日本海沟板块汇聚与地震活动的磁学记录—IODP 343航次后研究

The 2011 Tohoku-Oki earthquake (Mw 9.0) off the Pacific coast of Japan highlighted many of the gaping unknowns in earthquake science. Physical, chemical and texture properties and their formation mechanisms of plate-boundary sediments is an important window to understand the kinetics of plate subduction and mechanism of large earthquakes in subduction zones. In the present proposal, magnetic fabric and rock magnetic studies will be conducted on deep-sea sediments from the Integrated Ocean Drilling Program (IODP) Expedition 343, Japan Trench Fast Drilling Project (JFAST) Site C0019 at the toe of the frontal prism near the Japan Trench, where the Pacific Plate subducts below the North American Plate. The relationship between magnetic properties, mineralogical, geochemical and isotope compositions, and textural features will be fully explored. The goals of these works are to address: (1) characteristics and mechanisms of structural deformation, as well as history and evolution of tectonic stress field at plate convergent boundary since the early Miocene, (2) magnetic responses of the fluid activities at convergent boundary and frictional heating within coseismic slip zone of the Tohoku-Oki earthquake, and (3) plate convergent process and seismic activities related information carried by magnetic properties of the plate-boundary sediments. All these results are expected to contribute important magnetic constraints for understanding the tectonic stress environment and slip mechanism of the Tohoku-Oki earthquake.

日本3.11大地震，向板块俯冲动力学机制与俯冲带地震孕育发生机理提出了一系列亟需解决的科学问题。汇聚板块边缘沉积物的物理化学性质、结构及其形成机制是认识与理解俯冲过程与俯冲带地震机理的重要切入点。本项目以综合大洋钻探(IODP) 343航次在位于3.11地震巨大滑移区的日本海沟俯冲增生楔前缘钻取的汇聚板块边缘沉积物为研究对象，以系统的磁组构和岩石磁学测量与分析为基础，以配套样品的矿物组成、化学成分、同位素特征和显微结构之间的相互关系为桥梁，结合区域构造背景，揭示俯冲增生楔的构造变形机制，重塑早中新世以来，北美板块与太平洋板块俯冲汇聚边缘构造应力场演化历史；探讨汇聚边缘流体活动与地震断层摩擦升温的磁学响应机制，解译沉积物磁性特征所蕴含的与板块汇聚过程和地震活动相关的地质信息，为深入认识与理解活动边缘地震孕育发生的构造应力环境与机理及断层滑动机制等提供依据。

汇聚板块边缘沉积物磁性特征蕴含着有关板块汇聚过程和地震活动的丰富信息，为认识与理解俯冲过程与俯冲带地震机理提供了重要切入点。本项目以综合大洋钻探(IODP) 343航次在位于日本3.11大地震巨大滑移区的日本海沟俯冲增生楔前缘钻取的汇聚板块边缘沉积物为主要研究对象，以系统岩石磁学测量与分析为主要手段，以配套样品的矿物组成和显微结构之间的相互关系为桥梁，结合高速摩擦实验，探讨汇聚边缘地震活动的磁学记录及其响应机制。取得了以下主要成果：（1）基于磁性矿物的热化学反应（或磁性相）稳定性，发展了地震断层带岩石磁学“地温计”，并利用该地温计在日本海沟滑脱带中，发现了300-500℃的地震摩擦生热记录；（2）在IODP343航次钻孔三个断层带发现了单斜磁黄铁矿，并建立了断层带黄铁矿向磁黄铁矿转化的热物理化学模型；（3）通过高速摩擦实验，揭示了地震断层带断层泥磁性增强机制。上述成果不仅表明，岩石磁学手段为追踪地震断层滑动面位置，确定地震断层摩擦生热水平提供了一种新的有效手段，而且为深入认识与理解日本海沟边缘地震孕育发生的构造应力环境与机理及断层滑动机制等提供磁学依据。