[S-CG46] Uncovering stress accumulation and fault strengthening of megathrust earthquakes

[SCG46-P09] Changing rock magnetic properties driven by subduction processes recorded in paleomagnetic signatures

*Taizo Uchida¹, Toshiya Kanamatsu², Sara Satolli³, Takeru Yoshimoto⁴, Marianne Conin⁵, Jamie Kirkpatrick⁶, Shuichi Kodaira², Patrick Fulton⁷, Christine Regalla⁸, Kohtaro Ujiie⁹, Lena Maeda², Natsumi Okutsu², IODP Expedition 405 Scientists - (1.Kochi University, 2.Japan Agency for Marine-Earth Science and Technology, 3.University of Chieti-Pescara, 4.Kobe University, 5.University of Lorraine, 6.University of Nevada, Reno, 7.Cornell University USA, 8.Northern Arizona University, 9.University of Tsukuba)

Keywords:paleomagnetism, plate subduction, secondary remanence, gyromagnetic effect

IODP Expedition 405, conducted in the Japan Trench subduction zone off the coast of Tohoku, aimed to investigate the conditions and processes within plate subduction interfaces that facilitate large shallow slip. Cores were retrieved from two sites near the trench: the incoming sediments and the prism sediments. Comparing the profiles from both sites provided insights into the subduction processes of the frontal prism.
Paleomagnetism was utilized to establish magnetostratigraphy to construct the age model, to restore the core orientation and to interpret tectonic evolution. Natural remanent magnetization (NRM) analysis was performed using stepwise alternating field demagnetization up to 80 mT for discrete specimens and up to 20 or 25 mT for archive halves, utilizing a passthrough superconducting quantum interference device (SQUID) rock magnetometer.
Demagnetization behaviors indicated that the dominant carrier minerals maybe magnetite (Fe₃O₄) in the incoming sediments and a combination of magnetite and greigite (Fe₃S₄) in the prism sediments. This suggests that magnetic mineralogy changed during the subduction processes and that secondary remanence may overlap or overprint the primary remanence carried by the original minerals present in the incoming sediments.
Regarding paleomagnetic direction, the characteristic remanent magnetization (ChRM) in the incoming sediments consistently varies with depth, enabling accurate magnetostratigraphy reconstruction in the incoming sediments site (C0026). In contrast, normal polarity is predominant in the prism sediments, with inclinations often differing from values calculated using the International Geomagnetic Reference Field (IGRF) model. Biostratigraphy dating also revealed multiple instances of age reversals with depth.
These findings suggest that structural evolution influence the paleomagnetic record in the prism sediments. Further rock magnetic studies are required to elucidate the magnetic mineralogy and accurately interpreting magnetic fabric data.