17:15 〜 19:15
[SCG46-P09] Changing rock magnetic properties driven by subduction processes recorded in paleomagnetic signatures
キーワード：古地磁気学、プレート沈み込み、二次磁化、ジャイロ磁気効果
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.
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.