ベイオブアイランズ・オフィオライトはColeman（1977）による世界の4つの主要なオフィオライトの1つである（概要は新版地学事典（平凡社）の拙著同名項目参照）．ニューファンドランド島の西岸沿いに北北東へ延びる120×25kmの範囲に分布し（図1），東から西へマントル超苦鉄質岩，超苦鉄質集積岩，斑れい岩，輝緑岩岩脈群，玄武岩枕状溶岩の順に重なるオフィオライト層序が発達する．このオフィオライトはフィヨルドやベイオブアイランズ湾によって北から南へテーブル山，ノースアーム山，ブローミーダウン山，ルイス丘陵の4つの山体に分かれる．表紙写真は北部のテーブル山岩体の超苦鉄質集積岩と斑れい岩との境界，すなわちモホ面付近の深成岩に発達するレーヤリング（層状構造）を示したものである．白い層はトロクトライト，かんらん石斑れい岩，斜長岩よりなり，褐色の層はダナイトやウェルライトからなる．テーブル山のマントル断面では全体として下部（衝上断層直上）から上部（モホ面直下）に向かって溶け残りかんらん岩の涸渇度が増す（Suhr and Robinson, 1994）．このオフィオライトは古生代前期に沈み込み帯上の拡大軸で形成されたと考えられており，とくに西部の海岸岩体やルイス丘陵は島弧の化学的特徴を示す（Edwards and Malpas, 1995; Kurth-Velz et al., 2004）．現地を案内していただいたS.J. エドワーズ博士に感謝する． （写真・解説：石渡 明 1994年8月11日撮影）
Episodic subsidence events at the east coast of Lake Mikata, Fukui Prefecture, suggesting fault activity in the Mikata fault zone, are recognized from two drilling cores, MK09 core (60-m long) and NEXCO core (100-m long). Detailed lithology including widespread tephra horizons is described for the MK09 core, with 18 AMS-14C dates. The radiocarbon ages and tephrochronology of the core sediments indicate the continuous sedimentary history of Lake Mikata back to about 130 ka. We recognize eight depositional units ranging in thickness from 5 to 10 m in the MK09 core, each of which shows a clear coarsening-upward sequence from clay to gravel beds. These units are also recognized in the NEXCO core drilled 50 m from the MK09 core site by the correlation of tephras and lithofacies. Analysis of sedimentary features suggests that each unit and its boundary reflect a rapid rise of lake level and subsequent progradation of alluvial fans. The mean sedimentary interval of individual units is equivalent to about 10.6 kyr, which is meaningfully shorter than astronomical cycles of climate change (∼20, 41 and ∼100 kyrs). Thus, we interpret these cyclic units to be the results of recurrent co-seismic subsidence due to Mikata fault zone activity. According to this interpretation, we could identify at least seven subsidence events besides the 1662 Kanbun Earthquake. The mean recurrence interval of these events is estimated to be about 7.7 kyr.
This paper is based on a questionnaire survey of residents in the Yakuin district of Fukuoka. It examines the characteristics of downtown residents and their reasons for living in a major city core. The focus is on differences between residents living in privately owned apartments and those living in rented apartments. To compare urban and suburban residents, we also surveyed residents of the Hakata-Minami district, which is a suburban area adjacent to a railroad station. Among the major findings is a difference in the age range between residents of privately owned and rented apartments in the Yakuin district. Privately owned apartment residents were aged between 30 and 60 years, whereas rented apartment residents were concentrated in the 20- to 30-year-old range. Although almost all of the rented apartment dwellers were either single residents or couples who did not intend to live in the area permanently, the majority of the privately owned apartment residents planned to settle in the area. Several previous studies on the migration of residents to downtown areas show that young residents make short-range and long-range moves. Our study revealed a similar duality. Short-range moves from one part of the city to another occur when young residents start attending a university or first enter the workforce. On the other hand, long-range moves from outside the city to downtown areas occur because of job transfers. Space requirements and affordability of housing are major factors influencing potential residents' decisions to live in downtown areas. The minimum space requirement depends on the number of children in a household. Although residents with no children or only one child have little difficulty finding a suitable downtown dwelling, most families with two or more children live in the suburbs, such as the Hakata-Minami district, where they can obtain an additional bedroom at almost the same housing cost as downtown.
This paper aims to depict the process of farmland accumulation by large-scale tenant farmers in the suburbs of Metropolitan Tokyo, focusing on social relationships among farmers. Data on farm management, changes in farming, distribution of farmland, percentage of leased land to total farmland, landowners renting out their cropland, and functions and membership of social groups were collected by conducting interviews with farmers and other people in the study area. Then, the data were analyzed to identify how closely tenants and landowners were related in social aspects. The study area is the Kitasuka district of Narita City, Chiba Prefecture facing by Lake Inba. Rice monoculture has been dominant in this area. When construction of Narita Airport created many off-farm employment opportunities in the 1970s, many farmers who were engaged in off-farm jobs wanted to rent out their farmland. On the other hand, rapid mechanization and increased application of agrichemicals enabled full-time farmers to expand the scale of their farming activities by leasing farmland. As a result, large-scale tenant rice farming grew in this area. The main findings of this study are summarized as follows: The people in the study area belong to various social groups, which are generally classified into the groups with territorial relations and those with kinship relations. The former include residents' associations of local districts or settlements, alumni association of an elementary school, patron of a local shrine, and membership of irrigation districts. The latter include ties between parents and children, ties among close relatives such as brothers, sisters, uncles, nephews, and cousins, relations between head and branch families, and relations by marriage. Lending and borrowing of farmland were mostly pursued within the district where tenant farmers lived, and were strongly influenced by close territorial and kinship relations. This type of farmland accumulation has long contributed to the development of large-scale tenant rice farming and to its sustainability. On the other hand, a new type of farmland accumulation has recently emerged as third-party organization has begun to mediate between tenants and landowners. The mediator in this case is the municipal government of Narita. Present successful tenant farmers have accumulated their farmlands by taking advantage of this opportunity in addition to traditional territorial and kinship relations.
The 2008 Iwate-Miyagi Nairiku earthquake (Mw 6.9) struck an area where no active fault was known prior to the earthquake. The earthquake was shallow and large enough to produce scattered and discontinuous surface ruptures. To better understand the source processes and seismotectonic setting of the earthquake, we mapped the coseismic surface ruptures observed at Oshu and Ichinoseki Cities, Iwate Prefecture. The NNE- to NNW-trending surface ruptures appeared at three localities: (1) between Mochikorobashi and Ochiai, (2) Itagawa, and (3) southwest of Shinyu hot spring. Most of the surface ruptures are fold scarps up on the west, which are consistent with source fault models based on geodetic and seismological data. These surface ruptures appear to be associated with a reverse slip on west-dipping faults imaged by seismic reflection profiling, which originated as Miocene normal faults and have been reactivated as reverse faults since the Pliocene. Some of the ruptures appeared along preexisting tectonic scarps, although these scarps are short and discontinuous. Geomorphic expression of the causative faults of the 2008 earthquake is weak. Integration of geomorphic, geologic and geophysical data is necessary to identify active faults in areas where crustal strain is distributed to multiple fault strands and thus the tectonic landform of each fault is not well developed.
This short article reviews studies on ophiolites over the last two decades. Chapter titles are; I. Introduction, II. Definition of ophiolites and differences from other mafic-ultramafic complexes, III. Classification of ophiolites and their petrologic diversity, IV. Ophiolites and the oceanic crust, V. Relationships between ophiolites and metamorphic rocks and accretionary complexes, VI. Time-space distribution of ophiolites, VII. Genetic settings of ophiolites (Miyashiro's debate and consequences), VIII. Conclusion. Special attention has been paid to recently discovered oldest and youngest ophiolites as well as the recent re-evaluation of “Troodos Debate” provoked by Miyashiro (1973), who threw a rock that stirred up the peaceful consensus among the world's geologists that correlated ophiolites to mid-ocean ridges, and stimulated extensive geochemical and geotectonic studies on both ophiolites and ocean floor. Geologists are eventually approaching a new consensus that most ophiolites formed at a subduction initiation along the edge of a new oceanic plate.
We analyzed changes in the relationship between geo-environment and agricultural land development around the central Jamuna (Brahmaputra) River, central Bangladesh. Based on sedimentary facies analysis and AMS radiocarbon dating, some conclusions are as follows: (1) Natural levees along the Jamuna River formed until ca. 12 to 11 k cal yrs BP; (2) Homesteads (bari-bhiti) were repaired by public works (mati-kata) following some flood events; (3) Paddy fields and homesteads were created since ca. 1.3 k cal yrs BP around central Bangladesh (central Bengal Delta).
Coral reef terraces are one of the best recorders of biological response to environmental change events (e.g., sea-level changes). Kikai Island provides a rare opportunity to show biological and ecological frameworks (e.g., competition, coexistence, and succession) during a recent geological period. The island is fringed by raised Holocene raised reef terraces, which formed as a result of periodic tectonic uplifts. This study aims to characterize the spatial and temporal changes of corals at this island during the Holocene. The analysis is based on topographical and biological data obtained for the three sites (Shidooke, Kadon, and Nakugama reefs). Three raised reef terraces (Terrace II, III, and IV) grew from 7300 to 4500 years ago (during 2800 years), from 4500 to 2900 years ago (during 1600 years), and from 2900 to 1800 years ago (during 1100 years), respectively. Terrace II and III were uplifted 1-2 m around 4500 years ago and around 2900 years ago. Terrace IV was uplifted 1-2 m around 1800 years ago. The modern reef has been composed of corals for 1800 years. Sixteen coral genera and 53 species were recorded from the reef terraces. Terrace III and IV were dominated by four coral species (A. digitifera, A. robusta, G. retiformis, and F. stelligera), but Terrace II was predominantly composed of A. digitifera and A. robusta. These biological and ecological variations between the terraces represent a growth strategy responding to differences of reef growth time and/or insolation.
Eighteen species of Early and Middle Ordovician conodonts belonging to sixteen genera were discovered in the Kaki Bukit Formation in the Langkawi Islands, northern peninsular Malaysia. These conodonts are classified into faunas A and B, and are contained in the lower and uppermost parts of the study section, respectively. Fauna A including Cooperignathus nyinti, Reutterodus sp. cf. R. andius, and Acodus deltatus indicates the late Floian age, the latest Early Ordovician. Jumodontus gananda and Tropodus laevis in fauna B show that the upper part of the section is of the earliest Middle Ordovician age. Thus, the Lower and Middle Ordovician boundary exists in the middle of the section. It is inferred from lithological analyses that limestones of the Kaki Bukit Formation were deposited on a carbonate platform on a shelf during the Early and Middle Ordovician age. Biogeographical evidence suggests Early Ordovician conodonts from Southeast Asia belong to the Australian province, which was characterized by a tropical climate and a shallow-water environment. Further research on Lower and lower Middle Ordovician limestones in the Langkawi Islands would clarify the conodont biostratigraphy and transitions of paleobiogeography and depositional environments in detail.
The Lake Mikata and Nakayama lowlands are situated along the central coast of Wakasa Bay, at the center of the Japan Sea side of Honshu Island, and fringed by the Mikata fault zone at the east side. The 1662 Kanbun Great Earthquake (estimated magnitude of 7.2 to 7.6) occurred there with vertical crustal movements up to 3 to 4 meters. Submerged topography with thick alluvial deposits in the lowlands might have been caused by Late Quaternary crustal movements of the Mikata fault zone. We analyzed the subsurface geology of the lowlands using many drilling cores recently obtained for scientific purpose such as the 60-m long MK09 core and highway construction (represented by NEXCO core). The main results are summarized below. (1) Subsurface geology beneath the eastern basin and eastern coast of Lake Mikata is clarified by detailed investigations of many drilling cores up to 100 m deep including the NEXCO core. Many radiocarbon (14C) dating and tephrochronological correlations have elucidated the standard chronostratigraphy during the Late Quaternary in this area. At least 100-m thick sediments have deposited continuously for about 130 kyr, recording climatic and seismic events etc. during the last glacial period. (2) The MK09 core is composed of a repeated coarsening-upward sequence with rapid facies changes at unit boundaries. We infer that these units reflect the rapid rise of a relative level of a paleo-lake and subsequent progradation of alluvial fans. The mean interval of individual units is estimated to be about 7.7 kyr from the sedimentary age of each unit, and is considered to be an upper limit of the recurrence interval of the Mikata fault zone. (3) The Mikata fault is recognized for observations of new outcrops and core inspections at the northeastern part of the Nakayama lowland. The vertical displacement across the Mikata fault is inferred to be 30-35 m, and probably reached further after formation of a higher terrace. (4) The concealed Mikata fault extending N-S parallel to the Mikata fault on the east, is estimated from our drilling data. On the basis of the displacements of tephra horizons such as the Aira-Tn volcanic ash beds (AT) in deposits of the Nakayama lowland, it is possible that activity of this fault is equivalent to or larger than that of the Mikata fault. Further detailed studies by deeper drilling cores and seismic refraction surveys are required to clarify the E-W direction underground structure across the Mikata fault zone. They will unravel major development histories of this lowland with many datable materials.
Many hummocky terrains are found preserved along the courses of debris avalanches. This research investigates the functional relation between the size (plane area) and the run-out distance of hummocks on debris avalanche depositional surfaces in Japan. The geomorphic meanings of coefficients of the function expressing this size-distance relation are discussed, along with the geomorphological role of catastrophic volcanic sector collapses. The sizes of hummocks decrease with distance and can be expressed by an exponential function of distance for individual debris avalanches, although their distribution patterns vary from one debris avalanche to the next. Concerning the function expressing the size-distance relation, slope coefficients, implying the rate of decrease of the sizes of hummocks with distance, show a correlation with the equivalent coefficients of friction. This indicates that the downsizing rate of hummocks is controlled by the mobility of debris avalanches. Intercept coefficients, which mathematically indicate the initial average size of hummocks at a distance of 0 km, show a high correlation with the volumes of collapsed masses. The initial average sizes of hummocks at the source area are potentially determined by the volume of collapsed mass of the volcanic body. During the last 500 years in Japan, catastrophic collapses at volcanoes have accounted for an annual denudation rate of ca. 0.5 mm/y per unit area of Quaternary volcanoes, which is comparable to the contemporary denudation rate of non-volcanic mountains in Japan. Furthermore, accompanying debris avalanches cause severe disasters in and around the volcanoes. In such situations, the size-distance relation of hummocks, even for part of a debris avalanche, would contribute to revealing the magnitude and damage of debris avalanches.
Tam Giang Lagoon, lying along the coast of Hue Province in Central Viet Nam, has undergone a rapid expansion of shrimp aquaculture over the last dozen years. Intensive shrimp farming has expanded not only in and along the littoral zone of the lagoon, but also over lowland paddy fields and on coastal sand dunes. In this research, the authors studied three areas that include the coastal sand dune area southeast of Tam Giang Lagoon, classifying shrimp farming into six types in terms of aquacultural facility and operational management, while identifying their geographical distributions and processes of development during the period 1999-2009 through on-site survey and interpretation of multi-temporal high-resolution satellite images of Landsat7-ETM, QuickBird and ALOS-AVNIR-2. Some environmental issues accompanying the development of the respective types of shrimp farming are schematized to make an assessment of lagoon environments from the perspective of sustainable resource management in this area. In the results, the following three conclusions are presented. (1) Overcrowded installation of aquacultural facilities in and along the littoral zone has deteriorated water quality in the lagoons, and such facilities may have increased recent flood levels during times of flooding. (2) Intensive aquaculture spreading over the lacustrine lowlands has caused further water pollution in the lagoons and outbreaks of shrimp diseases, and this type of aquaculture may lead to saltwater intruding into the groundwater under adjacent paddy fields. (3) Recent development of large-scale intensive shrimp farming on coastal sand dunes may cause a shrinkage of the freshwater lens existing under the sand dunes, as well as leakage of saltwater from shrimp ponds into the freshwater lens.
The bottom topography of lake Ashi-no-ko, in the Hakone volcano caldera, was examined using the SeaBat8101 narrow multi-beam sonar system. The purpose of micro-topographical observations of the lake bottom is to understand the tectonic relations between the historical formation-process of the caldera and the strike-slip motion in the Tanna-Hirayama active fault system. At the southeastern part of the lake, west of the Dougashima, undulations at the bottom such as swells, hollows, and stairs extending in the northwest-southeast direction, were observed. These bottom-forms show a northern extension of the Hakonemachi active fault, running along the east side of lake Ashi-no-ko. They may correspond to one of the north spray faults of the Kita-Izu active fault system, which is the source of the A.D.1930 Kita-Izu earthquake, and to motions of the western margin of the Manazuru micro-plate, in relation to caldera formation as a strike-slip basin.
In Japan, permafrost had been believed to be present on Mt. Fuji (3776 m asl.). However, available information on the permafrost on Mt. Fuji is mostly derived from studies undertaken in the 1970s and there has been no direct monitoring of ground temperature remaining at or below 0°C over one year. From 2008, we started monitoring ground temperature and related meteorological parameters near the summit. Surprisingly, our 3-m deep borehole showed the absence of permafrost even though the site is located in a cold climate with a mean annual air temperature below -5°C. Thus, we focused on the unknown mechanism that is keeping the ground warm on Mt. Fuji. In this study, we compare temporal variations of ground temperature with rainfall events during the ground-thawing period in 2009. A rapid increase of ground temperature immediately followed rainfall. Thawing of the ground was promoted by warm rainfall or cold rainfall getting heat from the warm surface soil when the water permeated the soil. Ground electrical resistivity indicates a dry surface above the seasonally frozen layer at the monitoring site in mid-summer. The high permeability of the volcanic debris usually keeps the ground dry and effectively allows heat transportation by rainfall infiltration. This study indicates that, in addition to heat conduction, heat transportation by rainfall infiltration has to be taken into account for ground thawing in the high mountains of Japan, because of warm humid conditions in summer.
In warm-humid and tectonically active regions such as Japan it is important to investigate the geomorphic development of recent alluvial plains controlled by sea-level changes to prevent natural disasters. This is because human activities concentrate coastal alluvial plains composed of unconsolidated soft sediments (alluvium) including inner-bay mud, which amplify seismic intensity. However, we know little about the general shape of the alluvium quantitatively although several river basins have been investigated in detail. We selected 33 trunk rivers throughout Japan and measured the Present River long-Profile (PRP) along with the Last Glacial River long-Profile (LGRP) defined by the depositional surface of basal gravel layers formed under the influence of falling sealevels during the last glacial period. The thickness of the alluvium given by the relative altitude between PRP and LGRP was basically in proportion to the size of the river basin, and it decreased linearly upstream with the exception of several rivers such as Shinano, Kiso, Oita, and Yoshino. Along these rivers, the alluvium is markedly thicker than expected probably because of tectonic subsidence occurring repetitively after formation of basal gravel layers. This implies that the fluvial response to sea-level change is sensitive enough to distinguish the effects of the accumulation of tectonic movements after deposition of basal gravels. In stable or slightly uplifted areas, the inland distribution of inner-bay mud is basically controlled by river basin size.
In May 2010, the mid-congress of INQUA, the International Focus Group on Tephrochronology and Volcanism (INTAV) was held in Kirishima City, Kagoshima Prefecture, Japan. The local organizing committee INTAV-J (based on the Japan Association for Quaternary Research) started to plan the conference in 2007. Financial support was provided by Tokyo Geographical Society in addition to INQUA, PAGES, WEST JEC, Asahi Insatsu Co., Ltd and Paleo Labo Co., Ltd. The conference focused on eight scientific themes and calls for papers. We also invited six keynote speakers. A total of 76 participants attended the conference from 11 countries (Australia: two, Canada: four, Germany: one, Indonesia: one, Japan: 42, New Zealand: four, Philippines: one, Russia: one, Sweden: three, UK: 12, USA: five), and 37 persons attended the post-conference excursion (15th, 16th, and 17th). Forty-nine oral presentations and 38 poster presentations were given over three days (10th, 11th and 14th May). On 12th and 13th May, one-day intra-conference field trips were organized for all participants. Papers emerging from the conference will be published in a special issue of Quaternary International, supported by INTAV executives.
The fourth international symposium of IGCP507, “Paleoclimates of the Cretaceous in Asia and their global correlation” was held in Kumamoto, on December 1-6, 2009. The purpose of the symposium, which was organized by the Scientific Committee of the IGCP507 Symposium, was to investigate Cretaceous biodiversity, faunal changes, environments, and climate in Asia through geological analyses of rock and fossil records. Seventy participants registered to attend the conference from 10 countries, and gave 28 oral and 28 poster presentations, two keynote speeches, one workshop “Cretaceous to Recent bryozoans”, and five sessions addressing (1) Vertebrate evolution in Asia during the Cretaceous (thematic session), (2) Paleontology, (3) Paleoclimate, (4) Sedimentology, and (5) Tectonics, on December 4-5. There was a pre-symposium field excursion, “Cretaceous non-marine and shallow marine facies and fossils in western Kumamoto, Kyushu, Japan”, to northern parts of Amakusa Islands (December 1-3). The excursion focused on sedimentary processes, faunal changes in benthic molluscan fossils, and biostratigraphy in the mid-Cretaceous Goshoura and the Upper Cretaceous Himenoura groups. During the post-symposium field excursion, “Mifune Dinosaur Museum and Upper Cretaceous Dinosaur site in the Mifune Group, Kumamoto (December 6)”, participants visited a typical terrestrial vertebrate site and museum exhibitions.