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，这样做对于降低风险仍然至关重要。随着人们对丰富多样的地震事件的日益认识，从像2004年苏门答腊岛地震这样的大规模破坏性地震到缓慢滑动地震，了解潜在的地质控制的必要性增加了。这些目标促使IODP考察队钻探和采样Bengal/Nicobar Fan序列和向下至印度洋玄武岩地壳的沉积物，以确定将进入俯冲带并将有助于末端地震的物质的特征。苏门答腊岛的地震引起了人们的特别关注，因为它比预期的更接近海沟，增加了地震和海啸的规模。最近发表的一篇论文提出，与传统模式不同，苏门答腊岛近海的浅层滑动是由深埋断层沉积的成岩强化作用驱动的，与俯冲前硅酸盐完全脱水有关。为了更好地理解这种非典型的浅层地震滑动，我们建议对岩心样品的微观组构和孔隙结构进行表征，其中一部分样品在这次考察期间冻结在现场LN2中，以便(1)寻找显微结构异常，结合地震和物理性质数据可能确定未来的断层定位和滑脱层位；(2)压实和小规模断裂作用的变形机制，有助于将岩石的力学性质从目前的海侵位置外推到苏门答腊岛俯冲杂岩中。为了实现这些目标，我们将使用(低温)宽束离子束抛光和(低温)扫描电子显微镜，研究在现场低温条件下保存的逐步压实和轻微变形的样品(即没有干燥损坏和保存孔隙水)和更多的缓慢干燥样品。我们将把这些结果与在实验室测量应力-应变路径的岩心样品的微观结构进行比较，以检验有关沉积部分在进入俯冲杂岩时将如何对额外的埋藏和剪应力做出反应的假设。

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.