Magnetic Properties Of Sediments From Nankai Subduction Zone And Their Geological Implications

The Nankai Trough subduction zone plays an important role in earth system sciences.A lot of scientific problems gather here.The tectonic and environmental evolution of subduction zone are the key to understand these scientific issues.Lack of samples and poor stratigraphic record make the study difficult.A series of scientific drilling has been conducted along the Nankai Trough subduction zone by the Nankai Trough Seismogenic Zone Experiment(Nan Tro SEIZE)of the Intergrated Ocean Drilling Program(IODP).Coring and logging of the Nankai Trough sedimentary system,including the Kumano forearc basin,the megasplay fault zone,the frontal thrust zone and Shikoku basin have generated a wealth of information about the stratigraphic and structural evolution of the Nankai margin and provided material to study the subduction.Magnetic minerals in marine sediments originate mainly from detrital input,and they are controlled by tectonic and climatic forcing and by diagenesis.Therefore,knowledge of the magnetic properties of marine sediments in subduction zone can provide new clues to explore subduction.In this study,we provide the detailed magnetic measurements for 3,297 discrete sediment samples along the margin-perpendicular Nan Tro SEIZE transect,including the Kumano forearc basin,the megasplay fault zone,the frontal thrust zone and Shikoku basin.The scientific objectives are to document the magnetic properties(mineral type,concentration,and particle size)of different tectonic units along the Nankai Trough.The vertical and lateral variation profiles of the rock magnetic properties for Nan Tro SEIZE transect were established,which revealed the responses of the magnetic properties in sediments to the diagenetic,climatic,and tectonic processes in subduction zones.The main conclusions are as follows:1)Terrigenous input was controlled by astronomical forcings with obvious precession and obliquity cycles;2)Increasing terrigenous input in northwest in the study area was due to the northward migration of the Kuroshio during strong East Asian Summer monsoon period;3)Magnetic parameters of the sediments in Shikoku basin indicate the variations of sediment sources caused by the motion of the Philippine Sea plate since Early Miocene;4)Fractures and faults may act as conduits for deep fluid migration;5)The distribution of fossil gas hydrate zones and present gas hydrate horizons were identified,and the migration of the gas hydrate zones in the sediment column may be due to the tectonic activity at one million years ago.This study helps to fully understand the tectonic and paleoenvironmental history and the sedimentary process of Nankai Trough.And it also provides a fundamental contribution to the marine environmental magnetism.