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[SGL23-P09] Middle Miocene folding in the Amakusa coalfield, western Japan revealed by paleomagnetic and structural data
Keywords:Fold test, Eocene formations, Paleogene, Japan Sea opening, Taiwan-Shinji fold
The folding of Eocene and Cretaceous formations in the Amakusa region [1], western Kyushu is a matter of debate: Otsuka (1935) related the folding to the movement of the MTL and other researchers to the Japan Sea opening [5, 6]. The timing of folding is important to resolve this controversy, but the timing has been unknown.
The purpose of this paper is to report the relative timing of folding and Middle and Late Miocene magmatism [7, 8] in the Amakusa region. To this end, we re-examined the geologic structures of the Gongen-yama Basalt, which yielded radiometric ages of 6.8 Ma [8], because Takai et al. (1997) suggested angular unconformity between the basalt and tilted Eocene strata. In addition, paleomagnetic measurements were made on the Miocene igneous rocks.
As a result, we found that the Gongen-yama Basalt was lava horizontally covering tilted Eocene formations. This was indicated by the horizontally lying lava with clinker at its base. Therefore, the folding is older than the basalt.
Oriented samples for paleomagnetic measurements were collected from 29 felsic intrusive rocks, 5 basaltic intrusive rocks, and one basaltic lava (the Gongen-yama Basalt). Stepwise thermal demagnetization successfully isolated high-temperature characteristic remanent magnetizations (ChRMs) from 10 sites of the felsic rocks and all sites of the basaltic rocks. Their high blocking temperatures of ~400–580℃ and linear behaviors towards origin suggest that they are primary thermoremanent magnetizations at the time of intrusion and lava emplacement. The ChRMs of the felsic ones were all reversely magnetized with scattered directions ranging from 155 to 220° in declination and −26 to −60° in inclination. The ChRMs of basaltic ones showed both normal and reversed polarity and scattered around north-south directions, except for a ~60° westerly deflected direction of a basaltic dike. In-situ and tilt-corrected site-mean directions were compared to check the relative timing of folding and magmatism. The directions from felsic intrusive rocks got more tightly clustered after tilt correction, though they did not pass the statistical test [10]. On the other hand, significant differences were not found in the case of basaltic rocks due to insufficient data.
The results of this study support that the folding in Amakusa was formed before the emplacement of the Gongen-yama Basalt and after the felsic magmatism. The timing is constrained between 6.8 and 14.5 Ma based on the radiometric ages [7, 8]. Therefore, the folding in Amakusa cannot be explained by the Japan Sea opening [5, 6] because the folding is younger than the clockwise rotation of the SW Japan arc [11]. The folding is also not coeval with the major movement of the MTL [12]. Alternatively, the folds in Amakusa may be an onshore part of the Taiwan-Shinji fold belt, the formation of which started at 12.5 Ma at the southern margin of the Japan Sea [13].
The purpose of this paper is to report the relative timing of folding and Middle and Late Miocene magmatism [7, 8] in the Amakusa region. To this end, we re-examined the geologic structures of the Gongen-yama Basalt, which yielded radiometric ages of 6.8 Ma [8], because Takai et al. (1997) suggested angular unconformity between the basalt and tilted Eocene strata. In addition, paleomagnetic measurements were made on the Miocene igneous rocks.
As a result, we found that the Gongen-yama Basalt was lava horizontally covering tilted Eocene formations. This was indicated by the horizontally lying lava with clinker at its base. Therefore, the folding is older than the basalt.
Oriented samples for paleomagnetic measurements were collected from 29 felsic intrusive rocks, 5 basaltic intrusive rocks, and one basaltic lava (the Gongen-yama Basalt). Stepwise thermal demagnetization successfully isolated high-temperature characteristic remanent magnetizations (ChRMs) from 10 sites of the felsic rocks and all sites of the basaltic rocks. Their high blocking temperatures of ~400–580℃ and linear behaviors towards origin suggest that they are primary thermoremanent magnetizations at the time of intrusion and lava emplacement. The ChRMs of the felsic ones were all reversely magnetized with scattered directions ranging from 155 to 220° in declination and −26 to −60° in inclination. The ChRMs of basaltic ones showed both normal and reversed polarity and scattered around north-south directions, except for a ~60° westerly deflected direction of a basaltic dike. In-situ and tilt-corrected site-mean directions were compared to check the relative timing of folding and magmatism. The directions from felsic intrusive rocks got more tightly clustered after tilt correction, though they did not pass the statistical test [10]. On the other hand, significant differences were not found in the case of basaltic rocks due to insufficient data.
The results of this study support that the folding in Amakusa was formed before the emplacement of the Gongen-yama Basalt and after the felsic magmatism. The timing is constrained between 6.8 and 14.5 Ma based on the radiometric ages [7, 8]. Therefore, the folding in Amakusa cannot be explained by the Japan Sea opening [5, 6] because the folding is younger than the clockwise rotation of the SW Japan arc [11]. The folding is also not coeval with the major movement of the MTL [12]. Alternatively, the folds in Amakusa may be an onshore part of the Taiwan-Shinji fold belt, the formation of which started at 12.5 Ma at the southern margin of the Japan Sea [13].