Akaishi Mountains Research Articles

Detrital zircon U–Pb geochronology of the trench-fill sandstone on the Cretaceous Shimanto accretionary complex in SW Japan: Implications for provenance and igneous activity in the eastern edge of East Asia

Cretaceous igneous rocks are widely distributed in the East Asian margin, and their formation is due to the large-scale migration of igneous activity areas from the continental interior to near the subduction margin. Detrital zircon U–Pb ages of trench-fill sandstones of the Shimanto accretionary complex in the Akaishi Mountains of southwest Japan can provide information on eroded igneous rocks, and their location at the eastern edge of East Asia makes them essential for better understanding the intensity and migration of the igneous activity. As a result of the dating, the Cretaceous zircons in the trench-fill sandstones were found to increase dramatically in the middle Cretaceous and to be abundant in the Late Cretaceous. In addition, these large amounts of Cretaceous zircons were found to be derived from igneous rocks such as the Abukuma Granites and Ryoke Granites distributed in the proto-Japan arc. These results indicate that migration of igneous activity from the continental side reached the eastern edge of East Asia in the middle Cretaceous and that large-scale igneous activity continued to occur in the eastern edge of East Asia until the Late Cretaceous. Thus, the detrital zircon age spectra in trench-fill sediments provide a clear picture of the degree of igneous activity in the source area and migration of the igneous activity on active continental margins.

The latest Early Cretaceous detrital zircons from clastic rocks in the Koshibu-gawa area of the central Akaishi Mountains

The latest Early Cretaceous detrital zircons from clastic rocks in the Koshibu-gawa area of the central Akaishi Mountains

Radiolarians from Jurassic accretionary complex of the Chichibu belt in the western Akaishi Mountains and chert pebbles of the Miocene Wada Formation in Minami-Shinano, central Japan

Radiolarians from Jurassic accretionary complex of the Chichibu belt in the western Akaishi Mountains and chert pebbles of the Miocene Wada Formation in Minami-Shinano, central Japan

New hypothesis to explain Quaternary forearc deformation and the variety of plate boundary earthquakes along the Suruga\u2013Nankai Trough by oblique subduction of undulations on the Philippine Sea Plate

Plate-boundary earthquakes of magnitude 8 or greater along the Suruga–Nankai Trough subduction zone have repeated at intervals of 90–150 years, but with widely varying magnitudes and rupture areas. We propose, based on geologic data on crustal movements of the forearc wedge, that these earthquake variations are controlled by two separate locked zones in the deeper part of the plate boundary. Long-wavelength topographic undulations, composed of alternating zones of uplift and subsidence along the forearc wedge, are associated with 2000 to 3000 m of vertical relief that has accumulated during Quaternary time. We suggest that this crustal deformation in the forearc wedge is caused not by stress loading and release during earthquake cycles, but rather by vertical displacements of the plate boundary caused by the westward movement of undulations in the obliquely subducting slab of the Philippine Sea Plate as it subducts beneath Southwest Japan. Dating of emergent marine shell fossil assemblages shows that the Kii Mountains, an uplift zone at the midpoint of the trough, has undergone uplift events at intervals of 400–600 year, and the latest event of those occurred when the 1707 Hoei earthquake ruptured the entire plate boundary along the trough. We infer that the plate boundary under the Kii Mountains is a locked zone and that slips of this zone, which accompanied ruptures of the entire plate boundary, caused uplift of the mountains by decreasing the plate boundary depth. A similar locked zone is inferred under the uplift zone of the Akaishi Mountains, along the eastern margin of the trough, and the 1854 Ansei earthquake pair was presumably caused by the slip of this zone. During the two 1854 events, the locked zone under the Kii Mountains presumably restricted the rupture propagation to the eastern half of the trough, then a rupture of the western half of the trough followed within 32 h. These locked zones are inferred to slip independently every few hundred years and determine the major patterns of characteristic ruptures along the Suruga–Nankai Trough.

赤石山地中央部に分布する秩父帯付加体の砕屑性ジルコンU-Pb年代

赤石山地中央部に分布する秩父帯付加体の砕屑性ジルコンU-Pb年代

Dendroclimatic Reconstruction of Summer Temperature at the Akaishi Mountains since A.D. 1774

Dendroclimatic Reconstruction of Summer Temperature at the Akaishi Mountains since A.D. 1774

Fossil trees in dammed lake deposits formed by the Dondokosawa rock avalanche of the late 9th century in the Akaishi Mountains, the Southern Japan Alps

Fossil trees in dammed lake deposits formed by the Dondokosawa rock avalanche of the late 9th century in the Akaishi Mountains, the Southern Japan Alps

赤石山地南部における四万十帯白亜系白根層群の地質，変形構造解析，および砕屑性ジルコンU-Pb年代

赤石山地南部における四万十帯白亜系白根層群の地質，変形構造解析，および砕屑性ジルコンU-Pb年代

Detrital zircon U-Pb ages of the Cretaceous Akaishi Group in the Shimanto Belt, Akaishi Mountains

Detrital zircon U-Pb ages of the Cretaceous Akaishi Group in the Shimanto Belt, Akaishi Mountains

伊豆-小笠原弧衝突により形成された地質構造-赤石山地西部に分布するジュラ紀付加体の例-

伊豆-小笠原弧衝突により形成された地質構造-赤石山地西部に分布するジュラ紀付加体の例-

Revisiting geomorphic changes caused by the Dondokosawa rock avalanche in the Akaishi Mountains, central Japan

Revisiting geomorphic changes caused by the Dondokosawa rock avalanche in the Akaishi Mountains, central Japan

赤石山地の和田層と赤石裂線・中央構造線・赤石山地の構造問題

赤石山地の和田層と赤石裂線・中央構造線・赤石山地の構造問題

26 後期白亜紀付加体中のin-situ緑色岩 : 紀伊半島および赤石山地の四万十帯

26 後期白亜紀付加体中のin-situ緑色岩 : 紀伊半島および赤石山地の四万十帯

403. 赤石山脈,御池山円形構造周辺に分布するガラス質物質

403. 赤石山脈,御池山円形構造周辺に分布するガラス質物質

Geomorphological and geological features and some problems by tunnel excavation in Akaishi Mountains

Geomorphological and geological features and some problems by tunnel excavation in Akaishi Mountains

Diagenesis and very-low grade metamorphism of the Chichibu and Shimanto Belts in the Akaishi Mountains, Southwest Japan: an illite crystallinity study

Diagenesis and very-low grade metamorphism of the Chichibu and Shimanto Belts in the Akaishi Mountains, Southwest Japan: an illite crystallinity study

中部山岳地域における気象観測の現状とその意義

The Japanese Alps collectively refer the Hida Mountains, Kiso Mountains, and Akaishi Mountains, ranging approximately 200 km north to south and 100 km east to west. In terms of span, the Japanese Alps region includes the Ryohaku Mountains to the west and Mt. Fuji, Mt. Yatsugatake, and Mikuni Mountains to the east, making the geographical extent of the region more than 200 km in the east-west direction. The maximum altitude of the Japanese Alps is around 3000 m; however, climatic conditions vary widely because the region lies at the center of Honshu, between the Japan Sea coast and the Pacific coast, and the northern area is known to experience one of the heaviest snowfalls in the world. It is thought that mountainous areas are particularly sensitive to global-scale environmental changes, such as warming. When attempting to evaluate the effect of a global-scale warming event on regional environmental change in the high-altitude Japanese Alps, we note a lack of high-altitude meteorological observation data. This presents difficulties when evaluating the effects of warming on ecological systems and water resources in mountainous regions. In this study, we discuss long-term variations of winter temperature, as well as the amount of snowfall and depth of snow cover at twelve observational stations in the Japan Alps region. At eleven sites other than Mt. Fuji, a trend of increasing annual minimum temperature is recognized, which is statistically significant at the 1% significance level. At locations such as Mt. Fuji, which are located at extremely high altitudes, the annual minimum temperature over the last several decades has not been seen to increase or decrease. The trend of decreasing annual cumulative snowfall is statistically significant at the 10% level at Takada, Toyama and Kanazawa stations. At the other eight sites, it is shown that the recent annual cumulative snowfall does not show either an increasing or decreasing trend. The increasing trend of the annual maximum snow depth is statistically significant at the 5% level at Mt. Fuji, whereas a decreasing trend of the annual maximum snow depth is statistically significant at the 10% level at Takada and Kanazawa. At the other nine sites, annual maximum snow depth has not shown a statistically significant change in recent decades. Recent studies report that the amount of snowfall in Japan will decrease as a result of global warming; however, these studies use data collected at low altitudes. It is, therefore, important to question whether the same theory can be applied to high-altitude mountainous areas. In high-altitude areas of the Japanese Alps region, it is reasonable to expect that a cold atmosphere will be maintained even with global warming, and an increase in evaporated moisture vapor in the air caused by warming on a global scale will increase the amount of snowfall in the region.

Process to extinction and genetic structure of a threatened Japanese conifer species, Picea koyamae

Since the end of the glacial age, Picea koyamae has been sparsely distributed in Japan as a relict species and is presently threatened with extinction. We investigated the population structure and genetic structure of nine populations of P. koyamae. Population size was assessed at 9–135 individuals in habitats ranging from 0.5 to 11.5 ha, and seedlings and saplings were observed in all but one particular population, which had a Sasa-type (bamboo grass) forest floor. The effective number of alleles per locus (Ne) within peripheral populations in the Yatsugatake Mountains was 1.8–2.7, much lower than that of core populations in the Akaishi Mountains (2.8–4.3) using five nuclear simple sequence repeat loci. This finding suggests that genetic variation in these populations has been reduced by isolation from other populations. The standardized genetic differentiation among populations (G′ST) was 0.410 and higher than that found in other Japanese conifers, suggesting that isolation and inbreeding have progressed in this species. In two isolated populations at the Yatsugatake Mountains, the fixation index (FIS) was 0.315–0.354, much higher than the values determined for the other populations (−0.188 to 0.263). This suggests that these two populations have survived several generations while increasing the degree of inbreeding. However, the highest seedling density was in a population with low genetic variation and high FIS. The most serious problems at present appear to be the declining number of mature trees and the deterioration of suitable environments for seedling establishment.

On the Topographicical Situations of the Slash-and Burned Fields in the Meiji Era in Former Ohkawa-mura, Shizuoka Prefecture, in the Southern Part of the Akaishi Mountains

On the Topographicical Situations of the Slash-and Burned Fields in the Meiji Era in Former Ohkawa-mura, Shizuoka Prefecture, in the Southern Part of the Akaishi Mountains

MicroRaman spectroscopy of anomalous planar microstructures in quartz from Mt. Oikeyama: Discovery of a probable impact crater in Japan

Abstract– Planar deformation features (PDFs) and planar fractures (PFs) have been found and confirmed by optical microscope observations and microRaman spectroscopy in quartz grains from Mt. Oikeyama (Akaishi Mountains, Central Japan), for which the semicircular topographic feature of the ridge suggests a crater formed by an impact event. According to the optical microscope observations, a low shock pressure (8–10 GPa) is estimated by the occurrence of basal or ω PDFs leading to lack of multiple sets of PDFs. In addition, a new type of planar microstructure was found in several quartz grains. The microRaman characteristics of PDFs in quartz from Mt. Oikeyama show the amorphous state indicating the presence of weak broad bands at 400 and 800 cm−1 peak positions. These characteristics are indicative of PDFs that are limited to shocked quartz. This indicates an impact origin for distinct planar microstructures in quartz from Mt. Oikeyama.