10:45 〜 12:15
[SCG45-P10] An exothermic event evidenced by thermoremanent magnetization to cataclasite with a fossil seismic fault in an exhumed accretionary complex: the Yokonami mélange, the Cretaceous Shimanto Belt, SW Japan
キーワード：古地磁気学、岩石磁気学、カタクレーサイト、熱残留磁化、熱イベント
An exothermic event which refrigerated in fault rocks have clues to understand various fault behaviors. Fossil seismic fault has evidence of frictional heating as a product of pseudotachylyte (Ikesawa et al., 2003). Increase in vitrinite reflectance can be found along faults from ocean drilling cores or exhumed accretionary complexes (Sakaguchi, 1996; Sakaguchi et al., 2011). To detect such exothermic events, the ferrimagnetic minerals can be employed when the thermoremanent magnetization (TRM) is identified which acquired by exothermic events. TRM, however, has not been examined directly on fault rocks formed along plate subduction interfaces. The TRM indicates not only the paleo temperature as the blocking-temperature but also the magnetic direction at the time of magnetization, which estimated by a stepwise thermal demagnetization (ThD).
We conducted the paleomagnetic and rock-magnetic studies on a cataclacite including a fossil seismic fault in an exhumed Cretaceous accretionary complex, Yokonami Mélange, the Shimanto Belt, southwest Japan. We carried out stepwise ThD and alternating-field demagnetization (AFD), and other rock magnetic experiments on the fault zone and surrounding host rocks. The fault zone is composed of a few cataclastic zones including thin seismic slip zones. The host rocks are composed of mélange. We also investigated at other three lithologies: deformed rocks, sandstone/mudstone alterations and sandstone in a coherent unit located at the northern side of the Yokonami Mélange.
According to the rock magnetic experiments, the major ferrimagnetic mineral carrying remanent magnetizations are identified maghemite from all lithologies. Based on the measurements of the remanent magnetizations, cataclasite only has a remanent magnetitic component which has blocking temperature (Tb) at 300–360 ℃, although the other lithologies do not have clear Tb during ThD up to 400 ℃. The specific magnetic component of cataclasite with the Tb directs to NW, which is not consistent with the other components from other lithologies. Therefore, the magnetic component only in the cataclasite can be TRM at 300-360 ℃. The temperature is higher enough than the paleo-maximum temperature of host rocks. Therefore, the TRM with the temperature range was acquired by a local exothermic event in the cataclasite. Because the direction of magnetization for the TRM orients to SW consistently among the cataclasite samples, the exothermic event can be related to the cataclasite formation. The rock-magnetic experiments showed that the heated temperature of the fault rock and host rocks is less than 400 ℃ because the thermal alteration of maghemite to magnetite were observed under the experiments. The results in this study indicate that the maghemite generated before the fault slip and a local exothermic event heated the cataclasite up to 300–360 ℃.
We conducted the paleomagnetic and rock-magnetic studies on a cataclacite including a fossil seismic fault in an exhumed Cretaceous accretionary complex, Yokonami Mélange, the Shimanto Belt, southwest Japan. We carried out stepwise ThD and alternating-field demagnetization (AFD), and other rock magnetic experiments on the fault zone and surrounding host rocks. The fault zone is composed of a few cataclastic zones including thin seismic slip zones. The host rocks are composed of mélange. We also investigated at other three lithologies: deformed rocks, sandstone/mudstone alterations and sandstone in a coherent unit located at the northern side of the Yokonami Mélange.
According to the rock magnetic experiments, the major ferrimagnetic mineral carrying remanent magnetizations are identified maghemite from all lithologies. Based on the measurements of the remanent magnetizations, cataclasite only has a remanent magnetitic component which has blocking temperature (Tb) at 300–360 ℃, although the other lithologies do not have clear Tb during ThD up to 400 ℃. The specific magnetic component of cataclasite with the Tb directs to NW, which is not consistent with the other components from other lithologies. Therefore, the magnetic component only in the cataclasite can be TRM at 300-360 ℃. The temperature is higher enough than the paleo-maximum temperature of host rocks. Therefore, the TRM with the temperature range was acquired by a local exothermic event in the cataclasite. Because the direction of magnetization for the TRM orients to SW consistently among the cataclasite samples, the exothermic event can be related to the cataclasite formation. The rock-magnetic experiments showed that the heated temperature of the fault rock and host rocks is less than 400 ℃ because the thermal alteration of maghemite to magnetite were observed under the experiments. The results in this study indicate that the maghemite generated before the fault slip and a local exothermic event heated the cataclasite up to 300–360 ℃.