著者

柳井 修一 青木 一勝 赤堀 良光

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.6, pp.1079-1124, 2010-12-25 (Released:2011-03-17)

参考文献数

121

被引用文献数

25 64

---

Median Tectonic Line (MTL) and Fossa Magna (Itoigawa-Shizuoka Tectonic Line) had long been considered to be the most critical fault boundaries controlling development of the Japanese Islands since Naumann (1885) and Kobayashi (1941). After the appearance of plate tectonics, several new interpretations emerged, e.g., sub-surface Benioff plane for the MTL. In this paper, we propose that those tectonic lines, major faults, and Tanakura Tectonic Line (TTL) were formed through a process at micro-plate boundaries during the opening of the Japan Sea in the Miocene. MTL could have been formed along the consuming boundary between the PHS plate and Japan Sea microplate, which has shifted southward to the Nankai trough, accompanying large-scale tectonic erosion. Fossa Magna was formed as a gigantic transform fault with a transtension component in the Medial-Japan Sea when opening was initiated. The eastern and western boundaries of the Japan Sea must be a strike-slip fault, corresponding to TTL to the east, and a newly proposed strike-slip fault called the West Kyushu Tectonic Line, respectively. Fossa Magna, a medial region defined by two NS-trending Miocene parallel faults in central Honshu, defined by Nauman (1885) could be interpreted to be the largest transform fault in the Medial-Japan Sea to offset the spreading axis when the Japan Sea opened. It should be emphasized that large-scale tectonic erosion occurred in front of consuming plate boundaries facing the PHS and PAC plates oceanward during the opening of the Japan Sea. The volume of tectonic erosion is calculated to be 17,581,500 km3, which is equivalent to 2/3 of the present-day Japan arc crust, which is sufficient to reach the depth of the megalith between the upper and lower mantle boundary, even with 10 km thickness of materials eroded and transported along the Benioff zone. Although MTL, Fossa Magna, and TTL are remarkable in the geology of Japan, these young faults never affected the orogeneses of Japan back to 520 Ma, which grew the continental crust of Japan. We propose that microplate boundary processes decreased the volume of the Japan crust.

著者

磯崎 行雄 丸山 茂徳 中間 隆晃 山本 伸次 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.120, no.1, pp.65-99, 2011-02-25 (Released:2011-05-20)

参考文献数

80

被引用文献数

32 48

---

The ca. 700 million year-long geotectonic history of the Japanese Islands comprises three distinct intervals; i.e., (1) the age of a passive continental margin off the South China continental margin (ca. 700-520 Ma), (2) the age of an active margin characterized by an arc-trench system (ca. 520-20 Ma), and (3) the age of an island arc off East Asia (20 Ma to the present). These three intervals are chronologically separated by two major boundaries with significant tectonic episodes; i.e., the ca. 520 Ma tectonic inversion from a passive to an active margin by the initiation of subduction from the Pacific side, and the ca. 20 Ma tectonic isolation of the modern island arc system from the Asian margin by the back-arc basin (Japan Sea) opening. Here, the evolutionary history of the Japanese Islands is revised significantly on the basis of new lines of information that derived from a new dating technique of detrital zircon in sandstone. Particularly noteworthy is the recognition of the Early Paleozoic to Middle Mesozoic arc batholiths that were exposed extensively in the past but not at all at present because the pre-Cretaceous granites merely occur as kilometer-size blocks in the modern Japanese Islands. As to these older granites, the remarkable disagreement between the current distribution and the predominance of their clastic grains in younger sandstones suggests the effectiveness of past tectonic erosion processes in the fore-arc domains. The newly documented historical change in sandstone provenance suggests that proto-Japan has experienced not only accretionary growth but also large-scale tectonic erosion in multiple stages. During the ca. 500 million-year history of the Japanese Islands, a large amount of juvenile arc (continental) crust was formed several times, however, most has already disappeared from the Earth's surface. In short, the orogenic growth of Japan, even in a long-lasting active continental margin setting, is explained as the intermittent repetition of ocean-ward continental growth and continent-ward contraction of an active arc-trench system. In contrast to these arc batholiths, the terrigenous flux from the neighboring two major continental blocks (South and North China) was less significant than previously imagined, except for the Jurassic to Early Cretaceous time when the collisional suture between North and South China blocks was selectively eroded to produce abundant terrigenous clastics. It is also significant that the eastern extension of this collisional suture was recognized in Japan as a chain of fragmentary remnants of the Triassic medium-pressure metamorphic belt. On the basis of these new lines of information, the South China-related origin of the main part of Japan is confirmed, whereas the Hida and Oki belts along the Japan Sea are identified as detached fragments of North China block. Summarizing all of these results, a series of revised paleogeographic maps of Japan from the Late Neoproterozoic to the Miocene is illustrated.

著者

柳井 修一 青木 一勝 赤堀 良光

出版者

Tokyo Geographical Society

雑誌

地學雜誌 (ISSN:0022135X)

巻号頁・発行日

vol.119, no.6, pp.1079-1124, 2010-12-25

被引用文献数

8 64

---

Median Tectonic Line (MTL) and Fossa Magna (Itoigawa-Shizuoka Tectonic Line) had long been considered to be the most critical fault boundaries controlling development of the Japanese Islands since Naumann (1885) and Kobayashi (1941). After the appearance of plate tectonics, several new interpretations emerged, <i>e.g.</i>, sub-surface Benioff plane for the MTL. In this paper, we propose that those tectonic lines, major faults, and Tanakura Tectonic Line (TTL) were formed through a process at micro-plate boundaries during the opening of the Japan Sea in the Miocene.<br> MTL could have been formed along the consuming boundary between the PHS plate and Japan Sea microplate, which has shifted southward to the Nankai trough, accompanying large-scale tectonic erosion. Fossa Magna was formed as a gigantic transform fault with a transtension component in the Medial-Japan Sea when opening was initiated. The eastern and western boundaries of the Japan Sea must be a strike-slip fault, corresponding to TTL to the east, and a newly proposed strike-slip fault called the West Kyushu Tectonic Line, respectively. Fossa Magna, a medial region defined by two NS-trending Miocene parallel faults in central Honshu, defined by Nauman (1885) could be interpreted to be the largest transform fault in the Medial-Japan Sea to offset the spreading axis when the Japan Sea opened.<br> It should be emphasized that large-scale tectonic erosion occurred in front of consuming plate boundaries facing the PHS and PAC plates oceanward during the opening of the Japan Sea. The volume of tectonic erosion is calculated to be 17,581,500 km³, which is equivalent to 2/3 of the present-day Japan arc crust, which is sufficient to reach the depth of the megalith between the upper and lower mantle boundary, even with 10 km thickness of materials eroded and transported along the Benioff zone.<br> Although MTL, Fossa Magna, and TTL are remarkable in the geology of Japan, these young faults never affected the orogeneses of Japan back to 520 Ma, which grew the continental crust of Japan. We propose that microplate boundary processes decreased the volume of the Japan crust.

著者

磯崎 行雄 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.378-391, 2010-04-25 (Released:2010-07-06)

参考文献数

72

被引用文献数

11 9

---

A new historical review is presented on the progress of the geological sciences in Japan since the Meiji revolution in 1868. Geological knowledge, particularly studies of the geotectonic evolution and orogenic aspects, of the Japanese Islands has progressed through three distinct phases; (1) non-science stage, (2) colonial science stage, and (3) independent science stage, as modeled by Basalla (1967), who demonstrated a general pattern of transplanting cutting-edge scientific/technological knowledge from western Europe to the rest of the world. During the “non-science” stage from the 1860s to the 1890s, major geological aspects of the Japanese Islands, together with discoveries of unusual rocks, fossils etc., were initially described by foreign geologists (e.g. E. Naumann). In contrast, almost nothing was contributed by domestic geologists. During the “colonial science” stage, from the 1900s to the 1980s, research and education systems were transplanted effectively from western European countries. For example, applying the purely imported concept of geosyncline, the geotectonic history of the Japanese Islands was summarized for the first time by domestic geologists (e.g., Kobayashi, 1941; Minato et al., 1965 etc.). The almost unidirectional acceptance of plate tectonics also followed at this stage, with the exception of the rare but outstanding contribution of A. Miyashiro during the 1960s-1970s. During the “independent science” stage from the 1980s, various new ideas and original techniques in geology were proposed by Japanese geologists with lesser help from the western countries than before; i.e., practical criteria for identifying ancient accretionary complex, exhumation tectonic of ultrahigh to high-P/T metamorphic rocks, and subhorizontal growth framework of subduction-related orogens. Furthermore, in the first decade of the 21st century, the geological science in Japan entered stage of (4), “exporting science” with the introduction of new paradigms, such as the application of detrital zircon chronology to subduction-related orogens, which efficiently recognizes new geotectonic subdivisions and allows paleogeographical reconstruction with much higher resolution than before. These new paradigms (ideas, techniques) from Japan are now on sale for applying to the rest of the world.

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.

著者

磯崎 行雄 丸山 茂徳 柳井 修一 下司 信夫 佐野 修 笠原 順三

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.187-189, 2010-04-25 (Released:2010-07-06)

参考文献数

15

著者

丸山 茂徳 磯崎 行雄 柳井 修一 下司 信夫 佐野 修 笠原 順三

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.6, pp.959-962, 2010-12-25 (Released:2011-03-17)

参考文献数

28

著者

青木 一勝 大藤 茂 柳井 修一 丸山 茂徳

出版者

Tokyo Geographical Society

雑誌

地學雜誌 (ISSN:0022135X)

巻号頁・発行日

vol.119, no.2, pp.313-332, 2010-04-25

被引用文献数

10 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 <i>et al.</i> (2007) recently demonstrated that protoliths of metamorphic rocks in the Oboke area of the belt in central Shikoku accumulated at the trench after <i>ca.</i> 90-80 Ma. Furthermore, Aoki <i>et al.</i> (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 (<i>sensu stricto</i>). It formed as an accretionary complex at <i>ca.</i> 140-130 Ma and subsequently experienced BS-EC facies metamorphism at <i>ca.</i> 120-110 Ma (Okamoto <i>et al.</i>, 2004). By contrast, the structurally lower segment termed the Shimanto BS facies metamorphic belt, formed as an accretionary complex after <i>ca.</i> 90-80 Ma and experienced peak metamorphism at <i>ca.</i> 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 <i>et al.</i>, 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 <i>et al.</i>, 2010). These results, therefore, indicate that the Shimanto metamorphic belt is exposed in Shikoku, Kii, and Kanto, thus the spatial distribution of Sanbagawa belt (<i>ss</i>) 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 <i>et al.</i>, 1996). In SW Japan, the Sanbagawa belt (<i>ss</i>) is paired with the Ryoke low-P/T metamorphic belt and with the <i>ca.</i> 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 <i>ca.</i> 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 (<i>ss</i>) and the Ryoke belts.

著者

下條 将徳 大藤 茂 柳井 修一 平田 岳史 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.257-269, 2010-04-25 (Released:2010-07-06)

参考文献数

32

被引用文献数

32 43

---

The Japanese Islands have grown through the formation of igneous rocks and accretionary prism caused by subduction of oceanic plates. However, the timing of the initiation of the subduction is not well defined. The South Kitakami Belt (SKB) in NE Japan is the best field to solve this problem. Here, basement igneous rocks are covered by successions of Ordovician to Early Cretaceous beds (450-100 Ma). We obtained LA-ICP-MS U-Pb zircon ages from the following localities. Along the Yakushigawa-Valley section in the northeastern part of the SKB, we examined the ages of (1) trondhjemite of the Kagura Complex of basement igneous rocks, (2) felsic tuff of the Koguro Formation conformably covering the Kagura Complex, and (3) Yakushigawa Formation covering the Koguro Formation and lying under the Silurian Odagoe Formation. In the Ohasama area in the northwestern part of the SKB, we examined the age of (4) Nameirizawa Formation, which probably lies below the Silurian Orikabetoge Formation. Moreover, we examined the ages of four samples from the Hikami Granite body in the central part of the SKB. The ages of trondhjemite (08331-5: 466±6 Ma) of the Kagura Complex and felsic tuff (08331-4b: 457±10 Ma) of the Koguro Formation indicate that the subduction of an oceanic plate had already started at 466 Ma, and that the Koguro Formation is the oldest age-known formation of the SKB. The tuffaceous sandstone of the Yakushigawa Formation (08331-3) has detrital zircons with the youngest age cluster of around 425 Ma, and is probably a Silurian formation. The tuffaceous sandstone of the Nameirizawa Formation (08331-9) has detrital zircons with the youngest age cluster of around 430 Ma. The formation is probably a Silurian formation and is correlated with the Yakushigawa Formation. Precise ages of around 412 Ma were obtained from the Hikamiyama body of the Hikami Granitic Rocks (08330-1,-3,-4), clearly suggesting that at least some parts of the body are not pre-Silurian basement.