Environmental Magnetism Research On The Sediments Of Site C0012 Of The IODP Expedition 322 In The Shikoku Basin,Philippine Seas

The Shikoku Basin is located in the northern Philippine Sea,and is subducting beneath the Eurasian Plate roughly orthogonal to the axis of the Nankai Trough.Sediments in the Shikoku Basin record information of the reorganization of the plate boundary and environment changes since Late Miocene.As part of the Nankai Trough Seismogenic Zone Experiment(NanTroSEIZE),the Integrated Ocean Drilling Program(IODP)Expedition 322 was designded to document characteristics of incoming sedimentary strata and the igneous basement prior to their arrival at the Nankai Trough subduction front,which drilled through the igneous basement,recovered two sediment cores from Sites C0011 and C0012,respectively.Site C0012 is located at the top of the crescent ridge of the Kashinosaki Knoll.The sedimentary sequence recovered from this site can be divided into five facies according to the lithological characteristics,including upper Shikoku Basin Facies(Unit A),middle Shikoku Basin Facies(Unit B),lower Shikoku Basin hemipelagic Facies(Unit C),lower Shikoku Basin turbidite Facies(Unit D)and Volcaniclastic-rich Facies(Unit E).In this study,we conducted detailed rock magnetic measurements on 391 discrete samples from Sites C0012 during Expedition 322 to characterize the changes in magnetic properties and mineralogy which could document sediment provenance variations in Shikoku Basin since Late Miocene.Rock magnetic analyses reveal that PSD magnetite dominates the magnetic properties of sediments in the upper Shikoku Basin Facies.Sediments in the middle Shikoku Basin Facies is characterized by high content of coarse-grain Ti-magnetite with a small amount of high-coercivity minerals.A significant change in the type,concentration,and size of the magnetic minerals occurred at the lower Shikoku Basin hemipelagic Facies.Paramagnetic mineral is prominent in lower the Shikoku Basin turbidite Facies.Volcaniclastic sandstone Facies is characterized by Ti-magnetite with a small amount of high coercivity mineral.Sediment provenance of the Shikoku Basin has undergone significant changes at 11 Ma.Detrital sediments of the East China Sea and the Shimanto belt in Japan were the the main source of hemipelagic sediments during 18.9-11 Ma,and the volcanic materials were from the southwestern Japan.The main source of hemipelagic sediments from 11 to 7.1 Ma was the Izu Bonin-Honshu collision zone and japanese archipelago.Sediments mainly came from the Izu Bonin-Honshu collision zone after 7.1 Ma as the Nankai Trough hindered the input of materials from japanese archipelago.