Provenance of sediments from IODP Expedition 358, (NanTroSEIZE Deep Riser Drilling: Nankai Seismogenic / Slow Slip Megathrust)

俯冲带是一个板块被迫压在另一个板块之下的地方，是最大规模地震的发生地。通常，这些地震发生在海底断层上，因此也有海啸的危险。最近的例子包括2011年日本近海的东北地震，2004年苏门答腊近海地震和2016年智利南部地震。这类海啸地震是由在所谓的“孕震区”内的俯冲板块上部沿着的滑动事件引起的。被称为“俯冲输入”的传入板块及其沉积物覆盖层的物质特性在海啸地震的生成中起着关键作用。因此，对活跃的板间孕震带进行钻探和仪器测量是了解孕震地震的优先事项。沿着日本西南部，菲律宾海板块向西北方向俯冲到欧亚板块之下，形成了南海俯冲带--这是研究这类带的最佳会聚板块边缘之一。1944年托南凯地震，一组低频地震事件的存在，以及在增生棱镜中记录的第一次构造震颤，表明该地区的地震非常活跃。NanTroSEIZE是一个多勘探钻探项目，旨在提高对整个活动俯冲巨型逆冲带的断层结构、断层和围岩成分的理解和表征。在过去的10年里，这些勘探工作已经在13个地点钻了超过45个孔，直接取样，现场测量和长期钻孔监测数据。IODP 358勘探重点是C 0002地点的更深部分（熊野弧前盆地），目标是高振幅地震反射层的穿透，据信这是主板块边界断层，海底1000米（mbsf）。之前的勘探338和348已经在现场C 0002钻到3，058.5 mbsf，并在该孔中安装了套管，钻到2，922.5 mbsf。这些考察表明，熊野弧前盆地主要被第四纪深海沉积物充填（例如，浊积岩和半远洋泥1），以及加积棱柱沉积物的上部（~ 2，000 - 3，000 mbsf）是层状和充分石化的泥岩和砂岩，它们是层状变形和胶结的。（322和333）在四国盆地表明非常斜俯冲（可能与左旋走滑有关）发生在中新世晚期，这样，陆源砂可能已经被供应到现在的本州近海南海增生棱镜的一部分，岩石地层学对俯冲带的物质性质和构造行为具有一级影响。粗粒与细粒沉积环境的三维位置和比例，例如，岩土工程师和地质学家认为海底通道或凸起会影响俯冲输入的摩擦系数和渗透率。这项拟议的研究将有助于分析增生棱柱下部（> 3，000 mbsf）的沉积物及其物源，并有助于理解日本西南部的板块构造演化，例如，这有助于限制与左旋走滑相关的斜向俯冲可能在晚中新世南开俯冲系西部起重要作用的假设，从而使早期沉积物从东海进入南开俯冲系。

Subduction zones, where one plate is forced beneath another, are the locations of the largest-magnitude earthquakes. Typically, these earthquakes occur on faults that are subsea, therefore there is also a hazard due to tsunamis. Recent examples include the Tohoku earthquake in 2011 offshore Japan, in 2004 offshore Sumatra, and in 2016 in southern Chile. Such tsunamigenic earthquakes are caused by slip events along the upper part of the subducting plate in the so-called "seismogenic zone". The material properties of the incoming plate and its sediment cover, referred to as "subduction inputs", play a critical role in the generation of tsunamigenic earthquakes. Therefore, drilling into and instrumenting an active interplate seismogenic zone is a high priority for understanding seismogenic earthquakes.Along SW Japan, the Philippine Sea Plate subducts NW beneath the Eurasian Plate, forming the Nankai subduction zone - one of the best convergent-plate margins to study such zones. The 1944 Tonankai earthquake, the presence of a cluster of a low-frequency seismic events, and the first tectonic tremor recorded in the accretionary prism, show that the area is very active seismically. The NanTroSEIZE is a multi-expedition drilling project built to improve understanding and characterisation of fault architecture, and fault and wall rock composition throughout an active subduction megathrust zone. These expeditions undertaken in the last 10 years, have resulted in >45 holes being drilled in 13 sites, with direct sampling, in-situ measurements, and long-term borehole monitoring data.The IODP Expedition 358 focusses on the deeper part of Site C0002 (Kumano forearc basin) and targets the penetration of the high-amplitude seismic reflector believed to be the main plate boundary fault at ~ 5,000 metres below sea floor (mbsf). Previous expeditions 338 and 348 have already drilled at Site C0002 to 3,058.5 mbsf with casing installed in that hole to 2,922.5 mbsf. These expeditions show that the Kumano forearc basin is mainly infilled by Quaternary deep-marine sediments (e.g., turbidites and hemipelagic muds 1), and that the upper part of the accretionary-prism sediments (~2,000- 3,000 mbsf) are bedded and well-lithified mudstones and sandstones, variably deformed and cemented.Results from previous IODP expeditions (322 & 333) in the Shikoku Basin suggest very oblique subduction (possibly associated with sinistral strike slip) took place during the late Miocene, such that terrigeneous sands may have been supplied to what is now part of the Nankai accretionary prism offshore Honshu-Shikoku from a sediment source in the East China Sea.Lithostratigraphy exerts a first-order influence over the material properties and tectonic behaviour of subduction zones. The 3-D location and proportion of coarse- versus fine-grained sedimentary environments, e.g., submarine channels or lobes, are appreciated among geotechnical engineers and geologists to affect coefficients of friction and permeability of the subduction inputs. This proposed research will contribute to the analysis of the sediment and their provenance in the lower-part of the accretionary prism (>3,000 mbsf), and help in understanding the plate-tectonic evolution of SW Japan, e.g., helping to constrain the hypothesis that oblique subduction associated with sinistral strike slip may have been important the western parts of the late Miocene Nankai subduction system such that early sediment routing to the Nankai system was from the East China Sea.