Deformation mechanisms in sediments entering a subduction complex to shallow seismogenic slip

地震挑战了我们的预测能力，但在现代社会，预测地震对降低风险仍然非常重要。随着人们对丰富多样的地震事件的认识不断提高，从2004年苏门答腊地震这样的大规模破坏性地震到缓慢滑动地震，了解潜在地质控制因素的必要性也在增加。这些目标促使IODP探险队钻探和取样孟加拉/尼科巴扇层序和沉积到印度洋玄武岩地壳，以表征将进入俯冲带并将导致端元地震的物质。苏门答腊地震之所以引起特别关注，是因为它发生在比预期更靠近海沟的地方，增加了地震和海啸的规模。最近发表的一篇论文表明，与传统模型相比，苏门答腊岛近海的浅层滑动是由深埋断层形成沉积物的成岩强化所驱动的，这些沉积物与俯冲前硅酸盐的完全脱水有关。为了更好地理解这种非典型浅层发震滑动，我们建议对岩心样品的微结构和孔隙结构进行表征。在这次考察中，其中的一部分在LN2中进行了现场冻结，目的是：(1)寻找微观结构中的异常，当与地震和物理性质数据相结合时，可能会定义未来的断层定位和滑脱层；(2)压实和小规模断层期间的变形机制，这将有助于推断岩石的力学特性，从它们目前的临海位置推断到苏门答腊俯冲复合体。为了实现这些目标，我们将使用（低温）宽离子束抛光和（低温）扫描电子显微镜，研究在低温条件下现场保存的逐渐压实和轻度变形的样品（即无干燥损伤和保存孔隙水）和更多缓慢干燥的样品。我们将把这些结果与实验室测量的岩心样品的微观结构进行比较，以检验沉积剖面在进入俯冲复合体时如何响应额外的埋藏和剪切应力的假设。

Earthquakes challenge our ability to predict, yet doing so remains keenly important for risk mitigation in modern society. With increasing awareness of the rich variety of seismic events, from massive, destructive earthquakes like the 2004 Sumatra earthquake to slow-slip quakes, the imperative to understand potential geologic controls increases. These objectives motivated an IODP Expedition to drill and sample the Bengal/Nicobar Fan sequence and sediments down to the Indian Ocean basaltic crust in order to characterize the materials that will enter the subduction zone and will contribute to an end-member earthquake. The Sumatra earthquake is of special interest because it occurred closer to the trench than expected, increasing earthquake and tsunami size. A recently published paper suggests that the shallow slip offshore Sumatra is driven by diagenetic strengthening of deeply buried fault-forming sediments associated with complete dehydration of silicates before subduction, contrasting with conventional models. To contribute to the better understanding of this atypical shallow seismogenic slip, we propose to characterize micro-fabrics and pore structures of core samples, a subset of which were frozen in LN2 on site during this expedition in order to (1) look for anomalies in microstructure that when combined with seismic and physical property data might define future fault localization and decollement horizons and (2) deformation mechanisms during compaction and small-scale faulting that will help extrapolate mechanical properties of rocks from their current seaward location into the Sumatra subduction complex. To fulfill these objectives, we will investigate progressively compacted and mildly deformed samples preserved on site in cryogenic conditions (i.e. no drying damage and preservation of pore water) and more numerous slowly dried samples, using (cryogenic) Broad Ion Beam polishing and (cryogenic) Scanning Electron Microscopy. We will compare these results with microstructures of core samples for which the stress-strain path is measured in the laboratory, to test hypotheses about how the sedimentary section will respond to additional burial and shear stress when entering the subduction complex.