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.118.646
K. Hanaoka
The purpose of this article is to evaluate a social simulation using a dynamic spatial microsimulation model for predicting demolitions of Kyomachiya, which are traditional wooden townhouses and core elements of the historical landscape of Kyoto City, Japan. This model is also applied to estimate the number of Kyomachiya surviving when preservation policies are introduced. The results are summarized as follows: (a)Spatially disaggregated synthetic microdata of Kyomachiya residents were constructed by combining multiple existing datasets in a manner whereby the sums of synthetic microdata agree with those of census datasets. Using synthetic microdata allows us to analyze detailed household demographics and the process of Kyomachiya demolitions at small area and individual levels. (b)Decision-making units such as individuals, households, and Kyomachiya can be modeled in the same way that they exist, behave, and interact with each other in the real world using object-oriented modeling. Another merit is that re-using and extending classes are possible due to object-oriented architecture. (c)The results of simulations show that, during the next 15 years, only 67.3% of Kyomachiya will be preserved and the proportion of the population aged 60 and over will increase from 43.9% to 51.6%. On the other hand, when a comprehensive preservation policy is implemented, those numbers are reduced to 82.5% and 49.2%, respectively. In this manner, a dynamic spatial microsimulation model is useful for understanding the process and cause-andeffect of Kyomachiya demolitions under the status quo. Furthermore, what-if simulations on the basis of Kyomachiya preservation policies help to evaluate which policy is most effective for reducing the number of demolitions.
{"title":"Social Simulation Using a Dynamic Spatial Microsimulation Model: Analysis of Kyomachiya (Traditional Wooden Townhouse) Demolitions","authors":"K. Hanaoka","doi":"10.5026/JGEOGRAPHY.118.646","DOIUrl":"https://doi.org/10.5026/JGEOGRAPHY.118.646","url":null,"abstract":"The purpose of this article is to evaluate a social simulation using a dynamic spatial microsimulation model for predicting demolitions of Kyomachiya, which are traditional wooden townhouses and core elements of the historical landscape of Kyoto City, Japan. This model is also applied to estimate the number of Kyomachiya surviving when preservation policies are introduced. The results are summarized as follows: (a)Spatially disaggregated synthetic microdata of Kyomachiya residents were constructed by combining multiple existing datasets in a manner whereby the sums of synthetic microdata agree with those of census datasets. Using synthetic microdata allows us to analyze detailed household demographics and the process of Kyomachiya demolitions at small area and individual levels. (b)Decision-making units such as individuals, households, and Kyomachiya can be modeled in the same way that they exist, behave, and interact with each other in the real world using object-oriented modeling. Another merit is that re-using and extending classes are possible due to object-oriented architecture. (c)The results of simulations show that, during the next 15 years, only 67.3% of Kyomachiya will be preserved and the proportion of the population aged 60 and over will increase from 43.9% to 51.6%. On the other hand, when a comprehensive preservation policy is implemented, those numbers are reduced to 82.5% and 49.2%, respectively. In this manner, a dynamic spatial microsimulation model is useful for understanding the process and cause-andeffect of Kyomachiya demolitions under the status quo. Furthermore, what-if simulations on the basis of Kyomachiya preservation policies help to evaluate which policy is most effective for reducing the number of demolitions.","PeriodicalId":356213,"journal":{"name":"Chigaku Zasshi (jounal of Geography)","volume":"11 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":"114813714","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.119.427
M. Satoh
We have developed a new type of atmospheric general circulation model, “global cloud-resolving model” which covers the globe with a mesh size of a few kilometers. This model enables us to directly simulate the multi-scale structure of the cloud systems of the Earth from meso-scale to planetary scale cloud disturbances. In particular, tropical cloud systems including Madden-Julian Oscillation and tropical cyclogenesis are realistically simulated by the global cloud-resolving model. It is contrasted with current atmospheric general circulation models that are used for climate projections, because they cannot simulate realistic tropical cloud disturbances in general. The global cloud-resolving model not only reproduces the geographical distribution of cloud-precipitation systems and their diurnal to intra-seasonal variabilities, but also more physically reproduces cloud properties by explicitly implementing cloud microphysical processes. It is expected that the global cloud-resolving model will play a major role in climate simulations, and that future climate projections will be more reliable than current approaches.
{"title":"Climate Study Using a Global Cloud-resolving Model","authors":"M. Satoh","doi":"10.5026/JGEOGRAPHY.119.427","DOIUrl":"https://doi.org/10.5026/JGEOGRAPHY.119.427","url":null,"abstract":"We have developed a new type of atmospheric general circulation model, “global cloud-resolving model” which covers the globe with a mesh size of a few kilometers. This model enables us to directly simulate the multi-scale structure of the cloud systems of the Earth from meso-scale to planetary scale cloud disturbances. In particular, tropical cloud systems including Madden-Julian Oscillation and tropical cyclogenesis are realistically simulated by the global cloud-resolving model. It is contrasted with current atmospheric general circulation models that are used for climate projections, because they cannot simulate realistic tropical cloud disturbances in general. The global cloud-resolving model not only reproduces the geographical distribution of cloud-precipitation systems and their diurnal to intra-seasonal variabilities, but also more physically reproduces cloud properties by explicitly implementing cloud microphysical processes. It is expected that the global cloud-resolving model will play a major role in climate simulations, and that future climate projections will be more reliable than current approaches.","PeriodicalId":356213,"journal":{"name":"Chigaku Zasshi (jounal of Geography)","volume":"121 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":"116046880","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.119.585
Shin'ichi Mori, K. Fujioka, M. Arima
This paper describes topographic and geologic features of the northern part of the Sagami Trough and surrounding areas, off central Japan, and discusses the role of the Philippine Sea plate in the development of the trough's topography and fault systems. In the study area, the Okinoyama Bank Chain, Miura Basin Chain, and several spurs are aligned NW-SE parallel to the axis of the Sagami Trough. We examine the lithological compositions of rock and piston-core samples collected from the R/V Hyper Dolphin during Japan Agency for Marine-Earth Science and Technology dives 906 and 907, and rock samples dredged during Ocean Research Institute of the University of Tokyo cruise KT88. These samples represent mainly volcaniclastic rocks derived from the volcanoes of the Izu arc and subordinate amounts of terrigenous sediment interpreted as derived from the Sagami and Sakawa Rivers. The topographic and geologic data define five fault systems in the eastern part of the northern Sagami Trough. We interpret the Okinoyama Bank Chain as an elevated outer-ridge belt developed at the landward slope of the plate boundary, which is represented by NW-SE trending right-lateral reverse faults (F1). At the northeast side of the Okinoyama Bank Chain, NW-SE trending normal faults (F2) formed and the Miura Basin Chain developed. The cutting relationship between these fault sets suggests that the collision of the Izu arc with the Honshu arc imposed a right-lateral shear stress field on the Okinoyama Bank and Miura Basin Chains. This shear stress may have led to clockwise rotation of these tectonic blocks formation of a set of NE-SW trending left-lateral reverse faults (F3). The initiation of the Izu block collision is interpreted as a turning point after which the northward motion of the Philippine Sea plate abruptly changed to NW and the Sagami Bay area came under a NW-SE compressional stress field, which resulted in the development of E-W trending right-lateral faults (F4) in the narrow shelf off the Miura Peninsula. A number of spurs subsequently developed along these faults. Finally, NNE-SSW trending right-lateral normal faults (F5) developed under an E-W extensional stress field, which caused the formation of N-S trending topographic depressions such as Tokyo Canyon and submarine highs such as Okinoyama and Oiso Spur.
{"title":"Topographical and Geological Characteristics of the Sagami Trough: Development of Five Fault Systems in the Northern Part of the Sagami Trough","authors":"Shin'ichi Mori, K. Fujioka, M. Arima","doi":"10.5026/JGEOGRAPHY.119.585","DOIUrl":"https://doi.org/10.5026/JGEOGRAPHY.119.585","url":null,"abstract":"This paper describes topographic and geologic features of the northern part of the Sagami Trough and surrounding areas, off central Japan, and discusses the role of the Philippine Sea plate in the development of the trough's topography and fault systems. In the study area, the Okinoyama Bank Chain, Miura Basin Chain, and several spurs are aligned NW-SE parallel to the axis of the Sagami Trough. We examine the lithological compositions of rock and piston-core samples collected from the R/V Hyper Dolphin during Japan Agency for Marine-Earth Science and Technology dives 906 and 907, and rock samples dredged during Ocean Research Institute of the University of Tokyo cruise KT88. These samples represent mainly volcaniclastic rocks derived from the volcanoes of the Izu arc and subordinate amounts of terrigenous sediment interpreted as derived from the Sagami and Sakawa Rivers. The topographic and geologic data define five fault systems in the eastern part of the northern Sagami Trough. We interpret the Okinoyama Bank Chain as an elevated outer-ridge belt developed at the landward slope of the plate boundary, which is represented by NW-SE trending right-lateral reverse faults (F1). At the northeast side of the Okinoyama Bank Chain, NW-SE trending normal faults (F2) formed and the Miura Basin Chain developed. The cutting relationship between these fault sets suggests that the collision of the Izu arc with the Honshu arc imposed a right-lateral shear stress field on the Okinoyama Bank and Miura Basin Chains. This shear stress may have led to clockwise rotation of these tectonic blocks formation of a set of NE-SW trending left-lateral reverse faults (F3). The initiation of the Izu block collision is interpreted as a turning point after which the northward motion of the Philippine Sea plate abruptly changed to NW and the Sagami Bay area came under a NW-SE compressional stress field, which resulted in the development of E-W trending right-lateral faults (F4) in the narrow shelf off the Miura Peninsula. A number of spurs subsequently developed along these faults. Finally, NNE-SSW trending right-lateral normal faults (F5) developed under an E-W extensional stress field, which caused the formation of N-S trending topographic depressions such as Tokyo Canyon and submarine highs such as Okinoyama and Oiso Spur.","PeriodicalId":356213,"journal":{"name":"Chigaku Zasshi (jounal of Geography)","volume":"43 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":"116234089","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.117.5
J. Kasahara, S. Unou, K. Tsuruga, T. Ike, K. Koda
The thickness of the Moho transition zone (MTZ) at the boundary between the Earth's crust and the subjacent mantle has a significant effect on seismic responses from the Moho. We examined the seismic characteristics of Moho reflections (hereafter PmP) using Multi-Channel Seismic (MCS) records obtained from high-quality seismic experiments in the western Pacific by Japan Oil, Gas and Metals National Corporation (JOGMEC). The MCS records show clear reflections at ~6⊖10 km in depth from the ocean bottom in the north and south of Ogasawara Plateau; however, considering horizontal variations in PmP intensity, the nature of the MTZ varies by location. In seismic profile D00-D, across Ogasawara Plateau in the N-S direction, the PmP abruptly disappears far from the nearby seamount where the overlain sedimentary section shows less change. In another case, shown in D00-C located 130 km west of D00-D, the PmP clearly shows a high-amplitude continuous reflection near the seamount's flank. Data acquisition is relatively constant for the Ogasawara MCS reflection lines ; therefore, the difference in PmP intensity between D00-D and D00-C might relate to the nature of the Moho. We calculated synthetic seismograms to evaluate the effects of MTZ thickness on seismic reflection records. The results suggest that if the thickness of the Moho transition zone is less than 1 km for the dominant frequency of 4 Hz, then PmP can be observed with the current MCS survey equipment. If the dominant frequency of the MCS reflection survey is~15 Hz, penetrating down to the Moho depth, then the thickness of the Moho required to identify the PmP should be less than a few hundred meters. Moreover, anisotropy assuming a strong olivine preferred orientation in peridotite might affect the change of PmP intensity. The MCS reflection records in the western Pacific and the western Philippine Sea Basin suggest that the thickness of MTZ varies from ~ 100 m to more than a few kilometers. This is consistent with petrological observations in * 日本大陸棚調査(株) ** 地球科学総合研究所 *** 独立行政法人 石油天然ガス・金属鉱物資源機構 **** 静岡大学理学部地球科学教室 * Japan Continental Shelf Survey Co. Ltd. ** JGI, Inc. *** Japan Oil, Gas and Metals National Corporation (JOGMEC) **** Institute of Geosciences, Shizuoka University
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