Solidification pressures and ages recorded in mafic microgranular enclaves and their host granite; An Example of the world's youngest Kurobegawa granite

Suzuki, Kota*; Kawakami, Tetsuo*; Sueoka, Shigeru; Yamazaki, Ayu*; Kagami, Saya; Yokoyama, Tatsunori; Tagami, Takahiro*

Island Arc, 31(1), p.e12462_1 - e12462_15, 2022/09

https://doi.org/10.1111/iar.12462

U-Pb ages of zircons from metamorphic rocks in the upper sequence of the Hidaka metamorphic belt, Hokkaido, Japan; Identification of two metamorphic events and implications for regional tectonics

Takahashi, Yutaka*; Mikoshiba, Masumi*; Shimura, Toshiaki*; Nagata, Mitsuhiro; Iwano, Hideki*; Danhara, Toru*; Hirata, Takafumi*

Island Arc, 30(1), p.e12393_1 - e12393_15, 2021/01

https://doi.org/10.1111/iar.12393

The Hidaka metamorphic belt is an excellent example of island-arc-type crust, and in this belt the metamorphic grade increases westwards from unmetamorphosed sediment up to the granulite facies. The metamorphic age of the belt had previously been considered to be ca. 55 Ma. However, zircons from the granulites in the lower sequence have given U-Pb ages of ca. 21-19 Ma and a preliminary report on zircons from pelitic gneiss in the upper sequence gave a U-Pb age of ca. 40 Ma. In this paper we provide new U-Pb ages for zircons from the pelitic gneisses in the upper sequence in order to assess the metamorphic age and also the maximum depositional age of the sedimentary protolith. The weighted mean Pb/U ages and 2 sigma errors for zircons from biotite gneiss in the central area of the belt are 39.6 0.9 Ma for metamorphic overgrowth rims and 53.1 0.9 Ma for the youngest inherited detrital cores. The ages of zircons from cordierite-biotite gneiss in the southern area are 35.9 0.7 Ma for overgrowth rims and 46.5 2.8 Ma for the youngest detrital cores. These results indicate that the metamorphism of the upper sequence took place at ca. 40-36 Ma, and that the sedimentary protolith was deposited after ca. 53-47 Ma. These metamorphic ages are consistent with the reported ages of ca. 37-36 Ma plutonic rocks in the upper sequence, but contrast with the ca. 21-19 Ma ages of metamorphic and plutonic rocks in the lower sequence. Therefore, we conclude, that the upper and lower metamorphic sequences developed independently but became coupled before ca. 19 Ma as a result of dextral reverse tectonic movements, as indicated by the intrusion of ca. 19-18 Ma magmas, possibly generated in the lower sequence, into the upper sequence.

Seismic subsidence near the source region of the 1662 Kanbun Hyuganada Sea earthquake; Geochemical, stratigraphical, chronological, and paleontological evidences in Miyazaki Plain, southwest Japan

Niwa, Masakazu; Kamataki, Takanobu*; Kurosawa, Hideki*; Kokubu, Yoko; Ikuta, Masafumi*

Island Arc, 29(1), p.e12341_1 - e12341_26, 2020/01

https://doi.org/10.1111/iar.12341

Based on detailed geochemical and paleontological analyses for lowland sediments in the Miyazaki Plain, southwest Japan, we clearly identified environmental change from the lower terrestrial sediments to the upper shallow marine sediments. Radiocarbon ages and tephra data suggest that the environmental change is attributed to local subsidence due to the 1662 Kanbun Hyuganada Sea Earthquake shown in historical records. This findings are important for historical earthquake studies because the study area is a part of the expected focal region of megathrust earthquakes along the Nankai Trough.

Provenance identification based on EPMA analyses of heavy minerals; Case study of the Toki Sand and Gravel Formation, central Japan

Shimizu, Mayuko; Sano, Naomi; Ueki, Tadamasa; Komatsu, Tetsuya; Yasue, Kenichi*; Niwa, Masakazu

Island Arc, 28(2), p.e12295_1 - e12295_13, 2019/03

https://doi.org/10.1111/iar.12295

no abstracts in English

Petrographic study of the Miocene Mizunami Group, Central Japan; Detection of unrecognized volcanic activity in the Setouchi Province

Sasao, Eiji

Island Arc, 22(2), p.170 - 184, 2013/06

https://doi.org/10.1111/iar.12019

Petrographic study of sandstones from the Miocene Mizunami Group distributed in Central Japan has been performed using core from a single borehole to evaluate the provenance of the sedimentary rock. Sandstones studied are divisible into three types based mainly on their heavy mineral and plagioclase compositions; one type is characterized by biotite and plagioclase in the range from albite to oligoclase, the second is characterized by dominance of amphibole and labradorite, and the third type is characterized by dominance of pyroxene and andesine with lesser labradorite, bytownite and anorthite. The first type was derived from basement granite whereas the others were derived mostly from volcanic ash. The volcanic activity, which supplied the volcanic ash to the Mizunami sedimentary basin, occurred in two phases, distinguishable by variations in mineralogical and geochemical compositions. This is an indication of change in the character of the volcanic activity.

hydraulic tests in the active fault survey tunnel, Kamioka mine, excavated through the active Mozumi-Sukenobe fault zone and their hydrogeological significance

Nohara, Tsuyoshi; Tanaka, Hidemi*; Watanabe, Kunio*; Furukawa, Noboru*; Takami, Akira*

Island Arc, 15(4), p.537 - 545, 2006/12

https://doi.org/10.1111/j.1440-1738.2006.00548.x

The spatial hydrogeological and structural character of the active Mozumi-Sukenobu Fault (MSF) was investigated along a survey tunnel excavated through the MSF in the Kamioka Mine, Central Japan. Major groundwater conduits on both sides of the MSF are recognized. One is considered to be a sub-vertical conduit between the tunnel and the surface, the other is estimated to be a major reservoir of old meteoric water alongside the MSF. Studies indicate that part of the MSF is a sub-vertical continuous barrier that obstructs younger meteoric water observed in the southeastern part of the Active Fault Survey Tunnel (AFST) and recharge to the rock mass intersected by the northwestern part of the AFST. It is considered that the MSF is a continuous barrier resulting in the storage of a large quantity of older groundwater to the northwest. The observations indicate that the major reservoir is not the fault breccia associated with the NE-SW oriented faults of the MSF. Rather, the reservoir is considered to be the zone in which blocks of fractured rocks occur beside high angle faults corresponding to X shears that coincide in orientation with the present-day regional stress field and antithetic Riedel shears of the MSF. It is considered that secondary porosity is developed in the major reservoir by the destruction of filling minerals and fracture development beside these shears. The results of borehole investigations in the AFST indicate that increase in hydraulic conductivity is not directly related to increased density of fractures around the MSF. Our results suggested that minor conduits are sporadically distributed in the sedimentary rocks around the MSF in the AFST.