters in Yamada Town and factors affecting these differences are explained geographically. The third scale is the shelter level; regional differences in rates of entry to school shelters in small areas are analyzed in relation to evacuees at two elementary schools. Furthermore, regarding evacuees in a school shelter where a daily list is available, a logistic regression analysis is performed to explain evacuees' decisions on whether or not to stay in the shelter on the basis of variables such as gender, age, and family situation. The regression analysis for the first scale reveals that the decreasing trend in the number of evacuees in shelters in Yamada Town has been slow compared to those in other affected coastal municipalities. The study reveals that progress in the construction of emergency temporary housing is the factor with the greatest impact on an evacuee's decision to leave a shelter. Regarding the second scale, the geographical distribution of the number of shelter residents in Yamada Town is analyzed on the basis of the scale of a district and a small area. The analysis reveals that regional differences in shelter entry rate reflect social network, topographical features, and developmental process of the settlement. Regarding the third scale, the relationship between distance from the shelter and entry rate of affected households is analyzed by small area using the rosters of Yamada Minami Elementary School and Orikasa Elementary School evacuation shelters. A significant correlation is found between average road distance from Yamada Minami Elementary School and affected household entry rate, and it is observed that many residents were from areas located within 1 km from the school. On the other hand, no significant correlation is found between average road distance from Orikasa Elementary school and affected household entry rate. Regarding the Orikasa Elementary School shelter, almost daily entry and exit records could be obtained for the period from April 9, 2011 to August 3, 2011. Using this record, age and family composition of withdrawers during this period could be identified. A logistic regression analysis was performed with gender, age group, marital status, and family type as explanatory variables. With respect to the influence of a resident's age and family structure on his or her exit time, it is found that the withdrawal rate of residents aged 75 years or more was high until the beginning of May. Households that stayed in the shelter with 0 to 18-year-old juveniles tended to find it better to stay in the shelter until their children's schools reopened. After resumption of schools, at the beginning of April; however, they tended to leave in higher numbers. The study concludes that, although schools should support evacuees during the earlier period of an evacuation as large-scale evacuation shelters, various types of shelter should be developed and reorganized to suit the evacuees' needs in case the evacuation is extended for a long period.

著者

小泉 格

出版者

Tokyo Geographical Society

雑誌

地学雑誌 (ISSN:0022135X)

巻号頁・発行日

vol.116, no.1, pp.62-78, 2007-02-25 (Released:2009-11-12)

参考文献数

69

被引用文献数

7 8

---

The results of hydro-environment researches into marine and lacustrine sediments revealed climatic variations which seem to have some relationships between turns in world history and climatic changes. In the Japan Sea and northeast Pacific off Kashima from the Late Glacial to the Holocene, diatom temperature, Td' = (frequency of warm-water species) - (frequency of warm-and cold-water species) -1?100, values show rhythmic fluctuations with durations of 1 kyr and 400-500 yrs (Koizumi et al., 2004, 2006). This indicates a strong and regular inflow of the warm Tsushima Current into the Japan Sea as a branch of the warm Kuroshio Current. The decreases of Td' values correspond to a period of climatic deterioration under which cultural changes occurred in Japan. The paleoclimatic variations and the triple events of high 14C in the atmospheric residual 14C record denoted by Stuiver et al. (1991) are recognized in the diatom variations.A similar periodicity is also recognized in North Atlantic drift ice (Bond et al., 2001), concentrations of sea salt and terrestrial dust in the GISP2 ice core (O'Brien et al., 1995), in lake deposits from the Jura in France (Magny, 1995), Tibet (Gasse et al., 1991; Ji et al., 2005), and Alaska (Hu et al., 2003), and oxygen isotope variations from stalagmites in southern China (Wang et al., 2005) and Oman (Neff et al., 2001).Climatic deteriorations are synchronous with periods when serious changes occurred in human intellectual achievements during the Holocene. The Agricultural Revolution (11, 000-10, 000yr BC), with the cultivation of wild plants and domestication of wild animals to produce and secure food, seems to coincide with the Younger Dryas event. Increased agricultural production made it possible to afford a population not engaged in agriculture, for example, people engaged in handicrafts, festivals, and politics. This seems to occur in the period coinciding with T2 event in the T1-T4 triplets, which have Maunder-and Sporer-type patterns occurring in sets of three, denoted by Stuiver and Braziunas (1993). The religious courtesy belief in the recycling and the reinforcing of life was converted into a rational and systematic consideration of life based on universal theory. This conversion of thinking seems to coincide with the T3 event. Modern science was established in Europe in the 17th century. And now human beings are destroying natural environments using highly developed scientific technologies of their own. This is a negative revolution that human beings have never previously experienced.In the south-to-north transect of the cores from both the Pacific and Japan Sea sides of Honshu, Td' values indicate that the Kuroshio Current and Tsushima Warm Current are both weakening at present. The present interglacial period might end and become a cooler climatic condition like a Little Ice Age within 300 years. The insolation at 37°N among the Earth's orbital parameters is weak at present (Koizumi and Ikeda, 1997). However, the mean global temperature has risen about 0.5°C during the last century. Warming of the Earth, which is caused mainly by carbon dioxide in the atmosphere, has made natural environments unstable.

著者

吉田 明夫

出版者

Tokyo Geographical Society

雑誌

地学雑誌 (ISSN:0022135X)

巻号頁・発行日

vol.103, no.3, pp.201-206, 1994-06-25 (Released:2009-11-12)

参考文献数

17

被引用文献数

1

---

Correlation between earthquakes in Kanto region and intermediate-depth earthquakes in Hida region, central Japan is re-investigated. We found that a significant correlation exists between earthquakes with depth of 70 km and deeper in western Kanto and intermediate-depth earthquakes in Hida region, but such a correlation is not seen for earthquakes in eastern Kanto. This result shows that earthquakes occurring in the Pacific slab are well correlated each other, but earthquakes which occur in relation to the subduction of the Philippine Sea plate are not correlated with intermediate-depth earthquakes in the Pacific plate. Intermediate-depth earthquakes in Hida region have a tendency to precede earthquakes in western Kanto. This feature may be related to the dynamics of the plate motion which oceanic plate is pulled by the subducted slab.

著者

早川 由紀夫 小山 真人

出版者

Tokyo Geographical Society

雑誌

地学雑誌 (ISSN:0022135X)

巻号頁・発行日

vol.106, no.1, pp.102-104, 1997-02-25 (Released:2009-11-12)

参考文献数

10

被引用文献数

4 1

著者

遠藤 毅

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.113, no.4, pp.534-538, 2004-08-25 (Released:2009-11-12)

参考文献数

10

被引用文献数

3

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

松原 聰

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.131, no.2, pp.147-161, 2022-04-25 (Released:2022-05-13)

参考文献数

28

被引用文献数

2

---

I here outline to the definition of a mineral, solid solutions in mineral nomenclature, rules of naming of minerals, and procedure for establishing of a new mineral by the Commission on New Minerals, Nomenclature and Classification, the International Mineralogical Association. Also discussed are the implications of new minerals and minerals found first in Japan. Detailed accounts are given of new minerals from Japan.

著者

香川 雄一

出版者

Tokyo Geographical Society

雑誌

地学雑誌 (ISSN:0022135X)

巻号頁・発行日

vol.110, no.3, pp.314-338, 2001-06-25 (Released:2009-11-12)

参考文献数

50

被引用文献数

5 1

---

Recently, studies of industrial spaces have taken several perspectives. For example, a Locality study, especially in the United Kingdom, studies not only economic aspects but also social and political aspects.This paper discusses the changing locality of the industrial city of Mizushima, Okayama pref. which was industrialized in a period of high economic growth. The indicators of a locality study are working and social trends, and the results of elections. However it is difficult to use same indicators for a Japanese industrial city, because locality research may be unique to English cases. Pollution problems, which had been a special topic of debate in local politics, are used as the indicator for industrialization. The topics of members of the assembly, which are recorded in proceedings, are important pointers to the change from an agricultural and fishing village to an industrial city.When it was an agricultural and fishing village (pre-modern period) there were such developments as a new rice field at Mizushima located beside the mouth of Takahashi-river. These are the bottom of industrial area, now. During World War II, a munitions factory was constructed on land reclaimed from the Takahashi river. Air raids destroyed most of the factories. After the war, Okayama pref. decided to invite many large companies. Machine, petrochemical, and steel manufacturing plants began operations. The resulting industrial complex made Mizushima an industrial city.In terms of social structure, old leaders retained power until the agricultural and fishing villages combined to form Kurashiki city. Kojima city and Tamashima city also combined to form part of Kurashiki city. In those areas, old industries supplied a large part of employment. Therefore, it has taken time to change urbanize communities while increasing the population through the inflow of workers for large factories.Secondly, to clarify local politics, there has been debate on pollution problems. Incidentally, labor movements, which are thought to be important in a locality study, did not have much influence on the pollution problems in Mizushima. The first pollution problems occurred with fish and crops. So farming and fishery organizations claimed compensation for damage. Furthermore, pollution problems harmed human health as industrial spaces grew. From the 1960's to the 1970's most Japanese industrial cities had anti-pollution movements, as was the case in Mizushima. There were neighborhood associations and working organizations forming anti-pollution organizations. These spread to form a network organization in Mizushima. Eventually, the anti-pollution movement was restricting to patients with diseases caused by pollution and their supporters. These changes can be seen in the proceedings about pollution problems.At the beginning of the period with pollution problems, there were debates about industrial change, damage to local people, and policy between parties. After pollution problems increased, this topic was shared by all of the inhabitants of Kurashiki city. The debate concentrated on pollution problems, regardless of resident or party or occupation. This led to an administrative policy for the pollution problem. Then pollution damage gradually decreased. The debate moved from pollution to environment problems, which also contains protection of nature, in the latter period. There was much debate by members of the assembly in a local reformist party, and residents of Mizushima were very concerned about improving the environment of living spaces affected by pollution problems.In conclusion, industrialization and urbanization in Mizushima not only involve construction of factories and increase of industrial labor, but urbanization also refers to the living environment sought by local residents.

著者

山本 純之 磯﨑 行雄

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.5, pp.791-806, 2013-10-25 (Released:2013-10-31)

参考文献数

86

---

Ever since the term stromatolith was first coined by Kalkowsky in 1908, research studies on stromatolites have continued for more than a century. This article reviews the study history of living and fossil stromatotelites. The history is divided into two parts by the discovery in 1961 by Logan of living stromatolites in Shark Bay, Western Australia, because this determined the subsequent main trend of studies on both living and fossil stromatolites. Major contributions during the last three decades include documentation of (1) a worldwide bloom of stromatolites in Proterozoic low-middle lateral shallow seas, and (2) oxygen-producing cyanobacterial activity related to stromatolites from the 1990s, resulting in various new aspects being clarified, which include in-vitro cultivation gradually revealing cyanobacterial calcification and dome-formation. Future studies will be directed towards reconciling morphological disparities and formation mechanisms among fossil, living, and cultured stromatolites.

著者

平田 大二 山下 浩之 鈴木 和恵 平田 岳史 李 毅兵 昆 慶明

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.1125-1160, 2010-12-25 (Released:2011-03-17)

参考文献数

142

被引用文献数

8 10 5

---

This paper reviews the geology, petrology, and tectonics of the Proto-Izu-Mariana arc against the Honshu arc, Japan, since 15 Ma, to formulate current topics related to the Pacific-type orogeny. Since the first pioneering work by Sugimura (1972), which placed the plate boundary on-land Japan north of the Izu peninsula collision-accretion tectonics of the Proto-Izu-Mariana arc against the Honshu arc started to be extensively investigated through multi-disciplinary methods. Aoike (1999) proposed a comprehensive scenario of five successive accretions of Proto-Izu to the present, with all bounded by top- and bottom-faults since 15 Ma. Oroclinal bending of the Honshu arc began simultaneously with the collision and indentation of the Proto-Izu arc, as is well documented by the Neogene paleomagnetic declination of sedimentary rocks in the Kanto district (Niitsuma et al., see summary, 1989). The entire Izu-Mariana arc is subducting without any accretion now under Honshu, as is well documented by seismic tomography imaging by Hasegawa et al. (see Hasegawa et al., 2010). Tanzawa Tonalite–Trondhjemite–Granodiolite (TTG) pluton intruded into the already-accreted Tanzawa Group at 4 Ma (Tani et al., 2010), resulting in a contact metamorphic aureole ranging from amphibolite through greenschist to zeolite facies at a depth of ca. 10 km, and tectonically denudated along the Kan-nawa fault to be exposed on the surface by 1.0 Ma. The Proto-Izu-Mariana arc was built on the Pre-Paleogene, presumably Cretaceous MORB crust on which boninitic and fractionated silicic lava flows accumulated since 48 Ma, which is older than the bending of the Hawaii–Emperor seamount chain at 43 Ma. Hence, we need another plate off Proto-Ogasawara arc, called North New Guinea plate (Seno, 1985), and ridge subduction underneath Ogasawara arc to initiate subduction magmatism. Both calk-alkali rock series (CA) and coeval blueschist–eclogite rocks are exposed along the inner wall of the Izu–Mariana trench. This strongly suggests that extensive tectonic erosion occurred, resulting in the coexistence of two rock units that were formed 100-150 km from each other horizontally, i.e., rocks on the volcanic front and rocks on the descending oceanic slab. Petrogenesis of juvenile crust and TTG rocks are being debate with two models: basaltic arc magma fractionation (Kawate and Arima, 1998) and delaminated mafic lower crust (Nakajima and Arima, 1998). In the latter model, a difficulty is derived from density at the Moho depth and small size of delaminated residue. Instead, extensive tectonic erosion that transports not only the upper crust but also the lower mafic crust together with parts of the descending slab could be more plausible than these two models in order to present a thinner mafic lower crust.

著者

尾方 隆幸

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.120, no.5, pp.846-852, 2011-10-25 (Released:2012-01-17)

参考文献数

13

被引用文献数

4 5

---

The Ryukyu Islands strongly promote the geodiversity of Japan. A sub-tropic natural environment forms coral reefs and various landscapes linked to surface processes, active tectonics, and Quaternary environmental changes. Field excursions permit geoscientific observations of such landscapes, which reveal geodiversity and a framework for geotourism. Fieldwork was undertaken to arrange an educational program for conserving geodiversity and for geotourism. This program demonstrates the geodiversity and geotourism of the Ryukyu Islands, in terms of geomorphology and related disciplines. Basic geotours observe landforms and sediments, and consider geomorphic processes: weathering, erosion, transport, sedimentation, uplift, and sea level change. Applied geotours observe landscapes indicating global environmental problems, such as global warming, climate change, sea level rise, acid rain, and ecosystem based on geomorphology and related geosciences. For instance, acid rain accelerates the chemical weathering (decomposition) of limestone and erosion of karst landforms that are distributed widely on raised coral reefs. Such geotours require both scientific and attractive geostories managed by geoscientists. Japanese geoscientists should propose models for geotours derived from field excursions for landscape observations that will contribute to education, conservation, and geotourism.