Structures and deformation mechanisms in a slowly slipping subduction thrust, Hikurangi Margin, New Zealand (IODP Expedition 375)

慢滑动事件是最近发现的现象，其中滑动以比板块边界变形更快的速率发生，但太慢而不能产生地震波。这些缓慢滑动事件的发现挑战了长期以来的范式，即断层要么以每年厘米的速度稳定地积累位移，要么在潜在的破坏性地震中偶尔失败。相反，现在有证据表明，断层的滑动速度可以跨越一个巨大的范围，从比指甲生长慢到破坏性地震中发生的每秒米的速度。然而，我们并不了解断层滑动行为在这一范围内的物理控制。新西兰希库朗吉边缘提供了一个独特的机会，可以钻探到一个已知可以容纳缓慢滑动事件的断层。虽然大多数缓慢滑动都是在超过10公里的深度发现的，超出了海洋钻探的范围，但希库朗吉边缘经历了缓慢滑动事件，传播到不到2公里的深度，甚至可能一直传播到海底。在375号考察队期间安装的观测站将更好地确定几何形状，但对这一提议至关重要的是，375号考察队还将对承载板块边界断层的物质进行采样，因此也会对缓慢滑动事件进行采样。利用375号考察队的样本，我将调查希库兰吉边缘承载缓慢滑动事件的岩石。利用卡迪夫大学最先进的扫描电子显微镜，可以非常详细地研究进入俯冲带的物质的成分和微观结构。计划中的钻孔与洋底和上覆的海洋和陆地沉积物相交，使我们能够知道发生滑动的岩石类型。钻孔还将在增生棱柱体的前部与最浅的活动断层相交，从而可以深入了解沉积物在发生断层时如何变形。特别是在这个建议中，我建议将这些岩石进入俯冲带时在进入的沉积物和海洋地壳中发展起来的结构重新定位。这种特征将显示变形如何分布在板块边界的最浅处，并允许推断在传入序列中板块边界断层的位置。重要的是，在微观尺度上观察什么样的结构也可以推断出在每个采样岩石单元中什么样的变形机制是活跃的。这个推论让我们知道哪些岩石可能会破裂，哪些岩石可能会因其他机制而变形。这些信息对于理解某些材料如何承载地震以及其他材料如何减缓滑动事件非常重要。

Slow slip events are a recently discovered phenomenon, where slip occurs at a rate faster than plate boundary deformation, but too slow to generate seismic waves. The discovery of these slow slip events has challenged the long-held paradigm that faults either accumulate displacement steadily, at rates of centimetres per year, or fail episodically in potentially damaging earthquakes. Instead, there is now evidence that faults can slip at rates that very across a huge spectrum, from slower than fingernail growth to the meter per second velocity that occurs in damaging earthquakes. We do, however, not understand the physical controls on this range in fault slip behaviour.The Hikurangi Margin, New Zealand, presents a unique opportunity in drilling into a fault that is known to accommodate slow slip events. Whereas most slow slip has been detected at depths in excess of ten kilometres, out of range of ocean drilling, the Hikurangi Margin experiences slow slip events that propagate to depths less than 2 km, and maybe even all the way to the sea floor. Observatories to be installed during expedition 375 will better define the geometry, but critically to this proposal, expedition 375 will also sample the materials that host the plate boundary fault, and therefore also the slow slip events.With samples from expedition 375, I will investigate the rocks that host slow slip events in the Hikurangi Margin. Using state-of-the-art scanning electron microscopy at Cardiff University, the composition and microstructure of the materials entering the subduction zone can be studied in great detail. The planned boreholes intersect the ocean floor and the overlying oceanic and terrestrial sediments, allowing us to know what rock type(s) slip is(are) occurring in. A borehole will also intersect the shallowest inferred active fault at the front of the accretionary prism, allowing insights into how the sediments deform when faulted. Specifically in this proposal, I propose to characterise the structures that have developed in the incoming sediments and oceanic crust as these rocks enter the subduction zone. This characterisation will show how deformation is distributed at the very shallowest level of the plate boundary, and allow inferences of where in the incoming sequence the plate boundary fault localises. Importantly, seeing what structures have developed at the microscale also allows inference of what deformation mechanisms are active in each sampled rock unit. This inference lets us know what rocks are likely to fracture, and what rocks may deform by other mechanisms. This information is important in understanding how some materials may host earthquakes, and others slow slip events.