Lithification processes in subduction zones and their influence on the structural style of accretionary prisms

矩震级Mw>9的大型逆冲断层地震主要发生在增生边缘，如2004年12月（印度尼西亚）发生的苏门答腊地震，造成了毁灭性的后果。地震时沿板块界面沿着的灾难性滑动起始于所谓的孕震区。由于迄今为止尚未对活动巨型逆冲断层带的孕震区进行采样，俯冲逆冲断层不稳定滑动开始的原因仍存在争议。对于地震核作用的发生，最被接受的假说之一是深部增生棱柱体的岩化作用。岩化作用还影响了棱柱体的结构样式，并导致锥角减小，这被认为与孕震带的出现有关。一套成岩低温（T）变质反应被认为是造成石化，但这些过程中的身份和时空分布的积极增生棱镜在很大程度上是未知的。该研究旨在确定南海增生棱柱体中的关键成岩-低温变质过程。对于研究而言，存在一个独特的样本集，包括1）来自综合大洋钻探计划（IODP）的样本，深度达海底以下2公里，2）来自泥火山的碎屑喷出物，源自2012年夏天用R/V索内回收的内楔内部，3）来自Shimanto带（日本）的陆上样本，在增生棱柱体的腹地有化石增生棱柱体。样品经历的温度达到孕震区的下倾极限，覆盖了俯冲前、外楔和内楔的不同构造段。一套全面的岩相分析，提出了识别和表征的成岩低温度变质过程，导致锂化。这项研究还辅之以实验室物理性质测量，以评估岩化作用对深部机械和水文地质性质的影响。利用随钻测井资料将制约楔体中岩化过程的时空发生。库仑楔分析将被用来评估俯冲带弧前的结构样式和地震成因的发生过程的影响。

Megathrust earthquakes with moment magnitudes Mw>9 occur mainly on accretionary margins such as the Sumatra event in Dec. 2004 (Indonesia) which had devastating consequences. The catastrophic slip along the plate interface during earthquakes initiates in the so-called seismogenic zone. The cause for the onset of unstable slip on the subduction thrust is still subject to debate, because the seismogenic zone of an active megathrust zone has not been sampled so far. One of the most accepted hypotheses for the onset of earthquake nucleaction is the lithification of the accretionary prism at depth. Lithification also affects the structural style of the prism and causes a decrease in taper angle, which is supposed to be linked to the occurrence of the seismogenic zone. A suite of diagenetic to low-temperature (T) metamorphic reactions is believed to cause the lithification but the identity and spatio-termporal distribution of these processes in active accretionary prisms is largely unknown. The proposed study aims to identify key diagenetic to low-T metamorphic processes in the active Nankai accretionary prism. For the research a unique sample set exits that comprise 1) samples from the Integrated Ocean Drilling Program (IODP) up to 2 km depth below the seafloor, 2) clastic ejecta from mud volcanoes that originate from the interior of the inner wedge recovered in summer 2012 with R/V Sonne and 3) onshore samples from the Shimanto Belt (Japan), a fossil accretionary prism in the hinterland of the accretionary prism. The samples experienced temperatures up to the downdip limit of the seismogenic zone and cover the different tectonic segments of pre-subduction, outer and inner wedge. A comprehensive suite of petrograhic analysis is proposed to identify and characterize the diagenetic to low-T metamorphic processes that cause lithifiation. The research is complemented with laboratory physical property measurements to assess the impact of lithification on mechanical and hydrogeological properties at depth. Utilization of logging-while-drillling data will constrain the spatio-temporal occurrence of the lithification processes in the wedge. Coulomb wedge analysis will be employed to assess the implications of the processes on the structural style of the subduction zone forearc and the occurrence of seismogenesis.