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.118.435
A. Ijiri
Submarine mud volcanoes are remarkable geological features on the seafloor, which are probably formed by mud breccia extruded from sub-seafloor sediment layers to the seafloor. Most of such volcanoes are found near the continental margin. The driving force of mud volcanism is thought to be unusually high pressure within the deep sedimentary layer and the release of that high pressure. It is important to know the origins of fluids in a mud volcano, because the production of low-density fluid and/or gas production in the deep sedimentary layer has been assumed to be one of the most probable sources of the pressure. Therefore, geochemical studies of pore fluids have been done at various mud volcanoes to identify the fluid origin. These studies revealed common chemical characteristics of the fluids, indicating the effects of dehydration of clay minerals. Also, the fluids contain hydrocarbon gases derived from thermocatalyte decomposition of sedimentary organic matter. These characteristics suggest that the mud volcano fluids must originate at a depth in the sedimentary layer greater than 2 km. In some mud volcano fields in the active continental margin, it is proposed that fluid in the mud volcano has migrated through faults from greater depths than the original depth of extruded sediments. Such fluid migration may be another source of high pressure in sedimentary layers.
{"title":"Origin of Fluid in Submarine Mud Volcanoes","authors":"A. Ijiri","doi":"10.5026/JGEOGRAPHY.118.435","DOIUrl":"https://doi.org/10.5026/JGEOGRAPHY.118.435","url":null,"abstract":"Submarine mud volcanoes are remarkable geological features on the seafloor, which are probably formed by mud breccia extruded from sub-seafloor sediment layers to the seafloor. Most of such volcanoes are found near the continental margin. The driving force of mud volcanism is thought to be unusually high pressure within the deep sedimentary layer and the release of that high pressure. It is important to know the origins of fluids in a mud volcano, because the production of low-density fluid and/or gas production in the deep sedimentary layer has been assumed to be one of the most probable sources of the pressure. Therefore, geochemical studies of pore fluids have been done at various mud volcanoes to identify the fluid origin. These studies revealed common chemical characteristics of the fluids, indicating the effects of dehydration of clay minerals. Also, the fluids contain hydrocarbon gases derived from thermocatalyte decomposition of sedimentary organic matter. These characteristics suggest that the mud volcano fluids must originate at a depth in the sedimentary layer greater than 2 km. In some mud volcano fields in the active continental margin, it is proposed that fluid in the mud volcano has migrated through faults from greater depths than the original depth of extruded sediments. Such fluid migration may be another source of high pressure in sedimentary layers.","PeriodicalId":356213,"journal":{"name":"Chigaku Zasshi (jounal of Geography)","volume":"6 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":"130359281","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.117.889
T. Matsu'ura, T. Ueki
The emplacement temperature of the AD915 Towada pyroclastic flow (To-a pyroclastic surge) is estimated from the emplacement temperature of crusts (pumices and a lithic fragment) within the surge deposits. The measured emplacement temperature of the pyroclastic surge varies vertically in the surge deposits. The lower part of the deposits shows low temperatures (300-500°C) because due to cooling by the cold ground surface. The middle part of the surge deposits, which was sandwiched by the lower and upper parts of the surge, shows high temperatures (350-680°C, mostly 620-650°C). The upper part of the surge deposits which was probably cooled by the atmosphere, shows moderate temperatures (less than 620°C, mostly 500-620°C).
{"title":"Emplacement Temperature and Cooling Process of the AD915 Pyroclastic Flow Deposits of Towada Volcano","authors":"T. Matsu'ura, T. Ueki","doi":"10.5026/JGEOGRAPHY.117.889","DOIUrl":"https://doi.org/10.5026/JGEOGRAPHY.117.889","url":null,"abstract":"The emplacement temperature of the AD915 Towada pyroclastic flow (To-a pyroclastic surge) is estimated from the emplacement temperature of crusts (pumices and a lithic fragment) within the surge deposits. The measured emplacement temperature of the pyroclastic surge varies vertically in the surge deposits. The lower part of the deposits shows low temperatures (300-500°C) because due to cooling by the cold ground surface. The middle part of the surge deposits, which was sandwiched by the lower and upper parts of the surge, shows high temperatures (350-680°C, mostly 620-650°C). The upper part of the surge deposits which was probably cooled by the atmosphere, shows moderate temperatures (less than 620°C, mostly 500-620°C).","PeriodicalId":356213,"journal":{"name":"Chigaku Zasshi (jounal of Geography)","volume":"166 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":"130966279","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.118.1131
Katsuhiko Suzuki, Kentaro Nakamura, Shingo Kato, A. Yamagishi
High-pressure and high-temperature hydrothermal experiments were initially conducted to measure mineral solubility and growth rate. Since then, considerable efforts have been made to characterize the alteration assemblages produced by a wide variety of hydrothermal fluids in different rock types. Based on such information, the conditions of sub-sea floor hydrothermal systems and the formation processes of ore deposits were investigated. These studies significantly depended on many important experimental results obtained by a batch (closed)-type experimental system which gives equilibrium conditions. On the other hand, attention has been also paid to a flow-type experimental system, because natural systems can not only constrained by experiments under equilibrium conditions but, more importantly, by non-equilibrium experiments. Recently, hydrothermal experiments were carried out to better understand interactions among rocks, hydrothermal fluids, and microbes. It has been suggested that microbial ecosystems might be widely distributed within oceanic crusts and be sustained by chemical energy derived from water-rock interactions. However, little is known about the flux of energy and materials involved in microbial activity within the crustal aquifer because of technical difficulties in accessing sub-seafloor environments. A flow-type cultivation system simulating natural hydrothermal environments including crustal aquifers could provide insights into the ecological significance of microorganisms and their contribution to the biogeochemical cycle in global oceans and crusts.
{"title":"Experimental Approach to Obtain a Comprehensive Understanding of the Biogeochemistry of a Seafloor Hydrothermal System","authors":"Katsuhiko Suzuki, Kentaro Nakamura, Shingo Kato, A. Yamagishi","doi":"10.5026/JGEOGRAPHY.118.1131","DOIUrl":"https://doi.org/10.5026/JGEOGRAPHY.118.1131","url":null,"abstract":"High-pressure and high-temperature hydrothermal experiments were initially conducted to measure mineral solubility and growth rate. Since then, considerable efforts have been made to characterize the alteration assemblages produced by a wide variety of hydrothermal fluids in different rock types. Based on such information, the conditions of sub-sea floor hydrothermal systems and the formation processes of ore deposits were investigated. These studies significantly depended on many important experimental results obtained by a batch (closed)-type experimental system which gives equilibrium conditions. On the other hand, attention has been also paid to a flow-type experimental system, because natural systems can not only constrained by experiments under equilibrium conditions but, more importantly, by non-equilibrium experiments. Recently, hydrothermal experiments were carried out to better understand interactions among rocks, hydrothermal fluids, and microbes. It has been suggested that microbial ecosystems might be widely distributed within oceanic crusts and be sustained by chemical energy derived from water-rock interactions. However, little is known about the flux of energy and materials involved in microbial activity within the crustal aquifer because of technical difficulties in accessing sub-seafloor environments. A flow-type cultivation system simulating natural hydrothermal environments including crustal aquifers could provide insights into the ecological significance of microorganisms and their contribution to the biogeochemical cycle in global oceans and crusts.","PeriodicalId":356213,"journal":{"name":"Chigaku Zasshi (jounal of Geography)","volume":"44 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":"128644070","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.117.124
Hiroshi P. Sato, H. Kumagai, N. Neo, Kentaro Nakamura
Mid-ocean ridge basalt (hereafter, MORB) is a final product of melt generated from the partial melting of mantle peridotite, following reaction with mantle and/or lower crustral rocks, fractionation at a shallower crust and other processes en route to seafloor. Therefore, it is difficult to estimate melting processes at the upper mantle solely from any investigations of MORB. In contrast to the restricted occurrence of peridotite of mantle origin in particular tectonic settings (e.g., ophiolites, fracture zones, or oceanic core complexes), the ubiquitous presence of MORB provides us with a key to understanding global geochemical variations of the Earth's interior in relation to plate tectonics. In fact, MORB has been considered to show a homogeneous chemical composition. In terms of volcanic rocks from other tectonic settings (e.g., island arc, continental crust, ocean island), this simple concept seems to be true. However, recent investigations reveal that even MORB has significant chemical variations that seem to correspond to location (Pacific, Atlantic, and Indian Oceans). These observations suggest that the mantle beneath each ocean has a distinct chemical composition and an internally heterogeneous composition. In this paper, global geochemical variations of MORB in terms of major and trace element compositions and isotope ratios are examined using a recently compiled database. The compilation suggests that MORB has heterogeneous compositions, which seem to originate from a mixture of depleted mantle and some enriched materials. Coupled with trace element compositions and Pb-isotope ratios, there seems to be at least two geochemical and isotopic domain of the upper most mantle: equatorial Atlantic-Pacific Oceans and southern Atlantic-Indian Ocean. Material (melt and/or solid) derived from plume, subducted slab, subcontinental crust, or fluid added beneath an ancient subduction zone is a candidate to explain the enrichment end-member to produce heterogeneous MORB. Because MORB is heterogeneous, using a tectonic discrimination diagram that implicitly subsumes homogeneous MORB or its mantle sources should be reconsidered. Further investigations, particularly of off-axis MORB, are needed to understand the relationship between heterogeneous compositions of MORB and geophysical parameters (e.g., degree of melting, temperature, spreading rate, crustal thickness, etc). In addition, the role of the MOHO transitional zone should be investigated to interpret the chemical characteristics of MORB.
{"title":"Variations of Chemical Compositions of Mid-ocean Ridge Basalts (MORB) and their Origin","authors":"Hiroshi P. Sato, H. Kumagai, N. Neo, Kentaro Nakamura","doi":"10.5026/JGEOGRAPHY.117.124","DOIUrl":"https://doi.org/10.5026/JGEOGRAPHY.117.124","url":null,"abstract":"Mid-ocean ridge basalt (hereafter, MORB) is a final product of melt generated from the partial melting of mantle peridotite, following reaction with mantle and/or lower crustral rocks, fractionation at a shallower crust and other processes en route to seafloor. Therefore, it is difficult to estimate melting processes at the upper mantle solely from any investigations of MORB. In contrast to the restricted occurrence of peridotite of mantle origin in particular tectonic settings (e.g., ophiolites, fracture zones, or oceanic core complexes), the ubiquitous presence of MORB provides us with a key to understanding global geochemical variations of the Earth's interior in relation to plate tectonics. In fact, MORB has been considered to show a homogeneous chemical composition. In terms of volcanic rocks from other tectonic settings (e.g., island arc, continental crust, ocean island), this simple concept seems to be true. However, recent investigations reveal that even MORB has significant chemical variations that seem to correspond to location (Pacific, Atlantic, and Indian Oceans). These observations suggest that the mantle beneath each ocean has a distinct chemical composition and an internally heterogeneous composition. In this paper, global geochemical variations of MORB in terms of major and trace element compositions and isotope ratios are examined using a recently compiled database. The compilation suggests that MORB has heterogeneous compositions, which seem to originate from a mixture of depleted mantle and some enriched materials. Coupled with trace element compositions and Pb-isotope ratios, there seems to be at least two geochemical and isotopic domain of the upper most mantle: equatorial Atlantic-Pacific Oceans and southern Atlantic-Indian Ocean. Material (melt and/or solid) derived from plume, subducted slab, subcontinental crust, or fluid added beneath an ancient subduction zone is a candidate to explain the enrichment end-member to produce heterogeneous MORB. Because MORB is heterogeneous, using a tectonic discrimination diagram that implicitly subsumes homogeneous MORB or its mantle sources should be reconsidered. Further investigations, particularly of off-axis MORB, are needed to understand the relationship between heterogeneous compositions of MORB and geophysical parameters (e.g., degree of melting, temperature, spreading rate, crustal thickness, etc). In addition, the role of the MOHO transitional zone should be investigated to interpret the chemical characteristics of MORB.","PeriodicalId":356213,"journal":{"name":"Chigaku Zasshi (jounal of Geography)","volume":"10 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":"128471118","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.117.110
S. Arai, N. Abe
This article reviews interpretations of the geological and petrological nature of the Moho, which is defined as a discontinuity in terms of Vp, with a view to preparing for the Mohole on the ocean floor in IODP. We strongly propose discarding non-seismic terms for the Moho, such as “petrologic Moho”. The nature of the Moho has been controversial for a long time; an isochemical phase transition boundary between gabbro (crust) and eclogite (mantle) was favored for the Moho by some researchers, while a chemical boundary between mafic rocks (crust) and peridotite rocks (upper mantle) is now favored by a majority of researchers. Boundaries between completely or partially serpentinized peridotite and fresh peridotite may be applicable as the Moho at some parts of the ocean floors of a slow-spreading ridge origin. Antigorite serpentinite can be expected to be observed at the lowermost crust if the Moho is the serpentinization front at the stability limit of serpentine. The Moho beneath the Japan arcs can be estimated using mafic-ultramafic xenoliths in Cenozoic volcanics. Peridotitic rocks scarcely mix with feldspathic rocks, indicating that the Moho at that location is the boundary between feldspathic rocks (mostly mafic granulites ; crust) and spinel pyroxenites (mantle). Possible fossil Mohos are observed in wellpreserved ophiolites, such as the Oman ophiolite. Two types of Moho are distinct in the Oman ophiolite ; gabbro-in-dunite Moho, where a gabbro band network in dunite changes upward to the layered gabbro within a few to several tens of meters, and dunite-in-gabbro Moho, where late-intrusive dunites intruded into gabbros. The former is of a primary origin at a fast-spreading ridge, and the latter is of a secondary origin at a subduction-zone setting in the obduction of the oceanic lithosphere as an ophiolite. The gabbro/peridotite (dunite) boundary as the primary Moho forms in embryo as a wall of melt conduit at fast-spreading ridges as well as at the segment center of slow-spreading ridges. The oceanic primary Moho is modified to various degrees by magmatism, metamorphism and tectonism in subsequent arc and continental environments. The gabbro-in-dunite Moho formation in the Oman ophiolite is an embryo of this modification. We expect in-situ sampling across the primary oceanic Moho formed at a fast-spreading ridge through the Mohole of IODP. Ultra-deep drilling at gabbro/peridotite complexes exposed on the ocean floor is indispensable for our understanding of the suboceanic upper mantle. Studies on appropriate ophiolites and deep-seated xenoliths from oceanic areas should complement the Mohole and other ultra-deep drillings to grasp the whole picture of the oceanic upper mantle. * 金沢大学自然科学研究科地球学教室 ** 海洋研究開発機構地球内部変動研究センター * Department of Earth Sciences, Graduate School of Natural Science and Technology, Kanazawa University ** Institute for Research on Earth Evolution (IFREE), Independent Administrative Institution/Japan Agency for MarineEarth S
{"title":"Investigation of the Petrologic Nature of the Moho toward the Mohole","authors":"S. Arai, N. Abe","doi":"10.5026/JGEOGRAPHY.117.110","DOIUrl":"https://doi.org/10.5026/JGEOGRAPHY.117.110","url":null,"abstract":"This article reviews interpretations of the geological and petrological nature of the Moho, which is defined as a discontinuity in terms of Vp, with a view to preparing for the Mohole on the ocean floor in IODP. We strongly propose discarding non-seismic terms for the Moho, such as “petrologic Moho”. The nature of the Moho has been controversial for a long time; an isochemical phase transition boundary between gabbro (crust) and eclogite (mantle) was favored for the Moho by some researchers, while a chemical boundary between mafic rocks (crust) and peridotite rocks (upper mantle) is now favored by a majority of researchers. Boundaries between completely or partially serpentinized peridotite and fresh peridotite may be applicable as the Moho at some parts of the ocean floors of a slow-spreading ridge origin. Antigorite serpentinite can be expected to be observed at the lowermost crust if the Moho is the serpentinization front at the stability limit of serpentine. The Moho beneath the Japan arcs can be estimated using mafic-ultramafic xenoliths in Cenozoic volcanics. Peridotitic rocks scarcely mix with feldspathic rocks, indicating that the Moho at that location is the boundary between feldspathic rocks (mostly mafic granulites ; crust) and spinel pyroxenites (mantle). Possible fossil Mohos are observed in wellpreserved ophiolites, such as the Oman ophiolite. Two types of Moho are distinct in the Oman ophiolite ; gabbro-in-dunite Moho, where a gabbro band network in dunite changes upward to the layered gabbro within a few to several tens of meters, and dunite-in-gabbro Moho, where late-intrusive dunites intruded into gabbros. The former is of a primary origin at a fast-spreading ridge, and the latter is of a secondary origin at a subduction-zone setting in the obduction of the oceanic lithosphere as an ophiolite. The gabbro/peridotite (dunite) boundary as the primary Moho forms in embryo as a wall of melt conduit at fast-spreading ridges as well as at the segment center of slow-spreading ridges. The oceanic primary Moho is modified to various degrees by magmatism, metamorphism and tectonism in subsequent arc and continental environments. The gabbro-in-dunite Moho formation in the Oman ophiolite is an embryo of this modification. We expect in-situ sampling across the primary oceanic Moho formed at a fast-spreading ridge through the Mohole of IODP. Ultra-deep drilling at gabbro/peridotite complexes exposed on the ocean floor is indispensable for our understanding of the suboceanic upper mantle. Studies on appropriate ophiolites and deep-seated xenoliths from oceanic areas should complement the Mohole and other ultra-deep drillings to grasp the whole picture of the oceanic upper mantle. * 金沢大学自然科学研究科地球学教室 ** 海洋研究開発機構地球内部変動研究センター * Department of Earth Sciences, Graduate School of Natural Science and Technology, Kanazawa University ** Institute for Research on Earth Evolution (IFREE), Independent Administrative Institution/Japan Agency for MarineEarth S","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":"115798811","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.119.519
R. Kataoka
Possible influences of cosmic rays on terrestrial climate have been studied by many researchers since a good correlation between neutron monitor counts and global cloud amount was reported by Svensmark and Friis-Christensen in 1997. The cosmic ray-cloud relationship may be best tested during Forbush decrease events, in which cosmic rays largely decrease for several days associated with coronal mass ejections. Some cloud parameters are likely to respond to the transient decrease of cosmic rays with a typical time delay of several days, although we do not know the physics behind the cosmic-ray cloud relationship.
{"title":"Cosmic Rays and Cloud Formation: Does Cloud Ammount Decrease during Forbush Decreases?","authors":"R. Kataoka","doi":"10.5026/JGEOGRAPHY.119.519","DOIUrl":"https://doi.org/10.5026/JGEOGRAPHY.119.519","url":null,"abstract":"Possible influences of cosmic rays on terrestrial climate have been studied by many researchers since a good correlation between neutron monitor counts and global cloud amount was reported by Svensmark and Friis-Christensen in 1997. The cosmic ray-cloud relationship may be best tested during Forbush decrease events, in which cosmic rays largely decrease for several days associated with coronal mass ejections. Some cloud parameters are likely to respond to the transient decrease of cosmic rays with a typical time delay of several days, although we do not know the physics behind the cosmic-ray cloud relationship.","PeriodicalId":356213,"journal":{"name":"Chigaku Zasshi (jounal of Geography)","volume":"223 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":"127627219","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}
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