Pub Date : 2021-10-25DOI: 10.5026/jgeography.130.615
Ken‐ichi Kano, A. Miyasaka, Genjyu Yamamoto, Kenji Kusunoki
The Suwa Basin is believed to be a typical pull-apart basin formed by the 12 km left-lateral motion on the ItoigawaShizuoka Tectonic Line active fault system. The starting age of the basin subsidence is considered to be after the violent volcanisms that formed the Lower Pleistocene Enrei Formation (Fm) distributed in and around the basin. However, the history of the basin development is still controversial mainly because of the insufficient geochronological data on the Enrei Fm in the SW Suwa Basin. The results of KAr dating are presented for two samples of andesite lavas of the Enrei Fm exposed on the northeastward-dipping slope of the SW Suwa Basin. One gives an age of 2.03 ± 0.19 Ma and the other an age of 1.51 ± 0.16 Ma. The former age corresponds closely to the oldest age of the Enrei Fm to the NE Suwa Basin, and suggests that the coeval and widespread initial volcanism at both sides of the basin occurred during the early Early Pleistocene (Gelasian) before the basin subsidence. The latter age closely corresponds to the peak periods of volcanic activity of the Enrei Fm. These lines of evidence suggest that the basin started to subside after 1.5 Ma, and that the position of basal unconformity, stratigraphy and geologic structures of the Enrei Fm in the SW Suwa Basin need to be re-examined.
{"title":"K–Ar Geochronology of the Andesite Lavas of the Lower Pleistocene Enrei Formation on the Southwestern Slope of the Suwa Basin, Central Japan","authors":"Ken‐ichi Kano, A. Miyasaka, Genjyu Yamamoto, Kenji Kusunoki","doi":"10.5026/jgeography.130.615","DOIUrl":"https://doi.org/10.5026/jgeography.130.615","url":null,"abstract":"The Suwa Basin is believed to be a typical pull-apart basin formed by the 12 km left-lateral motion on the ItoigawaShizuoka Tectonic Line active fault system. The starting age of the basin subsidence is considered to be after the violent volcanisms that formed the Lower Pleistocene Enrei Formation (Fm) distributed in and around the basin. However, the history of the basin development is still controversial mainly because of the insufficient geochronological data on the Enrei Fm in the SW Suwa Basin. The results of KAr dating are presented for two samples of andesite lavas of the Enrei Fm exposed on the northeastward-dipping slope of the SW Suwa Basin. One gives an age of 2.03 ± 0.19 Ma and the other an age of 1.51 ± 0.16 Ma. The former age corresponds closely to the oldest age of the Enrei Fm to the NE Suwa Basin, and suggests that the coeval and widespread initial volcanism at both sides of the basin occurred during the early Early Pleistocene (Gelasian) before the basin subsidence. The latter age closely corresponds to the peak periods of volcanic activity of the Enrei Fm. These lines of evidence suggest that the basin started to subside after 1.5 Ma, and that the position of basal unconformity, stratigraphy and geologic structures of the Enrei Fm in the SW Suwa Basin need to be re-examined.","PeriodicalId":45817,"journal":{"name":"Journal of Geography-Chigaku Zasshi","volume":" ","pages":""},"PeriodicalIF":0.3,"publicationDate":"2021-10-25","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"48088704","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 : 2021-10-25DOI: 10.5026/jgeography.130.683
H. Moriwaki, Toshiro Nagasako, Takehiko Suzuki, S. Terayama, Jun Matsukaze, Ryuhei Oda
― 683 ― Abstract Coastal sand dunes dating to the latter part of the late Pleistocene were constructed at sub-stantially lower sea levels on Kikaijima Island. They are preserved because of large-magnitude uplifts ascertained from occurrences of outstanding late Pleistocene and Holocene coral terraces. In addition, because Holocene sand dunes occur close to those of Marine Oxygen Isotope Stage 3 （ MIS 3 ） , the island is ideal for examining the environments of the formation of sand dunes, which were constructed under different climatic and sea-level conditions. Distributional and depositional features of sand dunes are clarified, and their chronology is constructed on the basis of tephrochronology, 14 C dating, and chronological relationships of the sand dunes with marine terraces of known age. The environments of sand-dune formation on Kikaijima are examined in relation to climatic and sea-level records within regional and global contexts. Nine tephra layers are recognized. These include two widespread tephras, Kikai-Akahoya tephra （ K-Ah ） and Aira-Tn tephra （ AT ） . The other seven tephras, consisting of fine ash layers, are newly recognized and named Kikaijima-1 tephra （ Kj-1 ） to Kikaijima-7 tephra （ Kj-7 ） , of which the upper six, lying between K-Ah and AT, are aged from 13,000 to 30,000 cal BP. Holocene sand dunes began to form at c . 8,000 cal BP before the culmination Island is mainly related to the occurrence of coastal sandy beaches during sea-level high stands. Although Holocene sand dunes are related to Holocene high stand, those of MIS 3 are related to high stands during cycles of sea-level fluctuations in MIS 3. The longitudinal landform of the sand dunes recognized in the late MIS 3 suggests that the prevailing winds in MIS 3 were stronger than in the Holocene. This study provides critical data for constructing a chronological framework that integrates various aspects of palaeoenvironments, as well as human interactions and responses in southern Kyushu.
Kikaijima岛的海岸沙丘可追溯至晚更新世后期，形成于海平面较低的地方。它们被保存下来的原因是，从突出的晚更新世和全新世珊瑚梯田的出现中确定了大规模的隆起。此外，由于全新世沙丘的形成与海洋氧同位素阶段3 (MIS 3)的沙丘非常接近，因此该岛是研究不同气候和海平面条件下沙丘形成环境的理想场所。阐明了沙丘的分布和沉积特征，并在地表年代学、14c测年以及沙丘与已知年龄的海相阶地的年代学关系的基础上构建了沙丘的年代学。在气候和海平面记录的区域和全球背景下，研究了菊岛沙丘形成的环境。识别出了9个tephra层。这些包括两种广泛分布的tephra, Kikai-Akahoya tephra (K-Ah)和Aira-Tn tephra (AT)。其余7个由细灰分层组成的岩带为新发现的岩带，命名为Kikaijima-1 ~ Kikaijima-7岩带(Kj-7)，其中位于K-Ah ~ AT之间的上部6个岩带年龄在13000 ~ 30000 cal BP之间。全新世沙丘开始形成于c。中天岛前8000 cal BP主要与海平面高架时期海岸沙滩的出现有关。全新世沙丘与全新世高海拔有关，而MIS 3的沙丘与MIS 3海平面波动周期中的高海拔有关。第三期晚期沙丘的纵向地貌特征表明，第三期的盛行风比全新世强。该研究为构建一个整合九州南部古环境各个方面以及人类相互作用和反应的时间框架提供了关键数据。
{"title":"The Formation of Late Pleistocene and Holocene Sand Dunes on Kikaijima Island, Nansei Islands","authors":"H. Moriwaki, Toshiro Nagasako, Takehiko Suzuki, S. Terayama, Jun Matsukaze, Ryuhei Oda","doi":"10.5026/jgeography.130.683","DOIUrl":"https://doi.org/10.5026/jgeography.130.683","url":null,"abstract":"― 683 ― Abstract Coastal sand dunes dating to the latter part of the late Pleistocene were constructed at sub-stantially lower sea levels on Kikaijima Island. They are preserved because of large-magnitude uplifts ascertained from occurrences of outstanding late Pleistocene and Holocene coral terraces. In addition, because Holocene sand dunes occur close to those of Marine Oxygen Isotope Stage 3 （ MIS 3 ） , the island is ideal for examining the environments of the formation of sand dunes, which were constructed under different climatic and sea-level conditions. Distributional and depositional features of sand dunes are clarified, and their chronology is constructed on the basis of tephrochronology, 14 C dating, and chronological relationships of the sand dunes with marine terraces of known age. The environments of sand-dune formation on Kikaijima are examined in relation to climatic and sea-level records within regional and global contexts. Nine tephra layers are recognized. These include two widespread tephras, Kikai-Akahoya tephra （ K-Ah ） and Aira-Tn tephra （ AT ） . The other seven tephras, consisting of fine ash layers, are newly recognized and named Kikaijima-1 tephra （ Kj-1 ） to Kikaijima-7 tephra （ Kj-7 ） , of which the upper six, lying between K-Ah and AT, are aged from 13,000 to 30,000 cal BP. Holocene sand dunes began to form at c . 8,000 cal BP before the culmination Island is mainly related to the occurrence of coastal sandy beaches during sea-level high stands. Although Holocene sand dunes are related to Holocene high stand, those of MIS 3 are related to high stands during cycles of sea-level fluctuations in MIS 3. The longitudinal landform of the sand dunes recognized in the late MIS 3 suggests that the prevailing winds in MIS 3 were stronger than in the Holocene. This study provides critical data for constructing a chronological framework that integrates various aspects of palaeoenvironments, as well as human interactions and responses in southern Kyushu.","PeriodicalId":45817,"journal":{"name":"Journal of Geography-Chigaku Zasshi","volume":" ","pages":""},"PeriodicalIF":0.3,"publicationDate":"2021-10-25","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"45782768","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 : 2021-08-25DOI: 10.5026/jgeography.130.543
Y. Ohara
― ― Abstract A significant fraction of the ocean floor is created in back-arc basins, where water plays a major role in generating back-arc basin basalts, contrasting strikingly with magmatic processes at mid-oceanic ridges. Furthermore, much of our understanding of all of the oceanic crust comes from ophiolites, which are largely attributed to supra-subduction zone environments. Therefore, studying the back-arc basin lower crust and uppermost mantle is arguably important, contribut-ing to the overall geology of the oceanic crust. The Godzilla Megamullion, located in the extinct Parece Vela Basin in the Philippine Sea, is the largest known oceanic core complex. It is an elongated massif with a distinct corrugated surface consisting of several individual domal highs. It records the secular evolution of the mantle melting beneath a dying back-arc spreading ridge along the length of the megamullion surface. Furthermore, strong heterogeneity in the P-wave velocity structure is observed along the length of the megamullion, with a normal oceanic crust-like structure in the distal （ i.e., near breakaway ） to medial parts, and a shallow high-velocity body in the proximal （ i.e, near termination ） part. The Godzilla Megamullion should arguably be the best place in the world to study the architecture of the back-arc basin lower crust and uppermost mantle, and the actual crust/mantle boundary through the International Ocean Discovery Program （ IODP ） . By locating three 400- to 800 m-deep drill holes along its length, key data are obtained to better understand and constrain the composition of the back-arc basin oceanic crust and uppermost mantle, as well as the architecture of oceanic core complexes. The extinct backarc basin environment at the Godzilla Megamullion provides a further unique opportunity to explore life in an oceanic crust after extinction of its hydrothermal activity.
{"title":"IODP Drilling at the Godzilla Megamullion: The Nature of the Backarc Basin Lower Crust and Upper Mantle","authors":"Y. Ohara","doi":"10.5026/jgeography.130.543","DOIUrl":"https://doi.org/10.5026/jgeography.130.543","url":null,"abstract":"― ― Abstract A significant fraction of the ocean floor is created in back-arc basins, where water plays a major role in generating back-arc basin basalts, contrasting strikingly with magmatic processes at mid-oceanic ridges. Furthermore, much of our understanding of all of the oceanic crust comes from ophiolites, which are largely attributed to supra-subduction zone environments. Therefore, studying the back-arc basin lower crust and uppermost mantle is arguably important, contribut-ing to the overall geology of the oceanic crust. The Godzilla Megamullion, located in the extinct Parece Vela Basin in the Philippine Sea, is the largest known oceanic core complex. It is an elongated massif with a distinct corrugated surface consisting of several individual domal highs. It records the secular evolution of the mantle melting beneath a dying back-arc spreading ridge along the length of the megamullion surface. Furthermore, strong heterogeneity in the P-wave velocity structure is observed along the length of the megamullion, with a normal oceanic crust-like structure in the distal （ i.e., near breakaway ） to medial parts, and a shallow high-velocity body in the proximal （ i.e, near termination ） part. The Godzilla Megamullion should arguably be the best place in the world to study the architecture of the back-arc basin lower crust and uppermost mantle, and the actual crust/mantle boundary through the International Ocean Discovery Program （ IODP ） . By locating three 400- to 800 m-deep drill holes along its length, key data are obtained to better understand and constrain the composition of the back-arc basin oceanic crust and uppermost mantle, as well as the architecture of oceanic core complexes. The extinct backarc basin environment at the Godzilla Megamullion provides a further unique opportunity to explore life in an oceanic crust after extinction of its hydrothermal activity.","PeriodicalId":45817,"journal":{"name":"Journal of Geography-Chigaku Zasshi","volume":" ","pages":""},"PeriodicalIF":0.3,"publicationDate":"2021-08-25","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"46994826","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 : 2021-08-25DOI: 10.5026/jgeography.130.559
T. Sano, M. Tejada, M. Nakanishi, T. Hanyu, S. Miura, D. Suetsugu, T. Tonegawa, A. Ishikawa, K. Shimizu, S. Shimizu
Large Igneous Provinces (LIPs), such as the Ontong Java Plateau (OJP) in the western equatorial Pacific, provide information on mantle processes and composition, and their formation may have global environmental consequences. The OJP is the largest oceanic plateau and is probably the most voluminous igneous edifice on Earth. Despite its importance, the size, volume, and formation rate of the OJP are not yet well constrained. The maximum extent of OJP-related volcanism may be even greater than currently estimated, because volcanological studies indicate that long lava flows (or sills) from the OJP may have reached the adjacent Nauru, East Mariana, and possibly Pigafetta basins. Moreover, the similarity in age and some geochemistry of lavas from the Ontong Java, Hikurangi, and Manihiki plateaus suggests that they once may have been part of a single LIP (Ontong Java Nui, OJN). If true, the massive volcanism may have covered * 国立科学博物館地学研究部 ** 海洋研究開発機構海域地震火山部門火山・地球内部研究センター *** 千葉大学大学院理学研究院 **** 海洋研究開発機構海域地震火山部門地震発生帯研究センター ***** 東京工業大学地球惑星科学系 ****** 海洋研究開発機構超先鋭研究開発部門高知コア研究所 ******* 千葉大学大学院融合理工学府 * Department of Geology and Paleontology, National Museum of Nature and Science, Tsukuba, 305-0005, Japan ** Volcanoes and Earth’s Interior Research Center (VERC), Research Institute for Marine Geodynamics (IMG), Japan Agency for Marine-Earth Science and Technology (JAMSTEC), Yokosuka, 237-0061, Japan *** Graduate School of Science, Chiba University, Chiba 263-8522, Japan **** Subduction Dynamics Research Center (SDR), Research Institute for Marine Geodynamics (IMG), Japan Agency for Marine-Earth Science and Technology (JAMSTEC), Yokohama, 236-0001, Japan ***** Department of Earth and Planetary Sciences, Tokyo Institute of Technology, Tokyo, 152-8550, Japan ****** Kochi Institute for Core Sample Research, Institute for Extra-cutting-edge Science and Technology Avant-garde Research, Japan Agency for Marine-Earth Science and Technology (JAMSTEC), Nankoku, 783-8502, Japan ******* Graduate School of Science and Engineering, Chiba University, Chiba, 263-8522, Japan 地学雑誌 Journal of Geography(Chigaku Zasshi) 130(4)559584 2021 doi:10.5026/jgeography.130.559
{"title":"Testing the Ontong Java Nui Hypothesis: The Largest Supervolcano Ever on Earth","authors":"T. Sano, M. Tejada, M. Nakanishi, T. Hanyu, S. Miura, D. Suetsugu, T. Tonegawa, A. Ishikawa, K. Shimizu, S. Shimizu","doi":"10.5026/jgeography.130.559","DOIUrl":"https://doi.org/10.5026/jgeography.130.559","url":null,"abstract":"Large Igneous Provinces (LIPs), such as the Ontong Java Plateau (OJP) in the western equatorial Pacific, provide information on mantle processes and composition, and their formation may have global environmental consequences. The OJP is the largest oceanic plateau and is probably the most voluminous igneous edifice on Earth. Despite its importance, the size, volume, and formation rate of the OJP are not yet well constrained. The maximum extent of OJP-related volcanism may be even greater than currently estimated, because volcanological studies indicate that long lava flows (or sills) from the OJP may have reached the adjacent Nauru, East Mariana, and possibly Pigafetta basins. Moreover, the similarity in age and some geochemistry of lavas from the Ontong Java, Hikurangi, and Manihiki plateaus suggests that they once may have been part of a single LIP (Ontong Java Nui, OJN). If true, the massive volcanism may have covered * 国立科学博物館地学研究部 ** 海洋研究開発機構海域地震火山部門火山・地球内部研究センター *** 千葉大学大学院理学研究院 **** 海洋研究開発機構海域地震火山部門地震発生帯研究センター ***** 東京工業大学地球惑星科学系 ****** 海洋研究開発機構超先鋭研究開発部門高知コア研究所 ******* 千葉大学大学院融合理工学府 * Department of Geology and Paleontology, National Museum of Nature and Science, Tsukuba, 305-0005, Japan ** Volcanoes and Earth’s Interior Research Center (VERC), Research Institute for Marine Geodynamics (IMG), Japan Agency for Marine-Earth Science and Technology (JAMSTEC), Yokosuka, 237-0061, Japan *** Graduate School of Science, Chiba University, Chiba 263-8522, Japan **** Subduction Dynamics Research Center (SDR), Research Institute for Marine Geodynamics (IMG), Japan Agency for Marine-Earth Science and Technology (JAMSTEC), Yokohama, 236-0001, Japan ***** Department of Earth and Planetary Sciences, Tokyo Institute of Technology, Tokyo, 152-8550, Japan ****** Kochi Institute for Core Sample Research, Institute for Extra-cutting-edge Science and Technology Avant-garde Research, Japan Agency for Marine-Earth Science and Technology (JAMSTEC), Nankoku, 783-8502, Japan ******* Graduate School of Science and Engineering, Chiba University, Chiba, 263-8522, Japan 地学雑誌 Journal of Geography(Chigaku Zasshi) 130(4)559584 2021 doi:10.5026/jgeography.130.559","PeriodicalId":45817,"journal":{"name":"Journal of Geography-Chigaku Zasshi","volume":" ","pages":""},"PeriodicalIF":0.3,"publicationDate":"2021-08-25","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"46334583","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 : 2021-08-25DOI: 10.5026/jgeography.130.585
Y. Tatsumi, Nobuaki Suenaga, S. Yoshioka, K. Kaneko
Water circulation, along with plate subduction, is considered based on the stabilities of hydrous phases and pressure temperature profiles of the sinking oceanic plate. Water in a rather hot slab like the present one may be largely liberated at shallow depths （ < 150 km ） and return to the ocean via. arc magmatism. On the other hand, stabilization of dense hydrous minerals under cooler conditions, which current subduction zones will soon experience, causes the transportation or reflux of seawater to the deep mantle, which reduces the total mass of surface seawater. Simple calculations accepting water contents in the subducting slab suggested by a recent seismic velocity structure model indicate that the Earth’s oceans are likely to disappear ~80 million years hence. Significant changes may happen such as the end of plate tectonics and the onset of snowball Earth, with associated catastrophes affecting life. The only way to confirm this picture of the future of the ocean planet Earth is to examine deep hydration taking place along the outer rise through direct analyses of the upper mantle across the
{"title":"Drilling into the Mantle: A Key to Prognosticating the Future of the Ocean Planet","authors":"Y. Tatsumi, Nobuaki Suenaga, S. Yoshioka, K. Kaneko","doi":"10.5026/jgeography.130.585","DOIUrl":"https://doi.org/10.5026/jgeography.130.585","url":null,"abstract":"Water circulation, along with plate subduction, is considered based on the stabilities of hydrous phases and pressure temperature profiles of the sinking oceanic plate. Water in a rather hot slab like the present one may be largely liberated at shallow depths （ < 150 km ） and return to the ocean via. arc magmatism. On the other hand, stabilization of dense hydrous minerals under cooler conditions, which current subduction zones will soon experience, causes the transportation or reflux of seawater to the deep mantle, which reduces the total mass of surface seawater. Simple calculations accepting water contents in the subducting slab suggested by a recent seismic velocity structure model indicate that the Earth’s oceans are likely to disappear ~80 million years hence. Significant changes may happen such as the end of plate tectonics and the onset of snowball Earth, with associated catastrophes affecting life. The only way to confirm this picture of the future of the ocean planet Earth is to examine deep hydration taking place along the outer rise through direct analyses of the upper mantle across the","PeriodicalId":45817,"journal":{"name":"Journal of Geography-Chigaku Zasshi","volume":" ","pages":""},"PeriodicalIF":0.3,"publicationDate":"2021-08-25","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"48770567","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 : 2021-08-25DOI: 10.5026/jgeography.130.457
S. Ono
{"title":"Introduction to the Special Issue “Hard-rock Drilling Science: Challenges of Mantle Drilling”","authors":"S. Ono","doi":"10.5026/jgeography.130.457","DOIUrl":"https://doi.org/10.5026/jgeography.130.457","url":null,"abstract":"","PeriodicalId":45817,"journal":{"name":"Journal of Geography-Chigaku Zasshi","volume":"50 1","pages":""},"PeriodicalIF":0.3,"publicationDate":"2021-08-25","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"82881069","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 : 2021-08-25DOI: 10.5026/jgeography.130.483
T. Morishita, G. Fujie, K. Hirauchi, I. Katayama, Y. Kouketsu, J. Kuroda, A. Okamoto, S. Ono, K. Michibayashi, Y. Morono, S. Yamamoto
{"title":"Crucial Scientific Issues in Earth Science Revealed Only by Mantle Drilling: Understanding the Current State of the Oceanic Plates of a Life-bearing Planet","authors":"T. Morishita, G. Fujie, K. Hirauchi, I. Katayama, Y. Kouketsu, J. Kuroda, A. Okamoto, S. Ono, K. Michibayashi, Y. Morono, S. Yamamoto","doi":"10.5026/jgeography.130.483","DOIUrl":"https://doi.org/10.5026/jgeography.130.483","url":null,"abstract":"","PeriodicalId":45817,"journal":{"name":"Journal of Geography-Chigaku Zasshi","volume":" ","pages":""},"PeriodicalIF":0.3,"publicationDate":"2021-08-25","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"47569576","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 : 2021-08-25DOI: 10.5026/jgeography.130.527
O. Ishizuka
― ― Abstract The Izu Bonin arc has been the target of several hard rock drilling expeditions, including those associated with the Ocean Drilling Program （ ODP ） and the Integrated Ocean Drilling Program （ IODP ） , as well as a drilling survey for delineating Japanese continental shelves undertaken by Japan’s Ministry of Economy, Technology and Industry. In 1989, ODP Legs 125 and 126 successfully recovered cores from the Izu Bonin forearc area and the backarc rift basin （ Sumisu Rift ） . These cores provided the first opportunity to investigate the early volcanic and tectonic history of the Izu Bonin arc in the Eocene and Oligocene. They also provided the earliest volcanic products of the Sumisu Rift, which is highly vesicular basalt. Drilling for the Japanese continental shelf survey was conducted in the most reararc side of the Izu Bonin arc. The cores from the reararc seamounts reveal that the across-arc variations in magma chemistry and age of volcanism observed along the reararc seamount chains continue further toward the spreading center of the Shikoku Basin. The Kinan Escarpment appears to be the westernmost （ i.e., most reararc ） location where a slab-derived geochemical signature can be recognized in the erupted magma. Three IODP drilling expeditions have been undertaken in the Izu Bonin arc region. Exp. 350 drilled in a small basin between the reararc seamount chains. Slightly over 1.8 km of sediment of mostly volcanic origin was drilled. This core preserves a continuous magmatic record in the reararc area since cessation of spreading of the Shikoku Basin, and provides critical information about how the reararc volcanoes were reestablished after the middle Miocene. Exp. 351 and 352 aimed to study subduction initiation processes. Exp. 352 was conducted in the Izu Bonin forearc. It recovered forearc basalt （ FAB ） and boninites associated with the seafloor spreading at subduction initiation. Based on their ages and geochemical characteristics, fast-and short-lived seafloor spreading is estimated to have occurred. Exp. 351 recovered ocean crust, which is interpreted to be the basement of the arc, from the Amami Sankaku Basin between the Kyushu Palau Ridge （ ancient Izu Bonin arc ） and the Daito Ridge （ Mesozoic remnant arc ） . This basement is similar in age and geochemistry to FAB, which implies the Izu Bonin arc basement is ocean crust produced following the onset of subduction. This expedition also provided for the first time a continuous volcanic record
――摘要伊豆-博宁弧一直是几次硬岩钻探探险的目标，包括与海洋钻探计划（ODP）和综合海洋钻探计划有关的探险，以及日本经济技术产业省为划定日本大陆架而进行的钻探调查。1989年，ODP Legs 125和126成功地从IzuBonin弧前地区和弧后裂谷盆地（Sumisu rift）回收了岩心。这些岩心为研究伊豆-博宁弧在始新世和渐新世的早期火山和构造历史提供了第一次机会。它们还提供了苏米苏裂谷最早的火山产物，这是一种高度多孔的玄武岩。日本大陆架调查的钻探是在伊豆-博宁弧的最弧后一侧进行的。弧后海山的岩心表明，沿弧后海山链观察到的岩浆化学和火山活动年龄的跨弧变化继续向四国盆地的扩展中心发展。基南断崖似乎是最西端（即最弧后）的位置，在这里可以在喷发的岩浆中识别出板块衍生的地球化学特征。IzuBonin弧区进行了三次IODP钻探考察。实验350在弧后海山链之间的一个小盆地中钻探。钻探了1.8公里多一点的沉积物，这些沉积物主要来自火山。自四国盆地停止扩张以来，该岩芯在弧后地区保存了连续的岩浆记录，并提供了关于弧后火山如何在中新世中期重建的关键信息。实验351和352旨在研究俯冲起始过程。实验352在伊豆博宁弧前进行。它回收了弧前玄武岩（FAB）和与俯冲开始时的海平面扩展有关的玻碳岩。根据它们的年龄和地球化学特征，估计已经发生了快速而短暂的海平面扩张。实验351从九州-帕劳山脊（古代伊豆-波宁弧）和大藤山脊（中生代残余弧）之间的Amami Sankaku盆地回收的海洋地壳，被解释为弧的基底。该基底在年龄和地球化学上与FAB相似，这意味着伊豆-波宁弧基底是俯冲开始后产生的海洋地壳。这次探险还首次提供了连续的火山记录
{"title":"Outcome of Hard Rock Drilling in the Izu–Bonin Arc","authors":"O. Ishizuka","doi":"10.5026/jgeography.130.527","DOIUrl":"https://doi.org/10.5026/jgeography.130.527","url":null,"abstract":"― ― Abstract The Izu Bonin arc has been the target of several hard rock drilling expeditions, including those associated with the Ocean Drilling Program （ ODP ） and the Integrated Ocean Drilling Program （ IODP ） , as well as a drilling survey for delineating Japanese continental shelves undertaken by Japan’s Ministry of Economy, Technology and Industry. In 1989, ODP Legs 125 and 126 successfully recovered cores from the Izu Bonin forearc area and the backarc rift basin （ Sumisu Rift ） . These cores provided the first opportunity to investigate the early volcanic and tectonic history of the Izu Bonin arc in the Eocene and Oligocene. They also provided the earliest volcanic products of the Sumisu Rift, which is highly vesicular basalt. Drilling for the Japanese continental shelf survey was conducted in the most reararc side of the Izu Bonin arc. The cores from the reararc seamounts reveal that the across-arc variations in magma chemistry and age of volcanism observed along the reararc seamount chains continue further toward the spreading center of the Shikoku Basin. The Kinan Escarpment appears to be the westernmost （ i.e., most reararc ） location where a slab-derived geochemical signature can be recognized in the erupted magma. Three IODP drilling expeditions have been undertaken in the Izu Bonin arc region. Exp. 350 drilled in a small basin between the reararc seamount chains. Slightly over 1.8 km of sediment of mostly volcanic origin was drilled. This core preserves a continuous magmatic record in the reararc area since cessation of spreading of the Shikoku Basin, and provides critical information about how the reararc volcanoes were reestablished after the middle Miocene. Exp. 351 and 352 aimed to study subduction initiation processes. Exp. 352 was conducted in the Izu Bonin forearc. It recovered forearc basalt （ FAB ） and boninites associated with the seafloor spreading at subduction initiation. Based on their ages and geochemical characteristics, fast-and short-lived seafloor spreading is estimated to have occurred. Exp. 351 recovered ocean crust, which is interpreted to be the basement of the arc, from the Amami Sankaku Basin between the Kyushu Palau Ridge （ ancient Izu Bonin arc ） and the Daito Ridge （ Mesozoic remnant arc ） . This basement is similar in age and geochemistry to FAB, which implies the Izu Bonin arc basement is ocean crust produced following the onset of subduction. This expedition also provided for the first time a continuous volcanic record","PeriodicalId":45817,"journal":{"name":"Journal of Geography-Chigaku Zasshi","volume":" ","pages":""},"PeriodicalIF":0.3,"publicationDate":"2021-08-25","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"44066312","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 : 2021-08-25DOI: 10.5026/jgeography.130.507
E. Takazawa
The ICDP Oman Drilling Project carried out onshore drilling of the world’s largest ophiolite, the Oman ophiolite （ also known as Samail ophiolite ） . This drilling project provided an opportu-nity to explore major key boundaries of the oceanic lithosphere, represented by the Oman ophiolite, by drilling cores and boreholes. Below the layered gabbro at the bottom of the crustal section is the Moho Transition Zone （ MTZ ） , which is mainly composed of dunite with small amounts of gabbroic sills. By drilling at the Wadi Zeeb CM site in the Wadi Tayin massif, cores were success-fully collected from a 150 m MTZ. Also collected were fragile altered rocks from wadi outcrops that are easily lost. The core description campaign was carried out aboard deep-sea scientific drilling vessel “Chikyu” anchored at Shimizu Port. The core observations were performed and described according to the IODP procedure, and the analysis was conducted using many instru-ments. The resulting data provide important insights and will contribute to future drilling of the Mohorovičić discontinuity in the ocean. The most striking fact is that MTZ dunites are strongly influenced by serpentinization. In particular, the upper part of the MTZ just below the boundary with the lower crustal gabbro was most strongly altered, and a fracture zone was also developed. Understanding when and how these alterations occurred at the boundary between the crust and the mantle is an important future task.
ICDP阿曼钻探项目对世界上最大的蛇绿岩阿曼蛇绿岩（也称为Samail蛇绿岩）进行了陆上钻探。该钻探项目提供了一个机会，通过钻探岩心和钻孔来探索以阿曼蛇绿岩为代表的海洋岩石圈的主要关键边界。在地壳剖面底部的层状辉长岩下方是莫霍过渡带（MTZ），该过渡带主要由含少量辉长岩岩床的纯岩组成。通过在Wadi Tayin地块的Wadi Zeeb CM现场进行钻探，成功地从150 m MTZ中完全收集了岩芯。此外，还收集了来自瓦迪露头的脆弱蚀变岩石，这些岩石很容易丢失。岩心描述活动是在停泊在清水港的深海科学钻探船“Chikyu”上进行的。根据IODP程序进行岩心观察和描述，并使用多种仪器进行分析。由此产生的数据提供了重要的见解，并将有助于未来在海洋中钻探Mohorovičić不连续面。最引人注目的事实是，MTZ dunite受到蛇纹石化的强烈影响。特别是，MTZ的上部，就在与下地壳辉长岩的边界下方，蚀变最强烈，还发育了断裂带。了解这些变化何时以及如何发生在地壳和地幔之间的边界是未来的一项重要任务。
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Pub Date : 2021-08-25DOI: 10.5026/jgeography.130.599
S. Umino, Y. Kusano
The style of crustal extension is governed by M, which is the ratio of magma consumed to relax the crustal strain caused by plate spreading. Bathymetric profiles across ridges are repro-duced well by changing M . However, what determines the value of M has not been explained. Fast-spread oceanic crust comprises dense sheet flows underlain by thin dense sheeted dikes compared to magmas. This density structure increases the magma extruded, allowing the crust to be extended solely by magmatic accretion; whereas, the intermediate-spread crust consists of less dense pillow lavas, yielding an apparent level of neutral buoyancy that traps magma to develop the sheeted dikes below. Consequently, the crust extends through dike intrusions in the lower levels and faults at shallow levels. Thus, the density structure of the oceanic crust determines the style of plate spreading, or the value of M . Because the spreading rate or the strain rate does not vary within the same ridge segment, intrasegment variations in crustal structure depend directly on the supply rate of magma, or the value of M , which decreases with the thin-ning of extrusive layers and the thickening of sheeted dikes along the Galapagos Spreading Center and the East Pacific Rise. This tendency is supported by the crustal architecture observed in holes 504B and 1256D, the Hess Deep and the Oman Ophiolite. The density structure of the upper crust can be discerned by the proportion of sheet flows among extrusive rocks on the ridge axis, which is drastically reduced with spreading rates from 10 cm/a to 7 cm/a. This spreading rate interval coincides with the change in axial magma chamber depth. Throughout this rate interval, the key observation is whether the style of crustal extension from magmatic accre-tion-dominant details ） . Sediments and lavas emplaced off ridge （ uppermost basement 100 m in Hole 1256D （ Umino et al ., 2008a ） and 50 m in Hole 504B （ Ayadi et al ., 1998 ） , respectively ） are excluded from density estimates. The 1256D crust on the East Pacific Rise spread at an ultrafast rate of 22 cm/a comprises high-density sheet flows, yielding almost the same lithostatic and magmastatic pressures. The 504B crust on the Costa Rica Rift spread at an intermedi-ate rate of 6.6 cm/a has an apparent level of neutral buoyancy （ LNB ） , where lithostatic pressure is lower than magmastatic pressure. Magma uprising from the axial magma chamber （ AMC ） is preferentially trapped in the LNB and forms the
地壳伸展的类型受M的支配，M是由板块扩张引起的地壳应变松弛所消耗的岩浆比例。通过改变M值，可以很好地再现脊上的水深剖面。然而，是什么决定了M的值还没有解释。与岩浆相比，快速扩张的海洋地壳由密集的片状流组成，下面是薄而密集的片状岩脉。这种密度结构增加了岩浆的挤压，使得地壳仅通过岩浆吸积而扩张;然而，中间伸展的地壳由密度较低的枕状熔岩组成，产生明显的中性浮力，使岩浆被困住，形成下面的片状岩脉。因此，地壳在较低层次上通过岩脉侵入，在较浅层次上通过断层伸展。因此，洋壳的密度结构决定了板块扩张的方式，即M值。由于在同一脊段内扩张速率或应变速率不变化，因此地壳结构的分段内变化直接取决于岩浆的供应速率或M值，M值随着沿加拉帕戈斯扩张中心和东太平洋隆起的挤压层变薄和片状岩脉变厚而减小。504B和1256D孔、赫斯深和阿曼蛇绿岩观测到的地壳结构支持了这一趋势。上地壳的密度结构可以通过在脊轴上的挤压岩石间的片流比例来识别，随着扩展速率从10 cm/a急剧减少到7 cm/a。这一扩张速率区间与岩浆库轴向深度变化一致。在整个速率区间内，关键的观察是地壳伸展的样式是否来自岩浆增生(以细节为主)。在密度估算中不包括山脊外的沉积物和熔岩(1256D洞最上层基底100 m (Umino et al .， 2008a)和504B洞50 m (Ayadi et al .， 1998))。东太平洋隆起1256D地壳以22 cm/a的超快速度扩张，包括高密度的片流，产生几乎相同的静岩和岩浆压力。哥斯达黎加裂谷504B地壳以6.6 cm/a的中等速率扩张，具有明显的中性浮力(LNB)水平，其中静岩压力低于岩浆压力。从轴向岩浆房(AMC)升起的岩浆优先被困在LNB中，并形成了地幔
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