著者

椚座 圭太郎 後藤 篤

出版者

公益社団法人 東京地学協会

雑誌

地学雑誌 (ISSN:0022135X)

巻号頁・発行日

vol.119, no.2, pp.279-293, 2010-04-25 (Released:2010-07-06)

参考文献数

50

被引用文献数

19 39

---

This paper examines the early stage of the geotectonic history of the Japanese Islands on the basis of finding hydrothermal jadeitite including zircons of ca. 520 Ma in serpentinite mélange of the Itoigawa-Omi area of the Hida-Gaien belt, Central Japan. Hydrothermal jadeitite contains euhedral jadeite in natrolite veins and patches, and consists of jadeite-albite and jadeite-natrolite without quartz. These minerals were crystallized from an aqueous fluid phase at the low-pressure and high-temperature side of the reaction boundary of albite = jadeite + quartz in the system NaAlSiO4-SiO2-H2O. The occurrence of rounded relict hornblende mantled by omphacite rimmed by fine-grained aggregates of jadeite in the matrix of jadeite and albite suggests a pervasive hydrothermal fluid flow, through which metabasite was extensively replaced by jadeitite. This rather high-temperature hydrothermal activity of ca. 520 Ma did not occur in an ordinary subduction zone but in a newly-formed mantle wedge suffering severe hydration from a subducting slab. Recently accumulated U-Pb ages of zircon of ca. 450-500 Ma from paleozoic sediments and granitic rocks of the Hida-Gaien belt were due to initiation of subduction followed by subduction zone magmatism. Protolith of serpentinite in the Hida-Gaien belt includes highly depleted harzburgite, thus requiring tectonic setting of a high-temperature-rift zone rather than a low-temperature-slow spreading ridge. Subduction was initiated at ca. 520 Ma along the boundary between low-density harzburgitic rift zone peridotite and lherzolitic spreading ridge peridotite with a slightly higher density, resulting in the common occurrence of harzburgitic serpentinite in the oldest part of the accretionary complex of Southwest Japan. An area including the Japanese Islands was born around the Yangtze block by the breaking up of the Rodinia supercontinent, because the oldest K-Ar age of biotite actinolit rock of 672 Ma (Matsumoto et al., 1981) and the subduction initiation of ca. 520 Ma are in accord with the paleogeographic history of the Yangtze block, and because ca. 300 Ma Renge schists of the Hida-Gaien belt did not suffer the ca. 280-200 Ma collision-type metamorphism of the Hida metamorphic belt that is an eastern extension of the suture between the Sino-Korea and Yangtze blocks.

著者

浜口 博之 長谷川 昭 植木 貞人 高木 章雄

出版者

Tokyo Geographical Society

雑誌

地学雑誌 (ISSN:0022135X)

巻号頁・発行日

vol.97, no.4, pp.333-340, 1988-08-25 (Released:2011-02-17)

参考文献数

15

著者

澤口 晋一

出版者

学術雑誌目次速報データベース由来

雑誌

地學雜誌 (ISSN:0022135X)

巻号頁・発行日

vol.104, no.6, pp.874-894, 1995

被引用文献数

4 8

---

The rates and processes of periglacial mass movement were measured on the vegetation-free debris-mantled slopes in middle reaches of Adventdalen and Reindalen in central Spitsbergen Svalbard (Fig. 1). All slopes are mainly covered with debris of sandstone and shale of Jurassic, Cretaceous and Tertiary age (Fig. 2). A grain size analysis shows that surface layers of these slopes are composed of rubble and frost-susceptible fine materials (Fig. 5). The maximum depth of active layer varies year by year, but the average depth is estimated about 1 meter. Total 17 painted-stone lines were installed nearly horizontally on the slope surface in above 2 areas : 12 lines during the summer, 1988 and -5 during the summer, 1989 in order to detect movement rates of surface materials (Table 1). Eight pieces of flexible glass-fibre tubes (5 mm in diameter) were inserted into the ground vertically in 1988 and 1989. Furthermore measurement of year-round ground temperature and frost heave was achieved at north-facing mountain slope (700 m a.s.l.) of Mt. Skolten, using a data logger at 3-hour intervals from August, 1990 to July, 1991. Sensors were installed at 0, 5, 20, 40, 60 and 100cm in depth.<BR>Annual averages of movement rates measured by painted stone-lines are 0.6 cm to 11.2 cm (Table 1). Movement rates of surface rubble and slope gradients have roughly mutual relation (Fig. 3). Deformation of all painted stone-lines were patterns parallel to the base lines, and this occurred on every type of slope materials independently of the thickness and size of surface rubble layer (Fig. 4). The vertical velocity profiles of the excavated tubes fall into three types : straight profile keeping tilt (Fig. 6 JH-1), concave profile indicating the greatest movement at the surface (5-1, S-2, S-4, R-5) and complex (concave/convex) profile indicating relatively larger movement at depth (AD-4, S-3, R-3). The movement of the tops of all tubes averaging 3.4 cm, which similar to the average rate of movement measured by painted stones. This implies that these three types of deformation were formed by the same processes, namely frost creep and gelifluction.<BR>Only eleven diurnal freeze-thaw cycles were recorded at the ground surface on the northfacing mountain slope (19°) of Mt. Skolten from August, 1990 to July, 1991 (Fig. 7-a). Maximum and average depth of ground freezing occurred during these short-term cycles were 6.5 cm and 3.2 cm respectively. Frost heave recorded as the average 0.47 cm per 1 event, occurred from middle August to middle September, corresponding to the diurnal freeze-thaw cycles (Fig. 8). On the other hand, the heave caused by seasonal frost occurred from 21 to 27 August and the amount was 3.0cm (Fig. 8). The seasonal heave occurred untill the frost table reached less than 30cm in depth. The cumulative amount of diurnal and seasonal frost-heave is 8.7 cm. Accordingly amount of potential frost creep for the year is calculated to 2.7 cm on the 19 degrees slope. The annual average movement rate which was measured by painted stonelines at the same slope is 4.0 cm. This value is lager than the potential frost creep. Since the movement of painted stone-line resulted from a combination of frost creep and gelifluction, the value (1.3 cm) which deduct potential frost creep from movement by painted line is regarded as movement by gelifluction.<BR>An increase of thickness of surface rubble layer without interstitial fine materials is the cause of decrease of rubble movement because ice segregation do not occur in such rubble layer (Fig. 9). Especially the surface rubble attaining more than about 45 cm in thickness suddenly decrease in movement rates by the reason of the layer underlying such thick rubbles maintain the frozen conditions for all the year round.

著者

早津 賢二 清水 智 板谷 徹丸

出版者

公益社団法人 東京地学協会

雑誌

地学雑誌 (ISSN:0022135X)

巻号頁・発行日

vol.103, no.3, pp.207-220, 1994-06-25 (Released:2009-11-12)

参考文献数

52

被引用文献数

6 8

---

The Myoko volcano group, consisting of five stratovolcanoes, Myoko, Kurohime, Iizuna, Yakeyama and Madarao, is situated in the northern part of central Japan. Twenty one volcanic rocks from the early stage of volcanic successions of Myoko (5 samples), Kurohime (4 samples), Iizuna (9 samples) and Madarao (3 samples) were dated with K-Ar methods. On the basis of previously well studied geology and petrology of the volcano group and chronology of marker tephra layers in this area together with newly obtained K-Ar ages, volcanic history of the Myoko volcano group was discussed in detail. The results confirmed the genetical story of the volcanoes by Hayatsu (1985), i. e., the Myoko volcano has grown by four stages of active volcanisms with three distinctive pauses and each stage has a chemical fractionation trend from basalt to dacite through andesite during a volcanism, and other volcanoes were also in the same story though number of stages was variable : three for Kurohime and two for Iizuna and Madarao, and the fractionation trend was little changeable. This paper calls this type of volcano “poly-generation volcano”.Growth mechanism of the poly-generation volcano was also revealed in detail : life span of each generation, that is time span of each stage of activity, is nearly constant, 20-50 kyrs though some exceptions but total amount of volcanic ejecta decreases with time, e. g., 40-20-7-5 km3 for Myoko, and pause period between any two active volcanic stages decreases with time from 100-160 kyrs of early pause to ca. 10 kyrs of late one.This poly-generation volcano is common in Japan and should be studied in further detail from view points of igenous petrology and tectonics of magmatism as well as geothermal energy and volcanic hazards.

著者

長岡 正利

出版者

公益社団法人 東京地学協会

雑誌

地學雜誌 (ISSN:0022135X)

巻号頁・発行日

vol.93, no.4, pp.Plate1-Plate2, 1984

被引用文献数

2

著者

米倉 二郎

出版者

公益社団法人 東京地学協会

雑誌

地学雑誌 (ISSN:0022135X)

巻号頁・発行日

vol.61, no.1, pp.8-13, 1952-03-30 (Released:2009-11-12)

参考文献数

15

被引用文献数

3

---

The Roman field was planned by Agrimensores with an instrument “Groma.” They made at first straight roads pointing north and south was called Kardos and east west road was called Limes Decumanus. With these two roads as axis they divided their field into sections 2400 Roman feet (nearly equal to 710.4 metres) square which was called Centuria. One Centuria was also divided into 100 Heredia by small lanes. Each Heredium had 240 Roman feet square. (See Map I) It is said that Romulus, the founder of Rome, gave one Heredium to each of his people. One Heredium consisted of two Jugera, and one Jngerum was the area which two oxen could till in one day. One Jugerum had two Acti. One Actus was the area of 120 Roman feet square.We can find as yet in many plains of Italy and other European countries, for example in England, the remnant of the Roman field system. The township system of northern America may be said the hereditary of the Roman field system.Ancient Chinese in Chou Dynasty also planned their field by making the main road crossing each other at right. angles. The north-south road was called T'sien and the east-west one Me. Later on with these two roads as axis they divided the field with basic block of one Ri or 300 Pu (nearly equal to 414.5 metres) square, called Tsing-t'ien. One Tsing-t'ien was composed of nine Fu which was 100 Pu square. Fu means one family's holding, so one Tsing-t'ien was the ideal type of small village which had eight families settled themselves at central Fu section, and had to farm other eight Fus. (See Map II).The field planning of Romans and that of ancient Chinese both with their square shaped field can also be called as square field system, each resembling so much that it seems these two methods must have a common origin.This field system was planned at first in the suburbs of Rome and Loyang, capital of Chou Dynasty, in the age of their city state. Therefore the origin of the former system may be traced back to the Orient where the foremost city state blossomed in human history.The square field system was planned with great accuracy by survey and arranged irrigation ditches alongside the roads and lanes, also the standard shape of the field was decided by ploughing with two oxen.These characteristics of agriculture are just the same which Prof. Gordon Chiide has called as the second revolution of human civilization. He says that this city revolution has occured at first in Mesopotamia, Egypt and India relating each other, and from there it spread out all over the world.We have found not yet the exact remnant of the square field system in these oldest countries. But some inscribed tablets show the continuity of the method adopted for example, Agrimensores of Rome, elected boundary stones after surveying at the corners of the field, and in Mesopotamia we know there were also boundary stones, and in the Sumerian city state the people (Sûb-lugal) were provided with uniformed feud.Chinese Tsing-Vien system also propagated to the east Asian countries, and especially in Japan we have a typical square field system “Jori.” Yamato basin, Nara prefecture, famous with many old temples, is the district in which Jori system was being held perfectly, Naka Gaido, the north-south road which runs in middle of the basin, was the principal meridian, and Yoko. Ohoji, east-west road connecting Sakurai, Yagi, and Takata towns which runs rather southern part of the basin was the base line of Jori of Yamato. Ground Block of Joni was called Ri and was one Ri square, the length Ri which was introduced from Ti of Chinese but prolonged a little, was about 640 metres. (See Map III).

著者

田中 阿歌麻呂

出版者

Tokyo Geographical Society

雑誌

地学雑誌 (ISSN:0022135X)

巻号頁・発行日

vol.18, no.6, pp.415-419, 1906

著者

青木 一勝 大藤 茂 柳井 修一 丸山 茂徳

出版者

公益社団法人 東京地学協会

雑誌

地学雑誌 (ISSN:0022135X)

巻号頁・発行日

vol.119, no.2, pp.313-332, 2010-04-25 (Released:2010-07-06)

参考文献数

88

被引用文献数

15 24

---

The Sanbagawa metamorphic belt in SW Japan was previously considered to extend in the E-W direction from the Kanto Mountains to Kyushu Island, a distance > 800 km. However, Aoki et al. (2007) recently demonstrated that protoliths of metamorphic rocks in the Oboke area of the belt in central Shikoku accumulated at the trench after ca. 90-80 Ma. Furthermore, Aoki et al. (2008) showed that these rocks suffered blueschist metamorphism at 66-61 Ma, which differs from the timing of the Sanbagawa metamorphism. Thus, these results show that the Sanbagawa belt in Shikoku is a composite metamorphic belt. We, therefore, redefine the traditional Sanbagawa belt; the structurally upper part is the Sanbagawa metamorphic belt (sensu stricto). It formed as an accretionary complex at ca. 140-130 Ma and subsequently experienced BS-EC facies metamorphism at ca. 120-110 Ma (Okamoto et al., 2004). By contrast, the structurally lower segment termed the Shimanto BS facies metamorphic belt, formed as an accretionary complex after ca. 90-80 Ma and experienced peak metamorphism at ca. 60 Ma. Our observations have important implications for the lateral extension of these two metamorphic belts in SW Japan. The accretionary ages of the traditional Sanbagawa belt in the Kanto Mountains are younger than the Sanbagawa peak metamorphic age (Tsutsumi et al., 2009), clearly indicating that the entire region of Kanto Mountains Sanbagawa must belong to the Shimanto metamorphic belt. The same timing relationships were also found for the Sanbagawa belt on Kii Peninsula (Otoh et al., 2010). These results, therefore, indicate that the Shimanto metamorphic belt is exposed in Shikoku, Kii, and Kanto, thus the spatial distribution of Sanbagawa belt (ss) is less than half of its previous extent. The metamorphic grade of the Kanto Mountains in the Shimanto metamorphic belt ranges from pumpellyite-actinolite facies to epidote-amphibolite facies. Therefore, the higher-grade rocks of the Shimanto metamorphic rocks are exposed in the Kanto Mountains in comparison with Shikoku and Kii Peninsula. Hence, these two distinct BS-EA-EC (?) metamorphic belts are virtually equivalent in terms of spatial distribution, metamorphic range of grade, and facies series. Pacific-type orogenic belts typically comprise accretionary complex, high-P/T metamorphic belt, fore-arc sediments, and batholith belt landward from the trench (Maruyama et al., 1996). In SW Japan, the Sanbagawa belt (ss) is paired with the Ryoke low-P/T metamorphic belt and with the ca. 120-70 Ma Sanyo TTG batholith belt. Furthermore the related fore-arc basin may have developed penecontemporaneously with the Shimanto BS-EA orogeny, which is paired with the late Cretaceous to early Tertiary San-in TTG belt, which extending along the Japan Sea coast. In-between the intervening Izumi Group, a fore-arc basin deposit formed during the Campanian to Maastrichtian. Thus, these two groups of orogenic units, which formed during independent orogenies were both extensively modified during the opening of the Japan Sea ca. 20 Ma. The southward thrusting of the Ryoke and Cretaceous TTG belts over the Sanbagawa extended beyond the southern limit of the Sanbagawa, leading the up-down relationship of the Sanbagawa (ss) and the Ryoke belts.

著者

大川 通久

出版者

Tokyo Geographical Society

雑誌

地学雑誌 (ISSN:0022135X)

巻号頁・発行日

vol.5, no.4, pp.194-196, 1893

著者

大藤 茂 下條 将徳 青木 一勝 中間 隆晃 丸山 茂徳 柳井 修一

出版者

公益社団法人 東京地学協会

雑誌

地学雑誌 (ISSN:0022135X)

巻号頁・発行日

vol.119, no.2, pp.333-346, 2010-04-25 (Released:2010-07-06)

参考文献数

41

被引用文献数

33 44

---

We measured the 206Pb/238U age distribution of detrital zircons in five psammitic schist samples from the Sanbagawa Belt in east-central Shikoku and the western Kii Peninsula to constrain their depositional age. The age-distribution diagrams for the five psammitic schist samples all show that detrital zircons of 100 to 90 Ma are most abundant and the age of the youngest zircon in each sample is less than 80 Ma. Considering the age of the retrogressive metamorphism of these psammitic schists, ca. 80-60 Ma, the protoliths age of the psammitic schists is constrained to 75-70 Ma, correlative to the age of the sandstone of the Middle Shimanto Belt (Yanai, 1984). A similar age-distribution has already been reported for two psammitic schist samples from the Central Unit of the Sanbagawa Belt in the Kanto Mountains (Tsutsumi et al., 2009). Thus the Sanbagawa Belt is most widely occupied by metamorphic rocks originating from rocks of the Middle Shimanto Belt. We also measured the 206Pb/238U age distribution of detrital zircons in Turonian sandstone from the Northern Shimanto Belt in the central Kii Peninsula. The age-distribution diagram shows that detrital zircons of around 128 Ma are most abundant and the age of the youngest zircon in the sample is about 100 Ma. A similar age-distribution has already been reported from a psammitic schist sample from the Southern Unit of the Sanbagawa Belt in the Kanto Mountains, overlying the Central Unit (Tsutsumi et al., 2009). The protolith age is still younger than the metamorphic age of the eclogites in central Shikoku, ca. 120-110 Ma (Okamoto et al., 2004), which occupy the uppermost portion of the Sanbagawa Belt. Although some previous studies suggested that the Sanbagawa Belt consists of metamorphosed Late Jurassic to Early Cretaceous accretionary complex, the present study shows that the belt is largely occupied by metamorphosed Late Cretaceous rocks: the Shimanto Metamorphic Rocks of Aoki et al. (2007). As a result, the Sanbagawa Belt consists of the following three units with different protolith ages: (1) Lower Unit of Shimanto Metamorphic Rocks with protoliths ages of 75-70 Ma and metamorphic ages of 70-60 Ma, (2) Upper Unit of Shimanto Metamorphic Rocks with protoliths ages of 95-85 Ma and metamorphic ages of 85-75 Ma, and (3) Sanbagawa Metamorphic Rocks (s.s.) with protoliths ages of Late Jurassic to Early Cretaceous and metamorphic ages of 120-110 Ma. The protoliths of the Upper and Lower units of the Shimanto Metamorphic Rocks are most likely rocks of the Northern Shimanto and Middle Shimanto belts, respectively.

出版者

Tokyo Geographical Society

雑誌

地学雑誌 (ISSN:0022135X)

巻号頁・発行日

vol.15, no.3, pp.273a-274, 1903

出版者

Tokyo Geographical Society

雑誌

地学雑誌 (ISSN:0022135X)

巻号頁・発行日

vol.19, no.10, pp.759a-759a, 1907

著者

鈴木 敏

出版者

Tokyo Geographical Society

雑誌

地学雑誌 (ISSN:0022135X)

巻号頁・発行日

vol.1, no.7, pp.307-317, 1889

出版者

Tokyo Geographical Society

雑誌

地学雑誌 (ISSN:0022135X)

巻号頁・発行日

vol.1, no.9, pp.440-440, 1889

著者

飯本 信之

出版者

公益社団法人 東京地学協会

雑誌

地学雑誌 (ISSN:0022135X)

巻号頁・発行日

vol.53, no.4, pp.153-172, 1941-04-15 (Released:2010-10-13)

著者

村内 必典

出版者

公益社団法人 東京地学協会

雑誌

地学雑誌 (ISSN:0022135X)

巻号頁・発行日

vol.73, no.2, pp.83-85, 1964-04-30 (Released:2009-11-12)

著者

秋元 菜摘

出版者

公益社団法人 東京地学協会

雑誌

地学雑誌 (ISSN:0022135X)

巻号頁・発行日

vol.125, no.4, pp.523-544, 2016-08-25 (Released:2016-09-12)

参考文献数

23

被引用文献数

8 7

---

Recently, governments of some cities have adopted urban policies, based on a compact city concept to facilitate accessibility, mainly for mobility-impaired people among Japan's ageing society. The government of Aomori city has planned a popular compact city model, or a centralized urban physical structure, by controlling the urbanized area of the Mid-city zone (population-incentive zone) in the city master plan established in 1999. The effects of measures for providing accessibility to the central railway station as a representative facility of the city center are analyzed, and the implications for urban policy based on a centralized compact city design in Aomori city are investigated. Accessibility, as represented by time-distance to the central station, is measured based on a road network analysis and timetables of train and bus services. As a result, it is observed at a level where most of the population can reach the facility. The simulations are based on the following measures: 1) supporting migration inside of the Mid-city from the Outer-city forming the suburbs; and 2) improving the frequency of bus transportation in the Toyama housing district, a case-study area in the suburbs. The major findings are as follows. 1) Accessibility to the central station by train is improved with migration and the effects are greater for elderly people. It seems to be useful to strengthen these effects if migration is only into the Inner-city. Although variations in migration measures are found under several conditions, accessibility is finally improved to almost the same level. Therefore, it is important to continue migration support measures, even if the pace is slow. 2) Accessibility to the central station by bus from the Toyama housing district is improved substantially under a situation where the frequency of bus services is increased until five-fold. The effect is also improved for elderly people. Some suburban cores such as large housing districts should be set or maintained at least the short-term aims of urban policy, because the population is already distributed in the suburbs and the level of functions of daily life should be maintained there. Lastly, it is important that the city master plan refers to the phasing of centralization from the suburbs, especially, about methods and times. If it is considered that the suburban cores continue independently in the process of compact city policy, the government of the city should include the cores in the urban structure and the city master plan should be revised.

著者

太田 陽子 柏木 修一 桜井 一賀 池田 潤

出版者

Tokyo Geographical Society

雑誌

地学雑誌 (ISSN:0022135X)

巻号頁・発行日

vol.97, no.1, pp.25-38, 1988-02-25 (Released:2009-11-12)

参考文献数

17

被引用文献数

3 2

---

Awashima is a small (7 × 2 km) island on the continental shelf along the wetsern coast of northern Honshu, and uplifted with northwestward tilt associated with the 1964 Niigata earthquake (Figs. 1 and 5). NAKAMURA et al. (1964) proposed a cumulative character of the coseismic uplift since the middle Miocene, of the island, based on the comparison of terrace profile and geologic structure of Tertiary sedimentary rocks with the coseismic tilt by the Niigata earthquake. This paper intends to establish such a progressive tilt of the island, on the basis of the detailed investigation of marine terraces.Marine terraces of Awashima are divided into H, M1, M2 and L terraces (Fig. 2). They are abrasion platforms and no tephra and terrace deposits are found, except thick deposits of L terrace at Uchiura area. The most extensive terrace, M1, can be regarded to be formed at the time of the last interglacial maximum (ca. 125 ka). The height of the former shoreline of M1 terrace ranges from ca. 75 in to the northeast to ca. 50 m to the southwest, indicating a notable northwestward tilt, which is similar to that of the 1964 earthquake, but having much steeper gradient. The gradient of the northwestward tilt of M1 terrace (20 × 10-3) is about 67 times of that of the coseismic tilt at the time of 1964 earthquake (0.3 × 10-3). It suggests that the similar coseismic uplift has repeated since the last interglacial maximum with interval of about 1900 years. Estimated resultant coseismic uplift is about 1.1 m at the northeastern part of the island, where M1 terrace is 75 m high, which is approximately same to the height estimated by the amount of coseismic resultant uplift of 67 times. Thus, average recurrence interval of the major earthquake resulting in the coseismic uplift of Awashima is estimated to be about 1900 years from the comparison of amount of both tilt and uplift.However, no significant difference is observed in terrace profiles of Ml, between A-B, parallel to strike of the tilt, and C?D or E?F, normal to it (Fig. 4). Therefore, terrace profile seems to be an unsuitable indicator for the detection of tectonic deformation, unless much steeper tilt has occured. Distribution of H and M2 terraces is too limited to discuss the deformation pattern.L terrace at Uchiura is underlain by thick marine deposits attaining ca 60 m thick, suggesting that it was formed in association with the postglacial sea level rise. No datable material was found despite examination of many borehole data and our excavation works. L terrace is subdivided into L1, L2 and L3 terraces. Episodic or intermittent emergence, probably coseismic, should have occurred at least 3 times since ca. 6000 yBP. It is consistent with the result obtained by Mi terrace. The height of L1 terrace is ca. 11 m above mean sea level, which means average uplift rate is ca. 1.5 m/ka, it is much larger than 0.7 m/ka for Mi terrace. Northwestward tilt of the lower terraces, however, is not confirmed by this study, owing to the limited distribution.

著者

末岡 茂 田上 高広 堤 浩之 長谷部 徳子 田村 明弘 荒井 章司 山田 隆二 松田 達生 小村 健太朗

出版者

公益社団法人 東京地学協会

雑誌

地学雑誌 (ISSN:0022135X)

巻号頁・発行日

vol.119, no.1, pp.84-101, 2010-02-15 (Released:2010-05-21)

参考文献数

62

被引用文献数

9 8

---

Quantitatively estimating denudation is generally difficult because it essentially involves the removal and loss of materials in situ. The denudation rate of mountainous areas in Japan has commonly been studied from the volume of sediment in a basin or catchment. Nonetheless, the availability of these methods is constrained spatially by upstream area and temporally by depositional age. In the last few decades, thermochronometric methods that evaluate thermal history using radiometric-dating methods have been used to evaluate the denudation and tectonic history of orogenic belts around the world. The advantages of thermochronometric methods are that we can calculate the denudation rate at each sampling point and that combining multiple methods and/or target minerals enables us to calculate denudation rates in multiple periods. However, thermochronometric methods have been applied to areas with extraordinarily high denudation rates in island arc areas such as Japan. Thus, the effectiveness of thermochronometric methods for estimating denudation rates in island arc areas has not been demonstrated. We applied apatite and zircon fission-track thermochronometry to granitic rock samples collected from outcrops and a borehole to estimate the tectonic history of the Rokko area, southwest Japan. Previous studies suggested that the Rokko Mountains have been uplifted by active faulting along their northern and southern margins during the Rokko movements, a Quaternary tectonic movement in the Kinki district. However, the tectonic history of the Rokko area prior to 1 Ma has not been revealed due to a lack of prevalent geologic markers. We estimated average denudation rates in various periods based on apatite and zircon FT ages and previously reported radiometric ages. We obtained denudation rates at about 0.04-0.10 mm/yr after about 30 Ma, 0.05-0.7 mm/yr during 50-30 Ma, 0.7-4.0 mm/yr during 70-50 Ma. The denudation rate after the deposition of the Kobe Group (36.9-30.4 Ma) is estimated to be in the 0.01-0.1 mm/yr order, while bedrock uplift rate after about 1 Ma is estimated to be about 0.5 mm/yr. Thus, the tectonic activity of the Rokko Mountains area prior to 1 Ma has been relatively low.

出版者

公益社団法人 東京地学協会

雑誌

地学雑誌 (ISSN:0022135X)

巻号頁・発行日

vol.11, no.12, pp.836a-839, 1899-11-25 (Released:2010-12-22)