10:00 〜 10:15
[SEM16-05] 走査型SQUID顕微鏡による陸上付加体の地震発生帯で形成された断層岩の岩石磁気特性の解明
キーワード：古地磁気学、岩石磁気学、走査型SQUID顕微鏡、地震発生帯
Rock magnetic properties of fault zones is an indicator of thermal histories or water-rock interactions associated with faulting processes (e.g., Chou et al., 2012; Mishima et al., 2009). Such geological evidence, particularly thermal histories, is crucial for understanding the factors controlling slip behaviors (Sibson, 1977), which are characterized by a wide range of slip parameters from slow to fast fault movements observed in different earthquake scaling laws (e.g., Ide and Beroza, 2023). Scanning SQUID microscopy (SSM), which enables the mapping of microscale rockmagnetic properties, is an effective method not only for investigating thermal histories but also for detecting magnetic mineralogical changes other than heating processes such as grain size reductions or mineral precipitations associated with faulting processes (Fukuzawa et al., 2017; Yang et al., 2020). The previous SSM studies, however, have not used the results to understand the fault slip behaviors. We applied SSM analysis to a fault rock that records a remanence component associated with faulting, which indicates a potential heating event (Uchida et al., 2024), and investigated magnetization processes during faulting to predict future slip behaviors.
The studied fault rock is a cataclasite developing within the tectonic mélange, the Yokonami mélange in the Cretaceous exhumed accretionary complex of the Shimanto Belt. The cataclasite is assumed to have formed along plate subduction interfaces based on paleo-stress and fluid inclusion analyses (Hashimoto et al., 2012; Hashimoto et al., 2014). The remanence component is unblocked during thermal demagnetization up to 360 °C, which is mainly carried by magnetite, although pyrrhotite and maghemite are also identified by rock magnetic analysis (Uchida et al., 2024). We conducted stepwise alternating field demagnetization (AFD) in the field at 5, 10, 20, 40, and 80 mT as well as natural remanent magnetization (NRM) before AFD, and magnetic images were obtained using SSM. Further, we obtained SSM images of rockmagnetic experiments. The results were investigated together with the optical images to associate mineral grains and magnetic dipoles.
AFD results show that most magnetic dipoles were steadily demagnetized with increasing applied field, suggesting that the natural remanent magnetization is mainly carried by a low-coercivity component, possibly magnetite. For rock magnetic analysis, we first applied a 1.2 T direct current field to impart saturation isothermal remanent magnetization (SIRM1.2T). After measuring the SIRM1.2T using a pass-through SQUID rock magnetometer and SSM, a back-field IRM0.12T was imparted and measured using the same protocol as for SIRM1.2T. The results show that high-coercivity ferrimagnetic minerals are localized along the mineral vein, suggesting that pyrrhotite crystallized due to water-rock interaction. In contrast, low-coercivity minerals are mainly located in the muddy matrix but are also found around the vein.
Since the low-coercivity component, possibly corresponding to magnetite, carries a unique NRM component that suggests heating within the cataclasite, it is inferred that the matrix was heated, leading to the acquisition of partial thermoremanent magnetization (pTRM). Nevertheless, the high-coercivity component is likely carried by pyrrhotite or maghemite, suggesting that these minerals may also have been heated simultaneously with magnetite. Therefore, although mechanical and geochemical changes induced by faulting vary throughout the seismic cycle (e.g., Chester et al., 1993; Yang et al., 2020), the magnetization possibly indicates that the maximum heating event, given the highest unblocking temperature. Since the heating signature related to faults provides crucial information for understanding coseismic slip behaviors, future work should focus on thermal demagnetization and the identification of minerals.
The studied fault rock is a cataclasite developing within the tectonic mélange, the Yokonami mélange in the Cretaceous exhumed accretionary complex of the Shimanto Belt. The cataclasite is assumed to have formed along plate subduction interfaces based on paleo-stress and fluid inclusion analyses (Hashimoto et al., 2012; Hashimoto et al., 2014). The remanence component is unblocked during thermal demagnetization up to 360 °C, which is mainly carried by magnetite, although pyrrhotite and maghemite are also identified by rock magnetic analysis (Uchida et al., 2024). We conducted stepwise alternating field demagnetization (AFD) in the field at 5, 10, 20, 40, and 80 mT as well as natural remanent magnetization (NRM) before AFD, and magnetic images were obtained using SSM. Further, we obtained SSM images of rockmagnetic experiments. The results were investigated together with the optical images to associate mineral grains and magnetic dipoles.
AFD results show that most magnetic dipoles were steadily demagnetized with increasing applied field, suggesting that the natural remanent magnetization is mainly carried by a low-coercivity component, possibly magnetite. For rock magnetic analysis, we first applied a 1.2 T direct current field to impart saturation isothermal remanent magnetization (SIRM1.2T). After measuring the SIRM1.2T using a pass-through SQUID rock magnetometer and SSM, a back-field IRM0.12T was imparted and measured using the same protocol as for SIRM1.2T. The results show that high-coercivity ferrimagnetic minerals are localized along the mineral vein, suggesting that pyrrhotite crystallized due to water-rock interaction. In contrast, low-coercivity minerals are mainly located in the muddy matrix but are also found around the vein.
Since the low-coercivity component, possibly corresponding to magnetite, carries a unique NRM component that suggests heating within the cataclasite, it is inferred that the matrix was heated, leading to the acquisition of partial thermoremanent magnetization (pTRM). Nevertheless, the high-coercivity component is likely carried by pyrrhotite or maghemite, suggesting that these minerals may also have been heated simultaneously with magnetite. Therefore, although mechanical and geochemical changes induced by faulting vary throughout the seismic cycle (e.g., Chester et al., 1993; Yang et al., 2020), the magnetization possibly indicates that the maximum heating event, given the highest unblocking temperature. Since the heating signature related to faults provides crucial information for understanding coseismic slip behaviors, future work should focus on thermal demagnetization and the identification of minerals.