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[SGL18-P07] Radial dikes and their central caldera around Yobuko, northwestern Kyushu: The Middle Miocene arc-parallel extension that formed the Chikuho-type structure
Keywords:Okinawa Trough, stress inversion, Zircon U–Pb and FT ages , Kyushu
Here, we report that late Middle Miocene dikes in northwestern Kyushu made a radiating pattern from a small caldera and that this region was subject to arc-parallel extension.
There are a number of NW-SE trending normal faults in the backarc regions from the western San-in region through northwestern Kyushu, called the Chikuho-type structures (Matsushita, 1951). This along-arc extension was recently attributed to the rifting in the northern Okinawa Trough (Ushimaru & Yamaji, 2023), which started in the Middle Miocene (Sibuet et al., 1987, 1995). Since Middle Miocene strata do not remain in northern Kyushu, there are few constraints on the timing of the normal faulting.
There are many basaltic to andesitic dikes in the Oligocene–Lowest Miocene Sasebo Group in the Yobuko area, Karatsu, northwestern Kyushu. However, their dominant orientations and ages are controversial. Based on the zircon fission-track ages of ~15 Ma from granite xenoliths, Watanabe & Ishibashi (1987) correlated the dikes with the Middle Miocene Hizen Dolerites (Yamazaki, 1959). Moreover, they pointed out the radial pattern made by the dikes. Matsumoto & Yano (1998) obtained the K–Ar ages of 12.3 and 12.6 Ma from the dikes. In contrast, Yamamoto (1991) regarded the dikes collectively as an NW-SE trending, parallel dike swarm, and interpreted that they extruded basalt to form a lava plateau in this area ~3 m.y. ago (Nakamura et al., 1986).
We measured the 77 attitudes of andesitic to basaltic dikes in the Yobuko area. We recognized that they made a radial pattern with a center at around the area, but a parallel pattern with NW-SE trends away from Yobuko, suggesting an NE-SW extensional far-field stress during the magmatism. The method of Yamaji & Sato (2011) was applied to the orientations of the dikes and determined the normal faulting regime of stress.
While previous studies reported only intrusive rocks around the center of the radial dikes (Kobayashi et al., 1955; Watanabe & Ishibashi, 1987), we found a volcanic depression that was carved into the gently dipping Sasebo Group at the center. Not only intrusive but volcaniclastic rocks were emplaced in this ancient caldera. The latter include tuff breccia, lava, and well-stratified fine tuff which was probably a lacustrine deposit. The volcaniclastic rocks had high-angle contacts with the Sasebo Group. In the vicinity of the boundary, there were sandstone breccias and meter-sized sandstone blocks, both of which were lithologically correlated with the group.
Zircon U–Pb and fission-track ages of a siltstone xenolith within an intrusive rock near the center were measured by Kyoto Fission-Track Co. The U–Pb ages ranged from 23 to 2,400 Ma with multiple age clusters. In contrast, the fission-track ages were clustered at around 13 Ma, which is consistent with the existing K–Ar ages of the Hizen Dolerites (Matsumoto & Yano, 1998).
The above-mentioned observations indicate the NE-SW extensional stress at 12–13 Ma in the Yobuko area. This stress condition is consistent with the model that northwestern Kyushu was affected by arc-parallel extension that formed the Chikuho-type structures in the Middle Miocene (Ushimaru & Yamaji, 2023). However, the contemporaneous stress had spatial variations in western Japan. This is because the NE-SW extension was simultaneous with the initial phase of folding along the southern margin of the Tsushima Basin (Kim et al., 2020), and was perpendicular to the extensional orientation in the western Seto Island Sea (Sato & Haji, 2021).
There are a number of NW-SE trending normal faults in the backarc regions from the western San-in region through northwestern Kyushu, called the Chikuho-type structures (Matsushita, 1951). This along-arc extension was recently attributed to the rifting in the northern Okinawa Trough (Ushimaru & Yamaji, 2023), which started in the Middle Miocene (Sibuet et al., 1987, 1995). Since Middle Miocene strata do not remain in northern Kyushu, there are few constraints on the timing of the normal faulting.
There are many basaltic to andesitic dikes in the Oligocene–Lowest Miocene Sasebo Group in the Yobuko area, Karatsu, northwestern Kyushu. However, their dominant orientations and ages are controversial. Based on the zircon fission-track ages of ~15 Ma from granite xenoliths, Watanabe & Ishibashi (1987) correlated the dikes with the Middle Miocene Hizen Dolerites (Yamazaki, 1959). Moreover, they pointed out the radial pattern made by the dikes. Matsumoto & Yano (1998) obtained the K–Ar ages of 12.3 and 12.6 Ma from the dikes. In contrast, Yamamoto (1991) regarded the dikes collectively as an NW-SE trending, parallel dike swarm, and interpreted that they extruded basalt to form a lava plateau in this area ~3 m.y. ago (Nakamura et al., 1986).
We measured the 77 attitudes of andesitic to basaltic dikes in the Yobuko area. We recognized that they made a radial pattern with a center at around the area, but a parallel pattern with NW-SE trends away from Yobuko, suggesting an NE-SW extensional far-field stress during the magmatism. The method of Yamaji & Sato (2011) was applied to the orientations of the dikes and determined the normal faulting regime of stress.
While previous studies reported only intrusive rocks around the center of the radial dikes (Kobayashi et al., 1955; Watanabe & Ishibashi, 1987), we found a volcanic depression that was carved into the gently dipping Sasebo Group at the center. Not only intrusive but volcaniclastic rocks were emplaced in this ancient caldera. The latter include tuff breccia, lava, and well-stratified fine tuff which was probably a lacustrine deposit. The volcaniclastic rocks had high-angle contacts with the Sasebo Group. In the vicinity of the boundary, there were sandstone breccias and meter-sized sandstone blocks, both of which were lithologically correlated with the group.
Zircon U–Pb and fission-track ages of a siltstone xenolith within an intrusive rock near the center were measured by Kyoto Fission-Track Co. The U–Pb ages ranged from 23 to 2,400 Ma with multiple age clusters. In contrast, the fission-track ages were clustered at around 13 Ma, which is consistent with the existing K–Ar ages of the Hizen Dolerites (Matsumoto & Yano, 1998).
The above-mentioned observations indicate the NE-SW extensional stress at 12–13 Ma in the Yobuko area. This stress condition is consistent with the model that northwestern Kyushu was affected by arc-parallel extension that formed the Chikuho-type structures in the Middle Miocene (Ushimaru & Yamaji, 2023). However, the contemporaneous stress had spatial variations in western Japan. This is because the NE-SW extension was simultaneous with the initial phase of folding along the southern margin of the Tsushima Basin (Kim et al., 2020), and was perpendicular to the extensional orientation in the western Seto Island Sea (Sato & Haji, 2021).