Environmental Magnetism Research Of Core C08A, IODP Expedition316

Hole C08A of Integrated Ocean Drilling Program (IODP) Expedition316islocated at the shallow portion of the accretionary prism of the Nankai Trough. Themegasplay fault zone was penetrated in this site. The variations of the magneticsusceptibility and NRM intensity show significant different characteristics at differentdepths. In order to understand the significance of the variation characteristics ofenvironmental magnetism parameters of the Hole C08A with environment andevolution of the megasplay fault, a detailed environmental magnetism research wascarried out on Hole C08A on the basis of lithologic units, inorganic geochemistry andorganic geochemistry results.The Hole C08A can be divided into three sections, section A (0-43mbsf)，B(43-160mbsf) and C (160-263mbsf) from top to bottom based on the variationcharacteristics of environmental magnetism parameters and their ratios.The values of χ, χARM, SIRM is very low in section A, which meansferrimagnetic content is low. The characteristics of thermomagnetic curves andhysteresis loops of representative samples indicate the sediments of section A maycontain pyrite. The values of χ, SIRM/χ is higher in section B. The samples fromsection B fall within the SD region of the Day plot. Greigite were identified in sectionB according to the thermomagnetic curves and hysteresis loops of representativesamples. The high values of χ, χARM, SIRM in section C may indicate the high contentof ferrimagnetic minerals. The samples from section C fall into the MD region of theDay plot. The characteristics of thermomagnetic curves and hysteresis loops show themagnetic characteristics of magnetite, so the main magnetic minerals are multidomainmagnetite in section C.Three stage variations of environmental magnetism parameters of Hole C08A areconsistent with the tectonic evolution of the splay fault in the upper part of theaccretionary prism. The section A corresponds to movement of splay fault after1.24Myr, when the high content of calcite was observed in the samples and the deposition occurred above the CCD. Ferrimagnetic relative content was low orferrimagnetic minerals has been dissolved, and pyritization process was completed.The section B corresponds to the movement of splay fault from1.55Myr to1.24Myr, when the uplift movement happened and deposition occurred near CCD.Ferrimagnetic relative content was low or ferrimagnetic minerals has been dissolved.Pyritization process was incomplete reaction by low sulfate concentration.The section C corresponds to movement stage of the splay fault from2.2Myr to1.55Myr when the deposition occurred below CCD. Fine grain was dissolved andmultidomain magnetite was left. Pyritization process was completed.