Pub Date : 1900-01-01DOI: 10.5026/JGEOGRAPHY.118.1186
J. Ishibashi, S. Nakai, S. Toyoda, H. Kumagai, T. Noguchi, O. Ishizuka
As our understanding of seafloor hydrothermal systems grows, we recognize they are not always stable and sometimes show dramatic changes. In this review, the authors present a compilation of geochemical and geochronological studies that are helpful when investigating the evolving processes of submarine hydrothermal systems. Chapter II describes the systematics and methodology of three dating techniques with discussions on their application to minerals formed by seafloor hydrothermal activities. The K-Ar (Ar-Ar) technique is popular for dating igneous rocks, but it is not appropriate for dating hydrothermal minerals because potassium is a trace component of sulfide/sulfate minerals. Following recent progress, micro-analytical techniques applying laser fusion are applicable for dating fluid inclusions and/or hydrothermal alteration minerals, which could provide important geochronological information. Uranium and thorium series disequilibrium dating have been employed for previous geochronological studies of hydrothermal minerals obtained from submarine ore deposits. To cover a wide time range, it is necessary to use various combinations of parent and daughter nuclides. Applying ESR dating to hydrothermal minerals is a rather new challenge. Although it needs several investigations to establish the methodology, it could be a useful rapid dating technique for a time range of less than one thousand years. Chapter III introduces studies focusing on the evolution of seafloor hydrothermal activities over a short time scale (one week to a few years). Detection of event plumes associated with seafloor lava eruption brought an awareness of episodic hydrothermal activity triggered by magmatic perturbation. Subsequent dive studies revealed evolving geochemical processes, such as major changes of volatiles and elemental species concentrations of venting fluid. With remote real-time monitoring of acoustic T-waves generated by seafloor seismic activities, event detection and response cruises have been conducted successfully to investigate various evolving processes in more detail. Chapter IV introduces studies focusing on the evolution of seafloor hydrothermal activities over a long time scale (tens of thousands of years). Radiometric dating studies of hydrothermal minerals such as sulfide and manganese oxide collected from the TAG mound, which is one of the largest hydrothermal mound structures, reveal an age distribution over at least 15000 years separated by quiescent intervals lasting up to 2000 years. On slow spreading ridges such as the Mid-Atlantic ridge, major fracture systems focus the hydrothermal discharge at one place for more than one thousand years with repeated reactivation. In Chapter V, the authors discuss the direction of future studies. Although hydrothermal systems on mid-oceanic ridges have been well studied, those related to arc-backarc magmatic activities could provide more appropriate fields for studying the evolutionary process of submari
{"title":"Geochemical and Geochronological Studies on the Evolution of Submarine Hydrothermal Systems","authors":"J. Ishibashi, S. Nakai, S. Toyoda, H. Kumagai, T. Noguchi, O. Ishizuka","doi":"10.5026/JGEOGRAPHY.118.1186","DOIUrl":"https://doi.org/10.5026/JGEOGRAPHY.118.1186","url":null,"abstract":"As our understanding of seafloor hydrothermal systems grows, we recognize they are not always stable and sometimes show dramatic changes. In this review, the authors present a compilation of geochemical and geochronological studies that are helpful when investigating the evolving processes of submarine hydrothermal systems. Chapter II describes the systematics and methodology of three dating techniques with discussions on their application to minerals formed by seafloor hydrothermal activities. The K-Ar (Ar-Ar) technique is popular for dating igneous rocks, but it is not appropriate for dating hydrothermal minerals because potassium is a trace component of sulfide/sulfate minerals. Following recent progress, micro-analytical techniques applying laser fusion are applicable for dating fluid inclusions and/or hydrothermal alteration minerals, which could provide important geochronological information. Uranium and thorium series disequilibrium dating have been employed for previous geochronological studies of hydrothermal minerals obtained from submarine ore deposits. To cover a wide time range, it is necessary to use various combinations of parent and daughter nuclides. Applying ESR dating to hydrothermal minerals is a rather new challenge. Although it needs several investigations to establish the methodology, it could be a useful rapid dating technique for a time range of less than one thousand years. Chapter III introduces studies focusing on the evolution of seafloor hydrothermal activities over a short time scale (one week to a few years). Detection of event plumes associated with seafloor lava eruption brought an awareness of episodic hydrothermal activity triggered by magmatic perturbation. Subsequent dive studies revealed evolving geochemical processes, such as major changes of volatiles and elemental species concentrations of venting fluid. With remote real-time monitoring of acoustic T-waves generated by seafloor seismic activities, event detection and response cruises have been conducted successfully to investigate various evolving processes in more detail. Chapter IV introduces studies focusing on the evolution of seafloor hydrothermal activities over a long time scale (tens of thousands of years). Radiometric dating studies of hydrothermal minerals such as sulfide and manganese oxide collected from the TAG mound, which is one of the largest hydrothermal mound structures, reveal an age distribution over at least 15000 years separated by quiescent intervals lasting up to 2000 years. On slow spreading ridges such as the Mid-Atlantic ridge, major fracture systems focus the hydrothermal discharge at one place for more than one thousand years with repeated reactivation. In Chapter V, the authors discuss the direction of future studies. Although hydrothermal systems on mid-oceanic ridges have been well studied, those related to arc-backarc magmatic activities could provide more appropriate fields for studying the evolutionary process of submari","PeriodicalId":356213,"journal":{"name":"Chigaku Zasshi (jounal of Geography)","volume":"3 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"1900-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"114225161","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 1900-01-01DOI: 10.5026/JGEOGRAPHY.118.1237
Masayuki Oishi
Azumaya Volcano is a stratovolcano located slightly at the back arc side of the volcanic front in central Japan. Previous studies led to the publication of a geologic map and the K-Ar age of some lavas, but the stratigraphic relationships of the volcanic products and their eruptive history are still unclear. Some tephras that possibly erupted from Azumaya Volcano have been found in the North Kanto region. But, there are relatively few descriptions of pyroclastic products. This study presents the lithological and petrographical characteristics of products from Azumaya Volcano, especially the refractive indices of phenocrysts, because these data are very important for identifying products and reconstructing eruptive history. This study examined the adequacy of the identification of tephras of the previous studies and whether the source of these tephras is Azumaya Volcano from the distribution of tephra and comparing mineral composition and refractive indices of orthopyroxene, plagioclase, hornblende, and cummingtonite phenocrysts in proximal products and distal tephras. Because the refractive indices of the phenocrysts in the products distributed in Azumaya Volcano have unique characteristics, refractive indices are useful for identifying and correlating products. Distal tephras correlated in previous studies have similar characteristics, increasing the probability that the source of the SgP.2 tephra bed is the Azumaya Volcano. In addition, the volume of the SgP.2 tephra bed was calculated to be 0.85 km3 dense-rock equivalents (DRE) using its distribution.
{"title":"Description of Characteristics of Volcanic Products and Distribution of Tephras from Azumaya Volcano, Central Japan","authors":"Masayuki Oishi","doi":"10.5026/JGEOGRAPHY.118.1237","DOIUrl":"https://doi.org/10.5026/JGEOGRAPHY.118.1237","url":null,"abstract":"Azumaya Volcano is a stratovolcano located slightly at the back arc side of the volcanic front in central Japan. Previous studies led to the publication of a geologic map and the K-Ar age of some lavas, but the stratigraphic relationships of the volcanic products and their eruptive history are still unclear. Some tephras that possibly erupted from Azumaya Volcano have been found in the North Kanto region. But, there are relatively few descriptions of pyroclastic products. This study presents the lithological and petrographical characteristics of products from Azumaya Volcano, especially the refractive indices of phenocrysts, because these data are very important for identifying products and reconstructing eruptive history. This study examined the adequacy of the identification of tephras of the previous studies and whether the source of these tephras is Azumaya Volcano from the distribution of tephra and comparing mineral composition and refractive indices of orthopyroxene, plagioclase, hornblende, and cummingtonite phenocrysts in proximal products and distal tephras. Because the refractive indices of the phenocrysts in the products distributed in Azumaya Volcano have unique characteristics, refractive indices are useful for identifying and correlating products. Distal tephras correlated in previous studies have similar characteristics, increasing the probability that the source of the SgP.2 tephra bed is the Azumaya Volcano. In addition, the volume of the SgP.2 tephra bed was calculated to be 0.85 km3 dense-rock equivalents (DRE) using its distribution.","PeriodicalId":356213,"journal":{"name":"Chigaku Zasshi (jounal of Geography)","volume":"34 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"1900-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"123828163","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 1900-01-01DOI: 10.5026/JGEOGRAPHY.119.153
Y. Ogawa
The present situation of studies on accretionary wedge formation and related phenomena is briefly summarized from various perspectives, ranging from theories, model experiments, observations on land and submarine, exhumation of high-pressure metamorphic rocks, fluid seepage, stress field, and asperity. Future perspectives are also considered from such recent results with potential areas of study. Gravity acts ubiquitously—everywhere and at all times—on the Earth's materials, so the role of gravity is also accounted for in the wedge development.
{"title":"Various Perspectives on Accretionary Wedge Formation and Related Phenomena","authors":"Y. Ogawa","doi":"10.5026/JGEOGRAPHY.119.153","DOIUrl":"https://doi.org/10.5026/JGEOGRAPHY.119.153","url":null,"abstract":"The present situation of studies on accretionary wedge formation and related phenomena is briefly summarized from various perspectives, ranging from theories, model experiments, observations on land and submarine, exhumation of high-pressure metamorphic rocks, fluid seepage, stress field, and asperity. Future perspectives are also considered from such recent results with potential areas of study. Gravity acts ubiquitously—everywhere and at all times—on the Earth's materials, so the role of gravity is also accounted for in the wedge development.","PeriodicalId":356213,"journal":{"name":"Chigaku Zasshi (jounal of Geography)","volume":"22 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"1900-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"125286512","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
{"title":"Pictorial 2: Secular Variations of Annual Net Balance for the 17 Glacier-Covered Regions and Annual Accumulation of Greenland Ice Sheet","authors":"A. Ohmura","doi":"10.5026/JGEOGRAPHY.119.IV","DOIUrl":"https://doi.org/10.5026/JGEOGRAPHY.119.IV","url":null,"abstract":"地球上の雪氷圏の状況を把握することは，気候変動を探るための重要な鍵となる．世界の各氷河にはそれぞれ独特の地域性があり，質量収支の経年変動も異なるが，その変動には総合的にみてどのような傾向が認められるだろうか（図1）．また，南極氷床に次ぐ体積を有するグリーンランド氷床の質量収支はどうだろうか．寒候期における涵養量（図2；その変動要因については「序説」（山川ほか, 2010）参照）と，主として暖候期における消耗量（気圧配置・天候によるところ大で，近年増加傾向）とのバランスはどのような状況なのだろうか．それらの実態をこの口絵と本文（大村, 2010）で明らかにしていきたい．","PeriodicalId":356213,"journal":{"name":"Chigaku Zasshi (jounal of Geography)","volume":"1 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"1900-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"133507342","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 1900-01-01DOI: 10.5026/JGEOGRAPHY.117.937
H. Hirano, K. Tanabe
{"title":"Report on the Seventh International Symposium, \"Cephalopods-Present and Past\"","authors":"H. Hirano, K. Tanabe","doi":"10.5026/JGEOGRAPHY.117.937","DOIUrl":"https://doi.org/10.5026/JGEOGRAPHY.117.937","url":null,"abstract":"","PeriodicalId":356213,"journal":{"name":"Chigaku Zasshi (jounal of Geography)","volume":"66 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"1900-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"134103788","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 1900-01-01DOI: 10.5026/JGEOGRAPHY.118.233
R. Nishii
Linear depressions and associated uphill-facing scarps, which result from the gravitational deformation of rock slopes, have widely developed along the main ridge in the Mt. Eboshi area, northern Japanese Alps. The role of landslides in the development of linear depressions in granitic rocks is evaluated from the distribution of linear depressions, characteristics of landslides (form, magnitude and frequency) and characteristics of bedrock (geology, discontinuity, degree of weathering and rock mass strength). The morphology of landslides and changes in the areas of landslides from 1958 to 2004 are compared to the development of linear depressions. Most linear depressions lie parallel to the main ridge and the strike of major rock joints, although their distribution differs between the northern and southern areas. In the northern study area, linear depressions are concentrated on the gentle upper slopes and landslides have intensively occurred on the lower slopes. In contrast, the southern area shows sporadic occurrences of linear depressions on the middle to upper slopes and has experienced only minor landslides. The intensive occurrences of landslides and linear depressions in the northern area are considered to result basically from debuttress and stress-release caused by the landslide activity. In addition, the lower rock mass strength in the northern study area also promotes the development of linear depressions.
{"title":"The Role of Landslides in Controlling the Distribution of Linear Depressions on Granitic Mountains in the Hida Range","authors":"R. Nishii","doi":"10.5026/JGEOGRAPHY.118.233","DOIUrl":"https://doi.org/10.5026/JGEOGRAPHY.118.233","url":null,"abstract":"Linear depressions and associated uphill-facing scarps, which result from the gravitational deformation of rock slopes, have widely developed along the main ridge in the Mt. Eboshi area, northern Japanese Alps. The role of landslides in the development of linear depressions in granitic rocks is evaluated from the distribution of linear depressions, characteristics of landslides (form, magnitude and frequency) and characteristics of bedrock (geology, discontinuity, degree of weathering and rock mass strength). The morphology of landslides and changes in the areas of landslides from 1958 to 2004 are compared to the development of linear depressions. Most linear depressions lie parallel to the main ridge and the strike of major rock joints, although their distribution differs between the northern and southern areas. In the northern study area, linear depressions are concentrated on the gentle upper slopes and landslides have intensively occurred on the lower slopes. In contrast, the southern area shows sporadic occurrences of linear depressions on the middle to upper slopes and has experienced only minor landslides. The intensive occurrences of landslides and linear depressions in the northern area are considered to result basically from debuttress and stress-release caused by the landslide activity. In addition, the lower rock mass strength in the northern study area also promotes the development of linear depressions.","PeriodicalId":356213,"journal":{"name":"Chigaku Zasshi (jounal of Geography)","volume":"172 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"1900-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"134104558","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 1900-01-01DOI: 10.5026/JGEOGRAPHY.117.387
H. Une
Recently, the circumstances of the world's national mapping organizations have changed due to various factors such as the computerization of mapping technologies, development of GIS and the Internet, global environmental problems and government restructuring. The new roles of national mapping organizations in the era of GIS should be to: 1) provide and maintain a unique framework for exchanging and sharing geo-spatial data as a social infrastructure and 2) contribute to sustainable development by providing accurate, current geographic information on the global environment. The Geographical Survey Institute (GSI), the national mapping organization of the Japanese Government, has adopted such roles by promoting the Digital Japan Project and the Global Mapping Project. GSI developed the Denshi Kokudo Web System to provide a platform for various geo-spatial data applying web-mapping technologies to realize the Digital Japan concept. This system enables users to dispatch original geographic information without having to prepare background map data. GSI also acts as the secretariat of the International Steering Committee for Global Mapping. The Global Mapping Project develops digital geographic information covering the earth's surface at 1km resolution with standardized specifications available to all through cooperation among national mapping organizations around the world. This paper outlines the background, history and current status of these projects.
{"title":"Digital Japan and Global Mapping: Role of National Mapping Organizations in the Era of GIS","authors":"H. Une","doi":"10.5026/JGEOGRAPHY.117.387","DOIUrl":"https://doi.org/10.5026/JGEOGRAPHY.117.387","url":null,"abstract":"Recently, the circumstances of the world's national mapping organizations have changed due to various factors such as the computerization of mapping technologies, development of GIS and the Internet, global environmental problems and government restructuring. The new roles of national mapping organizations in the era of GIS should be to: 1) provide and maintain a unique framework for exchanging and sharing geo-spatial data as a social infrastructure and 2) contribute to sustainable development by providing accurate, current geographic information on the global environment. The Geographical Survey Institute (GSI), the national mapping organization of the Japanese Government, has adopted such roles by promoting the Digital Japan Project and the Global Mapping Project. GSI developed the Denshi Kokudo Web System to provide a platform for various geo-spatial data applying web-mapping technologies to realize the Digital Japan concept. This system enables users to dispatch original geographic information without having to prepare background map data. GSI also acts as the secretariat of the International Steering Committee for Global Mapping. The Global Mapping Project develops digital geographic information covering the earth's surface at 1km resolution with standardized specifications available to all through cooperation among national mapping organizations around the world. This paper outlines the background, history and current status of these projects.","PeriodicalId":356213,"journal":{"name":"Chigaku Zasshi (jounal of Geography)","volume":"91 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"1900-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"131682026","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 1900-01-01DOI: 10.5026/JGEOGRAPHY.119.1
Chuki Hongo
The importance of Holocene sea-level change has long been a central theme of Quaternary Science. Holocene sea-level records provide direct evidence of the progress of the melting of the ice sheet during the Holocene. Although the correlation between ice and ocean volumes is incontrovertible, casual links are commonly obscured. Some regional studies of coral-reef sites based on analyses of boring cores have been carried out from reef flat to reef slope at present-day reefs, demonstrating a long-term (1000-10000 years) and large-amplitude (10-100 m) melt-water history. However, short-term (< 100 years) and small-scale (< 1 m) sea-level changes that detail past sea-level records and play a major role in predicting sea-level fluctuations in the near future are not observed from reef cores. This paper is based principally on a re-examination of sea-level records from the literature and presents the following suggestions to reconstruct high-resolution Holocene sea-level records: (1) Identifying species from boring core samples is effective to reconstruct sea-level changes more precisely during the Holocene. (2) Relative abundance of data for each species is essential to determine position and course of sea-level curve within the envelope of their living depths. (3) The accuracy of reconstructing the sea-level record depends on the distribution pattern of corals; the vertical distribution in a present-day reef obtained from a site close to a given boring site is all that is required. The sea-level curve based on agreement with the above requirement is characterized by smaller fluctuations (±0.5 - ±2.5 m) during the Holocene, thus studies on the high-resolution sea-level record will provide predictions for research on the spatial and temporal histories of sea-level change to Holocene sciences and management of conservation of land in the near future.
{"title":"High-resolution Holocene Sea-level Change Based on Coral Reefs and Hermatypic Corals","authors":"Chuki Hongo","doi":"10.5026/JGEOGRAPHY.119.1","DOIUrl":"https://doi.org/10.5026/JGEOGRAPHY.119.1","url":null,"abstract":"The importance of Holocene sea-level change has long been a central theme of Quaternary Science. Holocene sea-level records provide direct evidence of the progress of the melting of the ice sheet during the Holocene. Although the correlation between ice and ocean volumes is incontrovertible, casual links are commonly obscured. Some regional studies of coral-reef sites based on analyses of boring cores have been carried out from reef flat to reef slope at present-day reefs, demonstrating a long-term (1000-10000 years) and large-amplitude (10-100 m) melt-water history. However, short-term (< 100 years) and small-scale (< 1 m) sea-level changes that detail past sea-level records and play a major role in predicting sea-level fluctuations in the near future are not observed from reef cores. This paper is based principally on a re-examination of sea-level records from the literature and presents the following suggestions to reconstruct high-resolution Holocene sea-level records: (1) Identifying species from boring core samples is effective to reconstruct sea-level changes more precisely during the Holocene. (2) Relative abundance of data for each species is essential to determine position and course of sea-level curve within the envelope of their living depths. (3) The accuracy of reconstructing the sea-level record depends on the distribution pattern of corals; the vertical distribution in a present-day reef obtained from a site close to a given boring site is all that is required. The sea-level curve based on agreement with the above requirement is characterized by smaller fluctuations (±0.5 - ±2.5 m) during the Holocene, thus studies on the high-resolution sea-level record will provide predictions for research on the spatial and temporal histories of sea-level change to Holocene sciences and management of conservation of land in the near future.","PeriodicalId":356213,"journal":{"name":"Chigaku Zasshi (jounal of Geography)","volume":"109 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"1900-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"132102035","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 1900-01-01DOI: 10.5026/JGEOGRAPHY.117.637
I. Suzuki, Y. Ota, T. Azuma
The area at the middle to lower reaches of the Shinano River is a well-known major Neogene thrust and fold belt in Japan. Deformed fluvial terraces, such as anticlinal ridges, synclinal valleys, and fault scarps along the Shinano River, provide a good record of recent tectonic activity in this belt. A large exposure (ca. 150 m long, and up to 10 m deep) was excavated by construction work on the eastern limb of the Tokimizu anticline, giving us an opportunity to observe various types of fault geometry. Four faults—F1, F2, F3, and F4—cut terrace deposits of ca. 130-150 ka (Koshijippara terrace) and underlying early Pleistocene Uonuma Formation. The westernmost fault, F1 is represented as a remarkable flexure dipping westward, suggesting the presence of a low angle thrust underneath. We found a very low angle fault dipping eastward from an additional 2 m deep excavation. The vertical slip at F1, judged from the height difference with the top of the gravel bed (Bed V), is 12 m. In contrast, faults F2 and F3 to the east of F1 follow the bedding plane of the steeply dipping Uonuma Formation, and are high angle reverse faults with the upthrown side to the east. The vertical slip is 3-4 m for F2 and 7.5 m for F3. Profiling across these faults shows that F1 is clearly expressed as a deformed terrace, but the topographical expression of F2 and F3 is not necessarily obvious. Similar faults to F2 are recognized in the study area from observations of the other three large exposures. We classify the faults in the study area into three types: Type 1 is a blind fault assumed at the base of the eastern limb of the Tokimizu anticline. This fault might be the most important contributor to the formation of the major tectonic relief in the study area, although we have no data to prove the nature of the fault plane itself from this study. F1 fault, demonstrated by Type 2, was found for the first time in this study, and is a low angle reverse fault truncating the structure of the Uonuma Formation with a vertical slip rate of 0.1 m/ka. The Type 3 fault is represented by F2, F3, and F4, and these are interpreted to be flexural slip faults along the bedding plane of the Uonuma Formation. Repeated faulting is confirmed from the progressive deformation of different beds not only for the F1 fault (Type 2) but also for the fold-related secondary faults, F2 and F3. No faulting has occurred since ca. 7,500 years BP, however.
{"title":"Interpretation of Various Types of Active Fault on Large Exposures within a Fold and Thrust Belt at the Eastern Limb of the Tokimizu Anticline in Central Japan","authors":"I. Suzuki, Y. Ota, T. Azuma","doi":"10.5026/JGEOGRAPHY.117.637","DOIUrl":"https://doi.org/10.5026/JGEOGRAPHY.117.637","url":null,"abstract":"The area at the middle to lower reaches of the Shinano River is a well-known major Neogene thrust and fold belt in Japan. Deformed fluvial terraces, such as anticlinal ridges, synclinal valleys, and fault scarps along the Shinano River, provide a good record of recent tectonic activity in this belt. A large exposure (ca. 150 m long, and up to 10 m deep) was excavated by construction work on the eastern limb of the Tokimizu anticline, giving us an opportunity to observe various types of fault geometry. Four faults—F1, F2, F3, and F4—cut terrace deposits of ca. 130-150 ka (Koshijippara terrace) and underlying early Pleistocene Uonuma Formation. The westernmost fault, F1 is represented as a remarkable flexure dipping westward, suggesting the presence of a low angle thrust underneath. We found a very low angle fault dipping eastward from an additional 2 m deep excavation. The vertical slip at F1, judged from the height difference with the top of the gravel bed (Bed V), is 12 m. In contrast, faults F2 and F3 to the east of F1 follow the bedding plane of the steeply dipping Uonuma Formation, and are high angle reverse faults with the upthrown side to the east. The vertical slip is 3-4 m for F2 and 7.5 m for F3. Profiling across these faults shows that F1 is clearly expressed as a deformed terrace, but the topographical expression of F2 and F3 is not necessarily obvious. Similar faults to F2 are recognized in the study area from observations of the other three large exposures. We classify the faults in the study area into three types: Type 1 is a blind fault assumed at the base of the eastern limb of the Tokimizu anticline. This fault might be the most important contributor to the formation of the major tectonic relief in the study area, although we have no data to prove the nature of the fault plane itself from this study. F1 fault, demonstrated by Type 2, was found for the first time in this study, and is a low angle reverse fault truncating the structure of the Uonuma Formation with a vertical slip rate of 0.1 m/ka. The Type 3 fault is represented by F2, F3, and F4, and these are interpreted to be flexural slip faults along the bedding plane of the Uonuma Formation. Repeated faulting is confirmed from the progressive deformation of different beds not only for the F1 fault (Type 2) but also for the fold-related secondary faults, F2 and F3. No faulting has occurred since ca. 7,500 years BP, however.","PeriodicalId":356213,"journal":{"name":"Chigaku Zasshi (jounal of Geography)","volume":"117 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"1900-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"129113737","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 1900-01-01DOI: 10.5026/JGEOGRAPHY.117.1063
R. Kawamura
In the tropics, the El Nino-Southern Oscillation (ENSO) occupies a major part of the interannual air-sea interactive system. The ENSO plays a vital role in triggering the occurrence of extraordinary anomalous climates and weather not only in the tropics but also in the extratropical regions. In the South Asian monsoon region, the ENSO can influence the interannual variability of the monsoon system through at least two different impacts. During the decay phase of ENSO (from winter to summer) its delayed impact operates through large-scale air-sea interaction in the tropical Indian Ocean and land-surface hydrological processes over the Asian Continent, eventually bringing about change in summer monsoon activity in June and July. During the ENSO growth phase (from summer to winter), in contrast, its direct impact is considered to account for monsoon interannual variability especially in August and September. East Asian monsoon variability is also significantly affected by ENSO-related tropical forcing. Especially in early winter, ENSO-related anomalous convection can give rise to a change in the East Asian winter monsoon system through stationary Rossby wave propagation along the South Asian waveguide, but the remote response depends on the geographical configuration of the anomalous tropical convection. In summer, the ENSO's delayed impact is associated with excitation of an extratropical teleconnection, which causes anomalous weather in northeastern Asia. Midlatitude air-sea interactions and their potential impact on large-scale atmospheric circulations are also discussed. The coexistence of the East Asian winter monsoon flow and western boundary current makes air-sea heat exchanges in the Kuroshio extension very active. Due to enhanced baroclinicity and surface heat fluxes from the ocean, a number of extratropical cyclones tend to develop explosively in the vicinity of Japan. The activity of these extratropical cyclones contributes to the downstream development of upper-level teleconnections.
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