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[SGL18-P02] Formation of the Median tectonic line and tectonics of the fore-arc region in central Japan
Keywords:Median Tectonic Line, central Japan, Fore-arc region, Tectonics
Regarding the tectonics of central Japan, the formation of the Paleogene median tectonic line (MTL) and the changes that the MTL underwent during the Miocene are important. To elucidate these tectonics, it is necessary to examine the geological history of the arc-trench system since the Late Cretaceous. Figure 1 lists the main geological events of interest here.
<Arc-trench system around 75-60 Ma in central Japan>
Figure 2 is a schematic cross-section of the arc-trench systems in central Japan at 75 and 60 Ma. Main geological bodies distributed in the forearc region between Ryoke and Sanbagawa metamorphic rocks are sedimentary and igneous rocks of the Ryoke marginal zone, metamorphic rocks and granitoids originating from the Higo-Abukuma belt, Permian Kinshozan quartz diorite, Atogura Formation, and Kiroko metamorphic rocks. In the Ryoke belt, Hiji tonalite intruded at about 70 Ma. In the Ryoke marginal zone, Name granite intruded at 70 Ma, and Kanohara conglomerate and Kotsutateyama tuff were deposited at about 67Ma. During this period, parts of the Sanbagawa metamorphic rocks were exhumed in the subduction zone. Around 60 Ma, felsic magmatism is assumed to occur in the Ryoke marginal zone and the Ryoke belt, because Paleogene thick acidic tuff and detrital zircon have been confirmed in the Ryoke marginal zone. On the other hand, in the vicinity of the Sanbagawa metamorphic rocks, the Kiroko metamorphic rocks and serpentinite were exhumed at about 60 Ma. The Sanbagawa metamorphic rocks were located very far away from the Ryoke metamorphic rocks and the Kashio mylonites in early Paleogene, 60 Ma.
<Contraction tectonics and the formation of the Paleogene MTL after 60 Ma>
The forearc region around 60 Ma subsequently underwent extensive contraction tectonics. As a result, many geological bodies move toward the oceanic plate. At the end of this tectonics, Atogura and Ryoke Nappes were formed. After the formation of the Atogura Nappe, the felsic igneous rocks of the Late Cretaceous were in contact with the Sanbagawa metamorphic rocks by a low-angle fault. When the Ryoke Nappe was subsequently formed, the Ryoke metamorphic rocks and the Sanbagawa metamorphic rocks joined underground, forming the low-angle Paleogene MTL. Its formation time is estimated to be around 50 Ma. The 60 Ma forearc region disappeared due to the formation of the Paleogene MTL.
<Miocene MTL in the Chubu district >
The MTL in the Ina Mountains of the Chubu district is an almost vertically inclined high-angle fault formed in the mid-Miocene. During the formation of this MTL, the entire area surrounding the low-angle Paleogene MTL suffered from intense tectonic movements, and many geological bodies were deformed, moved, and disappeared. The intense tectonics also extended to the Ryoke belt near the present MTL. Hence, the Ryoke belt near the MTL of the Ina City is considered to be a mid-Miocene deformation zone.
<Late Cretaceous Nappe tectonics>
The Kiroko greenstone mélange of the Atogura Nappe is composed of Kiroko metamorphic rocks, serpentinite, and tectonic blocks. The Kiroko metamorphic rocks contain large tectonic blocks originating from the mid-Cretaceous Higo-Abukuma belt. The Higo-Abukuma metamorphic rocks were formed far from the Mikabu unit of the mid-Cretaceous Sanbagawa belt. However, the Kiroko metamorphic rocks in Figure 2 are located near the Mikabu unit. The Higo-Abukuma metamorphic rocks had migrated to the vicinity of the Mikabu unit before the end of the Cretaceous. It is thought that nappe tectonics occurred repeatedly in the forearc region during the Late Cretaceous (Figure 3).
<Arc-trench system around 75-60 Ma in central Japan>
Figure 2 is a schematic cross-section of the arc-trench systems in central Japan at 75 and 60 Ma. Main geological bodies distributed in the forearc region between Ryoke and Sanbagawa metamorphic rocks are sedimentary and igneous rocks of the Ryoke marginal zone, metamorphic rocks and granitoids originating from the Higo-Abukuma belt, Permian Kinshozan quartz diorite, Atogura Formation, and Kiroko metamorphic rocks. In the Ryoke belt, Hiji tonalite intruded at about 70 Ma. In the Ryoke marginal zone, Name granite intruded at 70 Ma, and Kanohara conglomerate and Kotsutateyama tuff were deposited at about 67Ma. During this period, parts of the Sanbagawa metamorphic rocks were exhumed in the subduction zone. Around 60 Ma, felsic magmatism is assumed to occur in the Ryoke marginal zone and the Ryoke belt, because Paleogene thick acidic tuff and detrital zircon have been confirmed in the Ryoke marginal zone. On the other hand, in the vicinity of the Sanbagawa metamorphic rocks, the Kiroko metamorphic rocks and serpentinite were exhumed at about 60 Ma. The Sanbagawa metamorphic rocks were located very far away from the Ryoke metamorphic rocks and the Kashio mylonites in early Paleogene, 60 Ma.
<Contraction tectonics and the formation of the Paleogene MTL after 60 Ma>
The forearc region around 60 Ma subsequently underwent extensive contraction tectonics. As a result, many geological bodies move toward the oceanic plate. At the end of this tectonics, Atogura and Ryoke Nappes were formed. After the formation of the Atogura Nappe, the felsic igneous rocks of the Late Cretaceous were in contact with the Sanbagawa metamorphic rocks by a low-angle fault. When the Ryoke Nappe was subsequently formed, the Ryoke metamorphic rocks and the Sanbagawa metamorphic rocks joined underground, forming the low-angle Paleogene MTL. Its formation time is estimated to be around 50 Ma. The 60 Ma forearc region disappeared due to the formation of the Paleogene MTL.
<Miocene MTL in the Chubu district >
The MTL in the Ina Mountains of the Chubu district is an almost vertically inclined high-angle fault formed in the mid-Miocene. During the formation of this MTL, the entire area surrounding the low-angle Paleogene MTL suffered from intense tectonic movements, and many geological bodies were deformed, moved, and disappeared. The intense tectonics also extended to the Ryoke belt near the present MTL. Hence, the Ryoke belt near the MTL of the Ina City is considered to be a mid-Miocene deformation zone.
<Late Cretaceous Nappe tectonics>
The Kiroko greenstone mélange of the Atogura Nappe is composed of Kiroko metamorphic rocks, serpentinite, and tectonic blocks. The Kiroko metamorphic rocks contain large tectonic blocks originating from the mid-Cretaceous Higo-Abukuma belt. The Higo-Abukuma metamorphic rocks were formed far from the Mikabu unit of the mid-Cretaceous Sanbagawa belt. However, the Kiroko metamorphic rocks in Figure 2 are located near the Mikabu unit. The Higo-Abukuma metamorphic rocks had migrated to the vicinity of the Mikabu unit before the end of the Cretaceous. It is thought that nappe tectonics occurred repeatedly in the forearc region during the Late Cretaceous (Figure 3).