Seismic imaging of the Macquarie Ridge Complex: A search for incipient subduction

俯冲描述了一个构造板块由于会聚而下降到毗邻构造板块下方的地幔的过程。目前在世界许多地区都可以发现活跃的俯冲带，包括南美洲西部、西北太平洋和东南亚。俯冲作用可能已经在地球上运行了30多亿年，在大陆的形成和演化、新的洋壳的产生、地球、海洋和大气之间的水交换以及地震和火山的全球分布中发挥着基本作用。因此，俯冲带是了解地球如何运行的主要关键之一，因此，它们一直是来自世界各地的地球科学家深入研究的主题。然而，俯冲循环的一个方面仍然知之甚少，那就是俯冲开始，在这个过程中，一个新的俯冲带开始形成。这不仅需要整个岩石圈厚度的局部减弱，以适应新的板块边缘的形成，而且还需要足够大的密度不稳定性，以克服边界处的摩擦力和板块的弯曲强度，以产生向下的板块。这些挑战导致许多研究得出结论，启动一个新的俯冲带是非常困难的，对于似乎影响俯冲启动的不同过程的相对重要性，仍然存在很大的分歧。实现贯穿整个岩石圈的薄弱地带的一种可能方式是继承先前存在的或化石边界，如老断裂带、转换断层和灭绝的扩张中心。如果有一个适当的区域应力场，就有可能引发俯冲。位于西南太平洋澳大利亚板块和太平洋板块交界处的麦格理山脊杂岩(MRC)是地球上一个被认为正在发生俯冲的位置。MRC最初看起来是一个扩张中心，但在过去的40 Ma，扩张方向经历了60-90度的变化，这导致了从分叉边缘到以变形(走滑)为主的边缘的逐渐转变。然而，MRC的方位沿走向变化很大，这导致了沿其几个段的扭压设置。据认为，挤压可以导致俯冲的启动，最近的一些研究表明，MRC的北段和南段可能正在经历早期的俯冲。然而，在包括麦夸里岛在内的MRC中部，没有足够的数据来确定是否已经开始俯冲。该项目将与澳大利亚合作伙伴合作进行一项大型地震实验，以成像麦夸里岛附近的MRC结构。通过将先进的地震成像方法应用于记录的数据，我们将能够解决关于MRC的形成和早期俯冲过程的基本问题。这包括：(I)为什么麦库里岛是世界上唯一一块暴露在海平面以上的蛇绿岩(洋壳)，其极端地形(高于周围洋盆约5公里)是如何支撑的；(Ii)为什么MRC是世界上最大的走滑地震的工厂，这是由增厚的地壳提供的；(Iii)MRC之下是否有穿透的断裂面或滑脱；(4)是否有证据表明地壳或地幔岩石圈存在密度不稳定？该提议的一个令人兴奋的方面是，它将利用(PI和澳大利亚合作者)成功竞标澳大利亚海洋国家设施船RV Investigator的机会，进行一次前往MRC的巡航，以便能够部署海底和陆地地震仪。

Subduction describes the process in which one tectonic plate descends into the mantle beneath an adjoining tectonic plate as a result of convergence. Currently active subduction zones can be found in many parts of the world, including western South America, the NW Pacific and southeast Asia. Subduction may have been operating on Earth for over three billion years, and plays a fundamental role in the creation and evolution of the continents, the generation of new oceanic crust, the exchange of water between the Earth, oceans and atmosphere and the global distribution of earthquakes and volcanoes. Subduction zones, therefore, are one of the primary keys to understanding how our Earth works, and as a result, they have been the subject of intensive study by Earth scientists from around the world. However, one aspect of the subduction cycle that is still poorly understood is subduction initiation, in which a new subduction zone starts to form. This requires not only localised weakening throughout the thickness of the lithosphere in order to accommodate the formation of a new plate margin, but also a sufficiently large density instability to overcome frictional forces at the boundary and the flexural strength of the plate in order to produce a downgoing slab. These challenges have lead many studies to conclude that it is very difficult to initiate a new subduction zone, and substantial disagreement remains about the relative importance of different processes which appear to influence subduction initiation. One possible way of achieving a weak zone that cross-cuts the entire lithosphere is from inheritance of pre-existing or fossil boundaries, such as old fracture zones, transform faults and extinct spreading centres. Given an appropriate regional stress field, it may then be possible to initiate subduction.The Macquarie Ridge Complex (MRC), which sits at the boundary between the Australian and Pacific plates in the south-west Pacific, is one location on Earth where subduction initiation is though to be taking place. The MRC initially appears to have formed as a spreading centre, but in the last 40 Ma has experienced a 60-90 degree change in spreading direction, which has resulted in a gradual transition from a divergent to a dominantly transform (strike-slip) margin. However, the azimuth of the MRC varies significantly along strike, which results in a transpressional setting along several of its segments. It is thought that transpression can result in subduction initiation, and several recent studies have suggested that the northern and southern segments of the MRC may be experiencing incipient subduction. However, in the central section of the MRC, which includes Macquarie Island, there is insufficient data available to determine whether subduction has begun.This project will undertake a large seismic experiment in collaboration with Australian partners in order to image the structure of the MRC in the vicinity of Macquarie Island. By applying advanced seismic imaging methods to the data which are recorded, we will be able to address fundamental questions regarding the formation of the MRC and the process of incipient subduction. This includes (i) why is Macqurie Island the only piece of ophiolite (oceanic crust) in the world exposed above sea-level, and how is its extreme topography (~5 km above the surrounding ocean basin) supported; (ii) why is the MRC a factory for the world's largest strike-slip earthquakes and is this accommodated by thickened crust; (iii) is there a penetrating fault surface or decollement beneath the MRC; (iv) is there evidence for a density instability in the crust or mantle lithosphere?An exciting aspect of the proposal is that it will take advantage of a successful ~£3.7M bid (by the PI and Australian collaborators) for the Australian Marine National Facility vessel the RV Investigator to undertake a cruise to the MRC to enable the deployment of both ocean bottom and land seismometers.