3:45 PM - 4:00 PM
[SGL18-06] Re-examination of the Akaishi Phase of the MTL activities in Chubu Region using Illite age analyses of fault gouge
Keywords:Median Tectonic Line, Illite age analysis, fault gouge, Akaishi phase, pre-Tobe phase
The Median Tectonic Line (MTL) has an active history of approximately 100 million years, and recent dating of fault gouges has divided it into different stages (Kubota et al., 2019). Previous dating methods cannot accurately evaluate the presence of detrital illite contaminated from the host rock. The significance of the age was given for illites which gives diagenetic IC (illite crystallinity) values (Takagi and Shibata, 1992). The illite age analysis (IAA) was proposed by Pevear (1992) to interpret the age by finding the intercept of the regression line where 1Md polytype is 100%. It has become necessary to reconsider age data obtained using traditional methods. Sugiyama et al. (2018) applied IAA to fault gouges derived from pelitic schist collected from the MTL to estimate the age of fault activity. We re-examine IAA for the same samples using the software, WILDFIRE©, and the results were slightly different from those of Sugiyama et al. (2018). Therefore, we report the revision of the age of the MTL Akaishi phase.
The ages of illite are 19.2 ± 1.0 Ma obtained from the samples from Anko-W outcrop in Oshika Village and 20.6 ± 5.8 Ma from Shimottaira outcrop in Sakuma Town. In addition, an age of 46.9 ± 2.3 Ma was obtained from a sample from the Taki outcrop in the eastern part of the Kii Peninsula. On the other hand, the ages of each detrital (2M1) illite determined from IAA are 69.3 ± 2.7 Ma for Anko-W, 62.6 ± 7.4 Ma for Shimottaira, and 65.7 ± 2.2 Ma for Taki outcrops. These results were almost consistent with the muscovite K-Ar age in the protolith schists.
From these results, the age of the Akaishi phase which was estimated previously as 23–19 Ma (Takagi and Shibata, 1992) is revised to 20–15 Ma including the age of the fault gouges from the Akaishi Tectonic Line (15 Ma: Tanaka et al., 1995). The Izu-arc collision has started after 17–15 Ma (Hoshi, 2018), but our data suggests that the MTL became active in the Chubu region from around 20 Ma. On the other hand, the 46.9 Ma obtained from the Taki outcrop located outside the Izu collision zone, and no activity of the Akaishi phase has been recorded there. Since this age at Taki outcrop coincides with the active MTL stage (47–46 Ma) in Shikoku (Kubota et al., 2019), pre-Tobe phase of the MTL activity can be narrowed down to 47–46Ma. This period coincides with the bending of the Pacific plate in its moving direction (47.5 Ma: O'Connor et al., 2013), and changes in the stress field caused by this may have brought about new MTL activity in Paleogene time.
References
Hoshi, H., 2018, J.Geol. Soc. Japan, 124, 805-817.
Kubota, Y., Takeshita, T., Yagi, K., and Itaya, T., 2019, Tectonics, 39.2.
O’Conner, J.M. et al., 2013, Geochemistry, Geophysics, Geosystems, 14, 4564-4584.
Pevear D. R., 1992, In: Proc. 7th Intern. Symp. Water-Rock Interaction. Balkema, Rotterdam, 1251–1254.
Sugiyama, K., Takagi, H. and Kawamoto, K., 2018, JpGU Abst. SGL31-P16.
Takagi, H. and Shibata, K., 1992, Mem. Geol. Soc. Japan, no. 40, 31–38.
Tanaka, H., Uehara, N. and Itaya, T., 1995, Contrib. Mineral. Petrol. 120, 150–158.
The ages of illite are 19.2 ± 1.0 Ma obtained from the samples from Anko-W outcrop in Oshika Village and 20.6 ± 5.8 Ma from Shimottaira outcrop in Sakuma Town. In addition, an age of 46.9 ± 2.3 Ma was obtained from a sample from the Taki outcrop in the eastern part of the Kii Peninsula. On the other hand, the ages of each detrital (2M1) illite determined from IAA are 69.3 ± 2.7 Ma for Anko-W, 62.6 ± 7.4 Ma for Shimottaira, and 65.7 ± 2.2 Ma for Taki outcrops. These results were almost consistent with the muscovite K-Ar age in the protolith schists.
From these results, the age of the Akaishi phase which was estimated previously as 23–19 Ma (Takagi and Shibata, 1992) is revised to 20–15 Ma including the age of the fault gouges from the Akaishi Tectonic Line (15 Ma: Tanaka et al., 1995). The Izu-arc collision has started after 17–15 Ma (Hoshi, 2018), but our data suggests that the MTL became active in the Chubu region from around 20 Ma. On the other hand, the 46.9 Ma obtained from the Taki outcrop located outside the Izu collision zone, and no activity of the Akaishi phase has been recorded there. Since this age at Taki outcrop coincides with the active MTL stage (47–46 Ma) in Shikoku (Kubota et al., 2019), pre-Tobe phase of the MTL activity can be narrowed down to 47–46Ma. This period coincides with the bending of the Pacific plate in its moving direction (47.5 Ma: O'Connor et al., 2013), and changes in the stress field caused by this may have brought about new MTL activity in Paleogene time.
References
Hoshi, H., 2018, J.Geol. Soc. Japan, 124, 805-817.
Kubota, Y., Takeshita, T., Yagi, K., and Itaya, T., 2019, Tectonics, 39.2.
O’Conner, J.M. et al., 2013, Geochemistry, Geophysics, Geosystems, 14, 4564-4584.
Pevear D. R., 1992, In: Proc. 7th Intern. Symp. Water-Rock Interaction. Balkema, Rotterdam, 1251–1254.
Sugiyama, K., Takagi, H. and Kawamoto, K., 2018, JpGU Abst. SGL31-P16.
Takagi, H. and Shibata, K., 1992, Mem. Geol. Soc. Japan, no. 40, 31–38.
Tanaka, H., Uehara, N. and Itaya, T., 1995, Contrib. Mineral. Petrol. 120, 150–158.