Outer Zone Of Southwest Japan Research Articles

Middle Miocene forearc alkaline magmatism in Amami-Oshima Island, central Ryukyu Arc: implications for paleoreconstruction of Shikoku Basin

During the middle Miocene, forearc alkaline magmatism occurred in the Outer Zone of Southwest Japan and the northern Ryukyu Arc, resulting in the formation of forearc alkaline basaltic rocks with ocean island basalt (OIB)-like chemical signatures. In contrast, subduction-related magmatism was present in the central Ryukyu Arc. However, the southwestern margin of the forearc alkaline magmatism was poorly constrained in the Ryukyu Arc. We examined two basaltic dikes in the Chichibu accretionary complex of Amami-Oshima Island, the central Ryukyu Arc. The dikes cut massive basalt, reddish chert, varicolored shale, and grey chert. The chemical compositions of basaltic dikes are characterized by the enrichment of incompatible trace elements, possibly representing a low degree of partial melting from a deep mantle source. 40Ar/39Ar dating analyses indicate that the basaltic dikes yield ages of 16.37 ± 0.14 Ma and 16.51 ± 0.10 Ma. The paleomagnetic analyses on stepwise thermal demagnetizations allowed extracting the direction of stable magnetizations with unblocking temperatures of 450–575 °C for the two dikes [(Dec, Inc) = (138.1°, − 13.3°), (124.0°, − 24.9°)]. The magnetization could be primary, acquired either as part of a secular variation or a geomagnetic excursion during reversed polarity chron/subchron. The 40Ar/39Ar ages and paleomagnetic directions within the error range imply that they intruded almost simultaneously during C5Cn.2r (16.532–16.434 Ma). The age and trace element patterns of the basaltic dikes are comparable to those of Miocene alkaline basaltic rocks, which resulted from forearc alkaline magmatism during the initial subduction of the young and warm Shikoku Basin. The discovery of alkaline basaltic dikes on Amami-Oshima Island suggests that the distribution of middle Miocene forearc alkaline magmatism may extend to the central Ryukyu Arc. Hence, the northern end of the Kyushu-Palau Ridge (i.e., southern end of Shikoku Basin) could have been located south of Amami-Oshima Island around 16.5–16.4 Ma, then moved eastward to the current location.Graphical

Metamorphic Thermal Structure of the Outer Zone of Southwest Japan in the Koshibu-gawa Section, Nagano Prefecture, Central Japan

Metamorphic Thermal Structure of the Outer Zone of Southwest Japan in the Koshibu-gawa Section, Nagano Prefecture, Central Japan

Timescale of magma chamber processes revealed by U–Pb ages, trace element contents and morphology of zircons from the Ishizuchi caldera, Southwest Japan Arc

AbstractU–Pb geochronology and trace element chemistry of zircons in a microscale analysis were applied to the Ishizuchi caldera in the Outer Zone of Southwest Japan in order to estimate the timescale of the magma process, in particular, the magma differentiation. This caldera is composed mainly of ring fault complexes, major pyroclastic flow deposits, and felsic intrusion including central plutons. Using SHRIMP‐IIe, our new U–Pb zircon ages obtained from the major pyroclastic flow deposits (Tengudake pyroclastic flow deposits), granitic rocks from central plutons (Soushikei granodiorite and Teppoishigawa quartz monzonite), and rhyolite from the outer ring dike (Tenchuseki rhyolite) and the inner ring dike (Bansyodani rhyolite) are 14.80 ±0.11 Ma, 14.56 ±0.10 Ma, 14.53 ±0.12 Ma, 14.55 ±0.11 Ma and 14.21 ±0.19 Ma, respectively. Based on the U–Pb ages, the Hf contents and the REE patterns of the zircons, three stages are recognized in the evolutionary history of the magma chamber beneath the Ishizuchi caldera: (i) climactic Tengudake pyroclastic flow eruption; (ii) Tenchuseki rhyolite intrusion into the outer ring dike and central pluton intrusion; and (iii) Bansyodani rhyolite intrusion in the inner ring dike. These results indicate a magma evolution history of the Ishizuchi caldera system which took at least ca 600 kyr from the climatic caldera‐forming eruption to the post‐caldera intrusions. Our new geochronological data suggest that the Ishizuchi caldera formed as part of the voluminous and episodic magmatism that occurred in the wide zone along the Miocene forearc basin of Southwest Japan during the inception of the young Philippine Sea Plate subduction.

Kabunoki Fault in eastern Kyushu as a member of the counterclockwise diagonal faults in the Outer Zone of Southwest Japan

Kabunoki Fault in eastern Kyushu as a member of the counterclockwise diagonal faults in the Outer Zone of Southwest Japan

Atogura Nappe and the late Cretaceous Mikabu thrust of the Outer zone of Southwest Japan

Atogura Nappe and the late Cretaceous Mikabu thrust of the Outer zone of Southwest Japan

Shimakuroxylon a new homoxylous Mesozoic wood genus from Asia, with palaeogeographical and palaeoecological implications

Revision of the Mesozoic fossil wood record for Eastern Asia led to the recognition of a new type of homoxylous wood radial pitting, called here “the japonicum type”. This anatomical feature has a limited distribution in both time and space. On this basis a new genus, Shimakuroxylon gen. nov., is recognized. It has limited biostratigraphical interest, ranging from the Jurassic to the Cretaceous. Paleogeographically, however, it is bound to these terranes which lined southernmost Eastern Asia during the Jurassic (Lhasa, Indochina, Semitau, etc.). Being also found in the Outer Zone of Southwest Japan, the origin of which is still a matter of debate, Shimakuroxylon points out a southern cradle for this tectonic unit. The genus distribution also suggests it is an indicator for warm and wet climates.

Palynofloras from the upper Barremian-Aptian Nishihiro Formation (Outer Zone of southwest Japan) and the appearance of angiosperms in Japan

Palynomorphs are reported for the first time from the Nishihiro Formation (Wakayama Prefecture, Outer Zone of southwest Japan). The Nishihiro Formation consists of brackish to shallow marine deposits, dated as late Barremian to Aptian from geological correlations. Spores prevail in the assemblage, representing Filicopsida (mainly Cyatheaceae and Anemiaceae), Marchantiopsida and Lycopsida. The pollen assemblage is dominated by Coniferales, whereas Gnetales and Bennettitales/Cycadales are only rarely observed. Moreover, we report angiosperm pollen grains of the genus Retimonocolpites for the first time in the Early Cretaceous sediments of Japan. Pollen grains of the Retimonocolpites Group are typical of early angiosperms and commonly found in assemblages from the early to mid-Cretaceous of all paleofloristic provinces. Until this paper, the oldest angiosperm fossils in Japan were represented by a single seed and a wood reported from the Albian of Hokkaido. The oldest reliable angiosperm pollen grains were reported in Hokkaido from the Cenomanian, and in Honshu from the Coniacian. Thus, Retimonocolpites pollen grains reported in the present study represent the oldest record of angiosperms in Japan. They indicate an appearance of the angiosperms in Japan older than thought until now, which is consistent with that proposed elsewhere in eastern Asia.

Lower Cretaceous (Upper Barremian-Lower Aptian?) Palynoflora from the Kitadani Formation (Tetori Group, Inner Zone of Central Japan)

The Tetori Group, which crops out in the Inner Zone of central Japan, has been extensively studied for its rich floral and vertebrate fossil assemblages. The authors provide the first contribution to the palynoflora of the Kitadani Formation, which has been dated as late Barremian to early Aptian on the basis of the fresh- water bivalves recorded from it. The studied assemblage yields 45 genera and 41 species of spores and gymno- sperm pollen grains, some freshwater algae, one epiphyllous fungus, and various plant fragments. No angiosperm pollen grains were observed. Some forms described here in detail are not yet known from the lit- erature. They probably correspond to new species, but the scarcity of the specimens has caused us to place them temporarily in open nomenclature. This palynological study adds new data to the present knowledge on the Barremian-Aptian Tetori-type Paleoflora of eastern Asia. The authors compare the palynological inventory with recently published data obtained from the Barremian strata of the Choshi Group, the Outer Zone of Southwest Japan. Then, they situate the results among the previous paleoenvironmental reconstructions of the Tamodani Flora, and confirm a warm temperate and moderately humid climate, with locally drier conditions. Even if some elements of the assemblage suggest transportation, all of them are of continental origin and con- firm a fluvio-lacustrine environment for the deposition.

なぜ，西南日本外帯で降雨時あるいは地震時に深層崩壊が多発するか？

なぜ，西南日本外帯で降雨時あるいは地震時に深層崩壊が多発するか？

268 Palynostratigraphy and paleoenvironments of the Lower Cretaceous Choshi Group, Outer Zone of south-west Japan

268 Palynostratigraphy and paleoenvironments of the Lower Cretaceous Choshi Group, Outer Zone of south-west Japan

Fault tectonic analysis of Kii peninsula, Southwest Japan: Preliminary approach to Neogene paleostress sequence near the Nankai subduction zone

AbstractThe southern part of the Outer Zone of Southwest Japan including the Kii peninsula belongs to the tectonic ‘shadow zone’, where fewer conspicuous active faults and less Quaternary sediments develop than in the Nankai subduction zone and Inner Zone of Southwest Japan. In order to study the paleostress sequence of the Kii peninsula, we analyzed fault‐slip data and tension gashes at pilot sites of Early–Middle Miocene forearc sediments and Late Cretaceous accretionary complex. According to the results, six faulting events are reconstructed in sequence: (i) east–west extension (normal faulting); (ii) east–west compression and north–south extension (strike‐slip faulting); (iii) NNW–SSE compression and ENE–WSW extension (strike‐slip faulting); (iv) northeast–southwest compression and northwest–southeast extension (strike‐slip faulting); (v) WNW–ESE compression (strike‐slip or reverse faulting); and (vi) NNE–SSW extension. The north–south to NNW–SSE trending dyke swarm of Middle Miocene age in the Kii peninsula is thought to be related to Event 3, implying that Event 3 was active at least during the Middle Miocene. Because Event 6 is recognized solely at a site, the overall latest faulting event seems to be Event 5. Assuming that the compression results from the motion of the crust or plate, the compression direction of Event 5 is in good accordance with the present‐day WNW crustal velocity vectors of the Kii peninsula. The stress trajectory map of Southeast Korea and Southwest Japan reveals that the current compression directions of the Kii peninsula correspond to the combinatory stress fields of the Himalayan and Philippine Sea tectonic domains.

Barremian palynofloras from the Ashikajima and Kimigahama formations (Choshi Group, Outer Zone of south-west Japan)

The Choshi Group, which crops out in the Outer Zone of south-west Japan, has been extensively studied for its rich macroflora by Makoto and Harufumi Nishida, among others, and was attributed to the Ryoseki-type Floristic Province by Kimura (1987). New microfloras were discovered in muddy, very fine-grained sandstones and mudstones of the marine Ashikajima and Kimigahama formations, representing the base of the Choshi Group. The authors provide a palynological inventory for these lithological units, which have been dated as Barremian on the basis of the ammonites recorded from them, and compare them with the paleofloristic associations of the South-Laurasian Province (Brenner 1976) and Euro-Sinian Region (Vakhrameev 1991). The studied assemblages yielded 53 genera and 89 species of spores and gymnosperm pollen grains, and also marine or freshwater algae and some epiphyllous fungi. No angiosperm pollen grain was observed. Four new species are described: Manumia japonica n. sp., Foveosporites ryosekiensis n. sp., Nodosisporites choshiensis n. sp. and N. makotoi n. sp. Other forms, probably new species, are described here in detail, but the scarcity of the specimens has led us to place them temporarily in open nomenclature. The spatio-temporal distributions of the genus Manumia Pocock, reported for the first time in Asia, and Cicatricosisporites sinuosus Hunt, 1985 are plotted on paleogeographical maps. With this palynological study, we add new data to the present knowledge of Barremian floras. This assemblage probably corresponds to a taphocenose. The authors suggest that the climate indicated had marked dry and more humid seasons, in accordance with the hypothesis of a moderate migration of the oceanic islands of the Outer Zone before their collision with the Eurasian continent, or moderate climatic change during the Early Cretaceous in Japan.

西南日本外帯，１００ＭａホルンブレンドＫ－Ａｒ年代を示す徳島県那賀町下雄アダカイト質花崗岩質岩

西南日本外帯，１００ＭａホルンブレンドＫ－Ａｒ年代を示す徳島県那賀町下雄アダカイト質花崗岩質岩

Chemical composition of whole-rocks samples and volcanic glasses of the Miocene Tamateyama and Sekibutsu Tuffs in the area surrounding the Nara Basin

We used XRF and INAA methods to analyze the chemical composition of whole-rock and volcanic glass samples from the Tamateyama and Sekibutsu Tuffs in the area surrounding the Nara Basin. Both tuffs have rhyolitic compositions with SiO2=73.6-75.0 wt.%, similar to that of the Muro Pyroclastic Flow Deposits (Muro PFD). The REE patterns of the tuffs are characterized by light REE enrichment, a pronounced negative Eu anomaly, and relatively constant chondrite-normalized middle to heavy REE abundances. These features are similar to those of S-type felsic rocks of the Outer Zone of southwest Japan. Our new petrochemical data provide additional constraints on correlations of the Tamateyama Tuff, Sekibutsu Tuff, and Muro PFD, and indicate that these units were derived from a large felsic igneous body in the Outer Zone of the Kii Peninsula.

Observation and judgment point of boring core for landslides in outer zone of southwest Japan

Observation and judgment point of boring core for landslides in outer zone of southwest Japan

P02 西南日本外帯の海溝寄り地域の中新世玄武岩質岩の地球化学的特徴(ポスターセッション,日本火山学会2007年秋季大会)

P02 西南日本外帯の海溝寄り地域の中新世玄武岩質岩の地球化学的特徴(ポスターセッション,日本火山学会2007年秋季大会)

U-Pb ages and trace element compositions of reddish, colorless, and detrital zircons in the Kumano Acidic Rocks in the Outer Zone of southwest Japan and origin of the host magma.

Sタイプ花こう岩類に含まれるジルコンの組成的特徴とその珪長質マグマの成因との関連性を明らかにするために西南日本外帯，熊野酸性岩から抽出した無色・赤色ジルコンについてLA-ICPMSによるU-Pb年代および微量元素組成を新たに測定した．ジルコン結晶内のクラックや微細包有物の効果を取り除いたU-Pb年代は15.5－16.7Maであり，同一試料から得られたFT年代に比べ若干（約1 Ma）古い年代を示した．無色・赤色ジルコンのREEパターンは右上がりで顕著なCeの正異常・Euの負異常を示し，これまで報告された花こう岩類のジルコンのREEパターンと比べ顕著な違いは見出されなかった．しかし，同ジルコンと平衡共存する珪長質メルトのREE組成を見積もると，無色ジルコンの共存珪長質メルト組成は花崗斑岩の全岩化学組成のより分化したものと類似し，赤色ジルコンの共存珪長質メルト組成はアダカイト的な特徴を示した．このことは熊野酸性岩の起源として異なる2種類の珪長質マグマの存在を示唆し，この組成差は付加体堆積物の部分溶融度の差を反映していると思われる．

Early Cretaceous non-marine mollusc faunas of Japan and Korea

A biostratigraphic correlation of Early Cretaceous marine and non-marine mollusc faunas of south-west Japan and exclusively freshwater faunas of Korea is proposed. The material originates from the Inner Zone of central Japan (Tetori, Sanchu), the Outer Zone of south-west Japan (Kyushu, Shikoku, Kii Peninsula), and the Kyongsang Basin of south-west Korea. It includes new collections of freshwater molluscs from the Monobegawa Group (Shikoku). The alternation of well-dated marine strata with non-marine beds in Kyushu, Shikoku and central Honshu (Sanchu area) provides accurate age constraints to the well-studied non-marine faunas. In the Inner and Outer Zones of Japan, four non-marine bivalve associations alternate with marine faunas: (1) Hauterivian: brackish–freshwater Hayamina naumanni– Unio ogamigoensis Assemblage of the Tatsukawa faunal association; (2) Hauterivian: brackish Hayamina carinata– Isodomella matsumotoi Assemblage of the Shobu faunal association; (3) Barremian: marine Yabea shinanoensis, Pterotrigonia kawaguchiensis and Nanonavis yokoyamai, with the ammonite Shasticrioceras nipponicum and radiolarians ( Archaeodictyomitra pseudoscalaris assemblage); (4) Late Barremian: brackish Hayamina matsukawai, Costocyrena radiatostriata, and Pulsidis tashiroi of the non-marine Sebayashi faunal association, with Nippononaia ryosekiana in Sanchu, passing upwards to marine deposits with ammonites; (5) Early Aptian: brackish Costocyrena minor, and Pulsidis rostratus of the non-marine Hibihara faunal association, passing upwards to marine Pterotrigonia pocilliformis and Goshoraia minor, with the ammonite Cheloniceras sp.; (6) Middle Albian: marine Inoceramus anglicus, and Parvamussium tosaense, with ammonites, radiolarians and calcareous nannoplankton. Viviparus onogoensis (gastropod) in the Hauterivian Tatsukawa association enables correlation of the Ryoseki and Tatsukawa formations (Monobegawa Group) with the Izuki, Kuwajima and Okurodani formations (Tetori Group) and the Myogog Formation (Korea). Plicatounio naktongensis links the Kitadani Formation (Tetori Group) and Sengoku Formation (Kanmon Group) directly with the Hasandon Formation (Korea). The Kitadani and Sengoku faunas share components with the Sebayashi fauna (Central Honshu), define the Late Barremian Hibihara faunal association, and correlate with the Yunoki-Hiura fauna (Monobegawa Group). The faunas are controlled by salinity and temperature. Relationships between the environmental conditions indicate a geographic continuity between the Early Cretaceous deposits of Japan and Korea that accumulated on the continental side of Asia.

Variation of Cl and SO3 Contents of Microphenocrystic Apatite in Intermediate to Silicic Igneous Rocks of Cenozoic Japanese Island Arcs: Implications for Porphyry Cu Metallogenesis in the Western Pacific Island Arcs

Abstract. Determinations of SO3 and Cl contents of igneous accessory apatite were carried out on Late Cenozoic intermediate to silicic intrusive and volcanic rocks in the Japanese island arcs of the western Pacific rim including the southwestern Kuril arc (eastern Hokkaido), Northeast Japan arc (southwestern Hokkaido through northeastern Honshu to central Honshu), Izu‐Bonin arc, Kyushu‐Palau ridge, Southwest Japan arc (northern Kyushu) and northern Ryukyu arc (southern Kyushu). These were compared to those from the Western Luzon arc, Philippines, to better understand the metallogenesis of porphyry Cu deposits in the western Pacific island arcs. In addition, SO3 and Cl contents of accessory apatite in the Cretaceous magnetite‐series granitic rocks in the Kitakami belt (northeastern Honshu) and the Miocene ilmenite‐series granitic rocks in the Outer Zone of Southwest Japan (southern Kyushu) were also examined.Microphenocrystic apatites in shallow intrusions associated with porphyry Cu deposits in the Western Luzon arc contain >0.1 wt% S as SO3. Such high SO3 contents of microphenocrystic apatite are a common characteristic of hydrous mag‐matism in the Western Luzon arc, from 15 Ma old tonalitic plutonic rocks of the Luzon Central Cordillera to present‐day volcanism at Mount Pinatubo. The accessory apatite in intrusive rocks associated with porphyry Cu deposits, especially those at the Santo Tomas II deposit, show significantly high Cl contents (>2 wt%).The SO3 contents of microphenocrystic apatite in most of the hydrous silicic rocks along the volcanic front, in andesites related to native sulfur deposits, and in Miocene and younger shallow granitic intrusions in northeastern Honshu, are generally <0.1 wt%. On the other hand, the SO3 contents of apatite in such rocks from eastern Hokkaido, southwestern Hokkaido, Izu, northern Kyushu and southern Kyushu are similar to those from the Western Luzon arc. The SO3 contents of accessory apatite in the Cretaceous magnetite‐series granitic rocks in the Kitakami belt are variable, whereas those of the Miocene ilmenite‐series granitic rocks in southern Kyushu are extremely low. The Cl contents of accessory apatite in some rocks of the Northeast Japan arc, Izu‐Bonin arc and Southwest Japan arc are significantly high.In terms of the Cl and SO3 contents of microphenocrystic apatite, Cenozoic Japanese arc magmatism show similarities with arc magmatism associated elsewhere with porphyry Cu mineralization, except for the most of northeastern Honshu of the Northeast Japan arc. Apatite commonly occurs as inclusions in other phenocrystic phases. Thus the variation in SO3 contents of apatite is a feature of early stage magmatic differentiation. The SO3 contents of microphenocrystic apatite are considered to reflect the redox state of the magma source region or fluids encountered during magma generation.

Strontiomelane, barianstrontiopiemontite and hennomartinite from the regional metamorphic manganese ore deposits in the outer zone of Southwest Japan

Strontiomelane, barianstrontiopiemontite and hennomartinite from the regional metamorphic manganese ore deposits in the outer zone of Southwest Japan