The Yaizu Plain in Shizuoka Prefecture, Japan, is located onshore of the Suruga Trough, which marks the convergent boundary between the Eurasian and Philippine Sea plates. The plain was inundated by tsunamis associated with the AD 1498 Meio earthquake (M 8.4) and the AD 1854 Ansei-Tokai earthquake (M 8.4); however, no previous study has investigated tsunami deposits in this area. Therefore, this study examined the distribution of possible tsunami deposits on late Holocene deposits, as observed in sediment cores (8–9 m long and 7 cm in diameter) collected at nine sites (elevations, 1.8–4.5 m) in the coastal area of the Yaizu Plain. At the northern area of the plain, the sediments in the cores are mainly back-marsh clayey deposits with thin gravelly channel-fill deposits. In the downtown area of Yaizu City, the sediments in the cores are mainly gravelly channel-fill deposits with thin backmarsh clayey deposits. The Kawagodaira pumice, which was erupted between 1,210 and 1,187 cal BC, was identified in the back-marsh clayey deposits at two sites. Possible tsunami deposits were not detected from all cores, over the past 3,200 years.
The coastal lowland areas of Minami-Izu and Shimoda, Shizuoka Prefecture, Japan, have been inundated by at least two tsunamis with wave heights of 5–6m during the past 400 years. Previous studies have analyzed the stratigraphy and paleoenviromental setting of Holocene terrestrial deposits in these areas based on one outcrop and sediment cores (8–11 m long) from 13 sites, searching for evidence of tsunami deposits. These analyses yielded no evidence for pre-historical tsunami deposits. In the present study, we examined sediment cores from new three sites in the coastal lowland areas of Minami-Izu and southwest Shimoda. As with the previous studies, we found no evidence for pre-historical tsunami deposits in the terrestrial deposits.
A method was developed for preparation of a large volume (~40 l) of low-trace metal seawater which can be used as a control medium for incubation experiments to estimate a role of trace metals in marine organisms. Seawater taken from a depth of 397 m in the Suruga Bay was filtered through a 0.2 µm-pore-sized polycarbonate filter, adjusted pH to 6 by addition of HCl and NaOH (metal-analysis grade, respectively), then loaded to an ion-exchange column. By loading a volume of 6 l of the filtered seawater to the column containing 500 mg of the chelating resin NOBIAS-CHELATE-PA1 at a flow rate of 2.5 ml min−1 three times, trace metal concentrations were decreased to <0.05 nmol l−1 (Cd, Cu, Mn and Zn) and 0.09 nmol l−1 (Fe). The column can be used repeatedly at least three times after washing with 1 mol l−1 HNO3. Contamination with organic carbon and major nutrients was quite small over the process. A volume of ~40 l of low-trace metal seawater can be prepared within 12 days by simultaneous use of three ion-exchange columns.
Numerical investigations of multiple phase saturation and fractional crystallization differentiation were done for a basaltic melt with major element composition identical to Innomarubi lava (INM melt), which is a phenocryst-poor, relatively undifferentiated basaltic lava erupted at ca. 1.5 ka from Fuji volcano, by using “PELE” (Boudreau, 1999), a thermodynamic phase equilibrium simulator for silicate melt, to understand the origin of whole rock compositional variation observed for volcanic rocks erupted in the last 2200 years from Fuji volcano. The results of multiple phase saturation simulation indicate that INM melt coexists with olivine, plagioclase and clinopyroxene, which are observed as phenocryst minerals in Innomarubi lava, under conditions of temperature of ca. 1160°C, pressure of ca. 580 MPa and H2O content in melt of ca. 0.85 wt.% with oxygen fugacity condition near Ni-NiO buffer. Fractional crystallization differentiation simulation indicates that plagioclase and clinopyroxene are major crystallized phases and, as a result, Al2O3 and CaO contents in melt decrease with cooling. However, the calculated LLD (liquid line of descent) failed to explain the whole rock compositional variation observed for natural volcanic rocks from Fuji volcano. Al2O3 and CaO contents are lower and TiO2, FeO* and K2O contents are higher for the calculated LLD melt than natural volcanic rocks at the same MgO content, indicating that some process other than fractional crystallization and mixing of LLD melts are required. Up to 25 wt.% addition of plagioclase component to calculated LLD melt can explain most of the compositional variation natural volcanic rocks reveal. This indicates that plagioclase accumulation is a primary process to form the observed whole rock compositional variation of natural volcanic rocks.
Picrite basalts are distributed in the Mt. Miwa-Takayama area of Shizuoka city, Shizuoka Prefecture. This area spans the Setogawa tectonic belt sediments composed of middle Eocene sandstone and shale. We found picrite basalt with pillow lavas at Yuyama near the Mt. Miwa-Takayama. Pillow lavas are found with weathered sediments with pillow breccia flows. This is the first record of picrite pillow lavas in Setogawa tectonic belt.
Miwa-Takayama picrite basalts contained many large phenocrysts of olivine, in which have high mg# [= Mg/(Mg+Fe) atomic ratio] and many chromian spinel inclusions. Some olivines have kink banding deformation characteristics. Whole rock chemical compositions showed that Miwa-Takayama picrite basalts are classified to komatiite or meimechite. Miwa-Takayama picrite basalts belong to ocean island basalts (OIB) by the chemical composition of Al2O3 and TiO2 of spinel inclusions trapped in the olivine. Equilibrium temperatures of the Miwa-Takayama picrite basalts are calculated to about 1140°C or over, relative to the olivine-spinel geothermometer. Miwa-Takayama picrite basalts are part of the accretionary prism in the Setogawa tectonic belt.
The Pliocene-Lower Pleistocene Komoro Group exposed in the northern Saku area, southeastern part of the North Fossa Magna region, central Japan, has recorded a unique tectonic history of the arc-arc collision zone. The Group consists of lake and fluvial deposits with abundant intercalations of volcaniclastic materials derived from the surrounding big on-land volcanoes, such as the Utsukushigahara, Yabashira and Eboshidake Volcanoes. The Komoro basin, that was filled with the Komoro Group, experienced two stages of irregularly-shaped collapses fringed by moderately- to steeply-dipping basin-walls forming abut-type unconformities. The primary collapse, more than 30 km in N-S length and 20 km in E-W width, was initiated at about 4 Ma by an E-W trending extensional stress in association with andesitic volcanisms around the basin. The secondary collapse, much smaller in scale than the primary one, modified the basin with in-situ andesitic volcanism. The extensional stress controlled the basin development during the Zanclean and Calabrian ages (approximately from 4.0 to 0.8 Ma), in spite of the E-W to NW-SE trending regional compressional stress field surrounding this basin. The stress regime abruptly changed from extensional to compressional at about 0.8 Ma. The N-S to NE-SW trending km-scale folds, flexures and faults were then formed locally in this basin under the E-W to NW-SE trending compressional stress-field during the Ionian. This change might have been related to the orthogonal collision of the Izu Block, the northernmost tip of the Izu-Bonin Arc, with the South Fossa Magna region of the Honshu Arc.
The Southern Alps of Japan, higher part of the Akashi Mountains, central Japan, is one of the highest mountain ranges in Japan. The upper reaches of the Ooi River in the southern part of the Southern Alps is deeply eroded area. The area is composed of the Upper Cretaceous-Paleogene Shimanto accretionary complex. After the accretion, the Shimanto rocks here strongly deformed during the Middle Miocene to form an arcuate bending toward the north. This was due to the initial orthogonal collision of the Izu-Bonin Arc with the Honshu Arc. Rapid uplifting, more than 3 mm/y., of the mountains initiated at about 1 Ma by the collision of the Izu block with the South Fossa Magna region, and the collision continues to present.
We can observe here many geological and geomorphological features that formed the Southern Alps. The features include the deformed rocks of the Shimanto accretionary complex, V-letter shaped, inclosedmeandering valleys, deep-seated landslides and related debris-flow landforms, and abundant lineardepressions by gravity collapses of ridges to form low-relief surfaces. These landforms are distinct evidence of rapid uplifting of the mountains. Hence, this paper introduces these geologically- and geomorphologically-interesting locations (geosites), and presents some geotour plans to observe them．