5:15 PM - 6:30 PM
[SMP25-P02] Geology and missing tectonics in the Inashi-Hase area of the eastern part of the Ryoke belt, central Japan
Keywords:Ryoke belt, Inashi-Hase, Hase unit, Kashio mylonite, missing tectonics
The Median Tectonic Line (MTL) in central Japan was formed during the opening of the Japan Sea, and large-scale tectonic movements and missing of geological bodies occurred near the MTL [1]. The eastern end of the Ryoke belt in the Ina-Valley area was strongly influenced by the missing tectonics [2, 3]. Here, we present an overview of the geology of the Hase-Ichinose area and confirm the missing tectonics.
Ryoke gneiss, Hiji tonalite and Awasawa metamorphic rocks are distributed in the Ryoke belt of the Hase-Ichinose area (Figure A). Many small meta-sediments derived from Ryoke metamorphic rocks are intercalated in the eastern margin of Hiji tonalite (the light blue region in Figure A). Regarding these xenoliths, pelitic and psammitic mylonites are distributed in Yamabuki Valley to the west of Awasawa village. On the other hand, chert and tuff-derived mylonites are common in the Magoi-Nakao area. The pelitic and psammitic mylonites near Yamabuki Valley are derived from Ryoke gneisses. This is based on the following geological facts. (1) Garnet-cordierite gneisses remain in two places (gneiss in Figure B). (2) Partialy resolved garnet porphyroclast (Figure C) is present in many rock samples, although garnet is not found in the matrix of mylonite.
For pelitic mylonites in and around Yamabuki Valley, the average grain sizes of matrix quartz and feldspar are about 20-30 µm in the upstream region and about 10-20 µm in the downstream region (Figure B). The increase of the grain sizes suggests the increase in metamorphic temperature. However, the metamorphic temperatures estimated by the feldspar geothermometer are almost the same for the two regions. That is, the Ab content of the matrix K-feldspar is almost constant for all the studied samples. Many tonalites in the upstream area are not suffered from mylonitization. For such tonalites, quartz and plagioclase of 70-100 µm in diameter are common.
Original rocks of meta-sediments in the Magoi-Nakao area are mainly mudstone, sandstone, chert, siliceous mudstone, siliceous tuff, conglomerate and mixed rock. These belong to the Hase unit of the accretionary complex [3]. Most of the pelitic mylonites derived from the Hase unit are very fine, and the grain sizes of quartz are about 5-15 µm. Moreover, fine sedimentary structure (Figure D) and radiolarian fossils (Figure E, H) were observed. It is reasonable to conclude that the meta-sediments were metamorphosed under lower temperatures than those of the garnet-cordierite zone. The main constituent minerals of the unaltered pelitic mylonites are biotite, muscovite, plagioclase and quartz. K-feldspar is rare and garnet is not recognized. However, fine garnets are present in calcite-rich pelitic mylonites. Coarse-grained andradite-rich garnets are found for a sample (Figure F).
The radiolarian fossils are not strongly deformed, and pressure shadows have not developed for many porphyroclasts (Figure G). Hence, shear deformation was not very strong as supposed before.
The Hase unit is distributed in the vicinity of MTL over a long distance from the Takato-Itayama area to the Hase-Awasawa area. However, the Hase unit is not confirmed in the Awasawa area, and it is presumed that the Hase unit in the Awasawa area was lost due to the Miocene tectonics. The Hase unit in the Magoi-Nakao area is narrow, and the strikes of foliations are significantly oblique to the directions of the MTL and Hase fault. The Hase unit was a large geological unit in the Paleogene, but most of it was lost by the Miocene tectonics [1-3].
[1] Ono, 2020, JpGU-AGU Joint Meeting 2020, SGL33-P07. [2] Ono, 2016, JpGU Meeting, SGL37- P16. [3] Ono, 2018, GSJ Meeting, R15-P13.
Ryoke gneiss, Hiji tonalite and Awasawa metamorphic rocks are distributed in the Ryoke belt of the Hase-Ichinose area (Figure A). Many small meta-sediments derived from Ryoke metamorphic rocks are intercalated in the eastern margin of Hiji tonalite (the light blue region in Figure A). Regarding these xenoliths, pelitic and psammitic mylonites are distributed in Yamabuki Valley to the west of Awasawa village. On the other hand, chert and tuff-derived mylonites are common in the Magoi-Nakao area. The pelitic and psammitic mylonites near Yamabuki Valley are derived from Ryoke gneisses. This is based on the following geological facts. (1) Garnet-cordierite gneisses remain in two places (gneiss in Figure B). (2) Partialy resolved garnet porphyroclast (Figure C) is present in many rock samples, although garnet is not found in the matrix of mylonite.
For pelitic mylonites in and around Yamabuki Valley, the average grain sizes of matrix quartz and feldspar are about 20-30 µm in the upstream region and about 10-20 µm in the downstream region (Figure B). The increase of the grain sizes suggests the increase in metamorphic temperature. However, the metamorphic temperatures estimated by the feldspar geothermometer are almost the same for the two regions. That is, the Ab content of the matrix K-feldspar is almost constant for all the studied samples. Many tonalites in the upstream area are not suffered from mylonitization. For such tonalites, quartz and plagioclase of 70-100 µm in diameter are common.
Original rocks of meta-sediments in the Magoi-Nakao area are mainly mudstone, sandstone, chert, siliceous mudstone, siliceous tuff, conglomerate and mixed rock. These belong to the Hase unit of the accretionary complex [3]. Most of the pelitic mylonites derived from the Hase unit are very fine, and the grain sizes of quartz are about 5-15 µm. Moreover, fine sedimentary structure (Figure D) and radiolarian fossils (Figure E, H) were observed. It is reasonable to conclude that the meta-sediments were metamorphosed under lower temperatures than those of the garnet-cordierite zone. The main constituent minerals of the unaltered pelitic mylonites are biotite, muscovite, plagioclase and quartz. K-feldspar is rare and garnet is not recognized. However, fine garnets are present in calcite-rich pelitic mylonites. Coarse-grained andradite-rich garnets are found for a sample (Figure F).
The radiolarian fossils are not strongly deformed, and pressure shadows have not developed for many porphyroclasts (Figure G). Hence, shear deformation was not very strong as supposed before.
The Hase unit is distributed in the vicinity of MTL over a long distance from the Takato-Itayama area to the Hase-Awasawa area. However, the Hase unit is not confirmed in the Awasawa area, and it is presumed that the Hase unit in the Awasawa area was lost due to the Miocene tectonics. The Hase unit in the Magoi-Nakao area is narrow, and the strikes of foliations are significantly oblique to the directions of the MTL and Hase fault. The Hase unit was a large geological unit in the Paleogene, but most of it was lost by the Miocene tectonics [1-3].
[1] Ono, 2020, JpGU-AGU Joint Meeting 2020, SGL33-P07. [2] Ono, 2016, JpGU Meeting, SGL37- P16. [3] Ono, 2018, GSJ Meeting, R15-P13.