17:15 〜 18:45
[SCG40-P36] The heating signature within fossil sesimogenic zone recorded as secondary magnetization: Cretaceous Shimanto Belt, Yokonami mélange, southwest Japan
キーワード：古地磁気学、付加体、岩石-水反応、岩石磁気学
Evaluation of thermal events within fault rocks is a key to understand slip behaviors quantitatively. Slip behaviors such as slip velocity and slip duration have been estimated by numerical method in previous studies. While the paleo-temperature of fault rocks have been studied via rock magnetic analyses, remanent magnetization of fault rocks has not been examined for the paleo-temperature studies. Remanent magnetization has the advantage comparing to the other methods for paleo-temperature constraints because remanent magnetization also show the paleomagnetic direction which can provide the clue for the tectonic event with the direction at the blocking temperature. Fault rocks composed of sediments however can be easily altered by weathering or rock-fluid interactions which may modify the magnetic minerals from the original setting at the thermal events. Therefore, the observations of reactions in the magnetic minerals must be conducted carefully to understand how the paleomagnetic orientations were preserved in the fault rocks.
We conducted paleomagnetic analyses with thermal and alternating-field demagnetization analyses and some rock magnetic experiments on a fossil seismogenic fault zone and the surrounding host rocks in the Cretaceous accretionary complex, Shimanto Belt, southwest Japan. The fault zone comprises a few cataclastic zones including thin-slip zones. The unblocking temperature at 300–360 °C was detected only in cataclasite as the secondary magnetic component from the paleomagnetic analyses. Various rock magnetic analyses show that the major carrier for the component is magnetite. In addition, characteristic remanent magnetization (ChRM) is carried by high coercivity component for all lithologies, indicating that single-domain magnetite is the major carrier also for the host rocks. For the cataclasite sample, however, electronic microscope observations with X-ray analysis show some pyrite particles are replaced by magnetite or other elements such as sodium and magnesium. There is also primary magnetite not replaced from pyrite in the cataclasite. On the other hand, pyrite particles in the host rocks are not altered, suggesting the local chemical reactions were occurred only in cataclasite. The remanent magnetization of cataclasite hence can be affected by the local chemical event. The secondary magnetization thus can be carried by the primary and the secondary magnetite.
The paleomagnetic direction of the secondary magnetization is NW-W which is specific only in the cataclasite. The unblocking temperature (300–360 °C) was higher than the paleo-maximum temperature of the host rocks (250 °C from vitrinite reflectance). Therefore, the unblocking temperature indicates a local thermal event associated with the cataclasite development during an event. Thermoremanent magnetization (TRM) were suggested as candidate mechanisms for the unblocking temperature.
Finally, a thermal events successfully detected by paleomagnetic method within the fault rocks although chemical reactions were observed in the fault rocks. Our results suggests that it is possible to detect microscale heating signature with the paleomagnetic method on the faults in the sedimentary rocks. Furthermore, the results imply that the paleomagnetic analyses can be applied to the fault rocks to detect fast slips associated with pseudotachylite.
We conducted paleomagnetic analyses with thermal and alternating-field demagnetization analyses and some rock magnetic experiments on a fossil seismogenic fault zone and the surrounding host rocks in the Cretaceous accretionary complex, Shimanto Belt, southwest Japan. The fault zone comprises a few cataclastic zones including thin-slip zones. The unblocking temperature at 300–360 °C was detected only in cataclasite as the secondary magnetic component from the paleomagnetic analyses. Various rock magnetic analyses show that the major carrier for the component is magnetite. In addition, characteristic remanent magnetization (ChRM) is carried by high coercivity component for all lithologies, indicating that single-domain magnetite is the major carrier also for the host rocks. For the cataclasite sample, however, electronic microscope observations with X-ray analysis show some pyrite particles are replaced by magnetite or other elements such as sodium and magnesium. There is also primary magnetite not replaced from pyrite in the cataclasite. On the other hand, pyrite particles in the host rocks are not altered, suggesting the local chemical reactions were occurred only in cataclasite. The remanent magnetization of cataclasite hence can be affected by the local chemical event. The secondary magnetization thus can be carried by the primary and the secondary magnetite.
The paleomagnetic direction of the secondary magnetization is NW-W which is specific only in the cataclasite. The unblocking temperature (300–360 °C) was higher than the paleo-maximum temperature of the host rocks (250 °C from vitrinite reflectance). Therefore, the unblocking temperature indicates a local thermal event associated with the cataclasite development during an event. Thermoremanent magnetization (TRM) were suggested as candidate mechanisms for the unblocking temperature.
Finally, a thermal events successfully detected by paleomagnetic method within the fault rocks although chemical reactions were observed in the fault rocks. Our results suggests that it is possible to detect microscale heating signature with the paleomagnetic method on the faults in the sedimentary rocks. Furthermore, the results imply that the paleomagnetic analyses can be applied to the fault rocks to detect fast slips associated with pseudotachylite.