Crush zone structures are heterogeneous due to occurrence of fault steps. This paper provides a geological description of crush zones in the western part of the active Atotsugawa Fault, Hida City, Gifu Prefecture of Japan, to understand the crush zone structure in a compressional step of a strike-slip fault. By means of aerial photograph interpretation and regional distribution of crush zones, we recognize that a compressional step occurs in the Sangawara area, whereas in the Itani area a long strand of the Atotsugawa Fault with ENE−WSW trend is extended. Detailed geological observation suggests that shear planes with NNE−SSW and NW−SE strikes, oblique to the trend of the Ato-tsugawa Fault at high angles, are characteristically developed in the Sangawara area, whereas shear planes with ENE−WSW strike dominate in the Itani area. The shear planes with NNE−SSW and NW−SE strikes in the Sangawara area can be a part of the composite planar fabric such as R2 surfaces which are dominantly developed within compressional steps. Based on the clay mineral composition including smectite and kaolinite, high density of sealed microcracks, and the K-Ar ages of fault gouge and andesite dike, fault breccia and gouge in the Sangawara area are mainly formed by fracturing at a shallow depth (under c.a. 100°C) after the dike intrusion during Oligocene to early Miocene time.
Molluscan fauna from the Mita Formation in Toyama Prefecture was examined for elucidating the timing and pass ways of the warm-current influx to the Japan Sea in the middle Pliocene. The Mita Formation consists of fine- to medium-grained sandstone with three tuff beds (OT3, MT1 and MT2). The horizons between OT3 (about 4 Ma) and MT2 (2.2-2.3 Ma) yielding many molluscan fossils can be assigned to the lower to middle part of NN16 nannofossil zone. The fauna includes many characteristic species of the Plio-Pleistocene Omma-Manganji fauna as well as some Miocene relict species. Besides 51 cold-water species, 24 warm-water species were found from the every stratigraphic horizon above OT3 and to that just above MT2. The Mita fauna also includes some common species with the Plio-Pleistocene warm-water Kakegawa fauna in the southwestern Pacific side of Japan. Such species composition might be affected by the shallow warmer current than today flowing over the cold-water mass. Judging from every horizons yielding the warm-current species, the warm current always flowed since about 4 Ma to 2.2Ma. Moreover, because of more numerous common warm-water species between the Mita and Kuwae faunas than the Kuwae and the northern Tentokuji ones, and of their biogeographical setting, it can be inferred that the warm current entered through around the Tsushima Strait, not through the Tsugaru Strait.
This study is the first report of radiolarians from the Early Cretaceous Gumizaki Formation in the northwestern part of Kagoshima Prefecture, Japan. Diverse radiolarian assemblages (including 16 genera and 25 species) were found in five mudstone samples collected from two localities in the B member of the Gumizaki Formation. These assemblages indicate the late Early Cretaceous (late early to middle Aptian) in age. Based on the age of these radiolarian assemblages, the B member of the Gumizaki Formation is correlated to the lower to middle members of the Hinagu Formation or the lower member of the Imaizumigawa Formation in the Yatsushiro area.
Newly found fossil shark teeth of a sleeper shark are described. This is probably an undescribed species of the genus Centroselachus belonging to the Family Somniosidae. These fossil teeth were discovered from the Middle Miocene Iseyama Formation (Northern Fossa Magna Region), Ueda City, Nagano Prefecture, central Japan. These teeth indicate the shape of a part of a symphysial tooth row, which belong to a left side of parasymphysial teeth and a left first anterior tooth. Judging from the main characters of a parasymphysial tooth, such as distal and mesial blades presented both joining in a notch, it is considered that the tooth differs from teeth of the genus Centroscymnus. This fossil is identified as the genus Centroselachus sp.. This paper constitutes the first fossil record of the genus Centroselachus from the Miocene of Japan.
Some 160 mesoscale faults were observed in Plio-Pleistocene strata to understand the state of stress associated with the formation of vein-type ores at∼1.9 Ma in the Nagano area, Japan, where NE−SW trending strike-slip and oblique normal faults were found to be abundant; but faults with other orientations and senses were also common. The strike-slip faults showed sinistral and dextral senses, suggesting that this area experienced multiple stresses since the late Pliocene. Five datasets were chosen from the Nagano Formation and Chayagatake Andesite, the depositional ages of which cover the late Pliocene to earliest Pleistocene. Each dataset was inverted to estimate paleostresses. The two datasets from the lower and middle Nagano Formation yielded the strike-slip faulting regime of stress with NNE−SSW and WNW−ESE trending σ1 and σ3 axes, respectively, and with intermediate stress ratios. The three datasets from the upper horizon yielded the opposite stress, suggesting that the state of stress changed sometime around 2 Ma. The veins around the Yamagano Mine have E−W trends, indicating a largely N−S trending σ3 at the time of vein formation, consistent with the stress determined from the upper horizon. The ores were deposited presumably under the strike-slip faulting regime of stress.