南海海槽俯冲带环境磁学特征及其对俯冲带地震和构造活动的响应-IODP315,316和322航次后研究

Earthquakes generated in the Subduction Zone and its triggering tsunamis bring a major disaster for humanity. However, we still have little understanding about the earthquake mechanisms in the Seismogenic Zone, a relatively small portion of the subduction zone where elastic strain accumulates and is eventually released to generate damaging earthquakes and tsunamis. The Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE) was one of the Priority Research Projects of the Integrated Ocean Drilling Program (IODP). The fundamental scientific goal of NanTroSEIZE is for the first time to sample and instrument the plate boundary system at several locations of the Kii Peninsula offshore, southwest Japan, where violent, large-scale earthquakes have occurred repeatedly throughout history. Up to now, 13 drilling sites of 9 IODP expeditions have been completed and a great progress has been made by the NanTroSEIZE project. Here we propose to conduct a systematic and comprehensive environmental magnetism, rock magnetism, grain size analysis, XRD and SEM on the precious core samples recovered by the IODP Expeditions 315, 316 and 322. The vertical and lateral variation profiles of the rock magnetic properties (composition, concentration, and grain size of magnetic materials) will be established for a transect of the Nankai Trough subduction zone, including pre-subduction inputs of sediment at Shikoku Basin, the rocks of the megasplay fault system and front thrust system at the accretionary prism, and the sediments of the Forearc basin of Kumano basin. Combining with available geological, geochemical and geophysical characteristics of the Nankai Trough, we are going to reveal the sediments magnetic property responses to the tectonic activities and fluid migration in subduction zones and their formation mechanisms. Results from this comprehensive multi-proxy approach and deeper level of analyses will increase our understanding of oceanographic, tectonic, sedimentary processes in the Nankai Trough region. The results will also provide a fundamental contribution toward the marine environmental magnetism and success of the NanTroSEIZE project.

板块俯冲带地震及其引发的海啸给人类带来重大灾难，但对其成因机制并不清楚。南海海槽发震带实验(NanTroSEIZE)是综合大洋钻探计划(IODP)优先研究领域，其通过对南海海槽俯冲带直接钻探取样、原位测量、监测、多学科综合研究，确定俯冲带地震和海啸产生机制。目前已完成9个航次13个站位钻探取样并取得重要进展。本项目拟对NanTroSEIZE项目IODP315、316和322航次所获得的珍贵岩芯开展系统的环境磁学、岩石磁学、粒度、粉晶衍射和扫描电镜分析，建立垂直俯冲带不同构造区沉积物，包括四国盆地、增生楔(大分支断层带和前缘逆冲断层带)和熊野盆地(弧前盆地)的垂向剖面(单钻孔)和横向剖面(不同钻孔)磁性矿物的类型、组合和粒度变化特征，结合其他学科所获得的成果，探讨沉积物磁学变化特征对俯冲带构造活动和流体运移的响应及成因机制。本项目成果将为海洋环境磁学和NanTroSEIZE项目做出一定贡献。

本项目对来自熊野弧前盆地、大分支断裂带、前缘逆冲带和四国盆地的3297块样品进行系统的岩石磁学和环境磁学研究，确定了日本南海海槽俯冲带不同构造区域的磁学特征（磁性矿物类型、含量、粒度），首次建立俯冲带不同构造区域沉积物的垂向剖面（单钻孔）和横向剖面（不同钻孔）的磁学特征变化，并揭示了其对俯冲带区域复杂的成岩、气候和构造作用的响应。本研究首次建立起俯冲带不同构造区域完整的磁学参数变化剖面，是拓宽磁学研究领域的有益尝试，为反演俯冲带构造演化和古环境变化提供新的证据。