The Louisville Ridge-Tonga Trench collision: Implications for subduction zone dynamics

板块构造范式提供了俯冲带洋壳岩石圈破坏的基本模式。但俯冲带的动力学也是岛弧岩石圈形成的原因，岛弧岩石圈的特征包括地球上一些最大和最活跃的火山和大多数大地震。由岛弧和深海海沟组成的弧形系统构成了西南太平洋，是全球俯冲系统中结构最复杂、地质最活跃的部分之一，也是地球上火山、地震和相关海啸灾害最集中的地区。了解这个系统的动力学是复杂的多样性的年龄，形态和构造环境的材料，是进入俯冲带，但它是这种材料的影响，这是一个主要因素，在确定的架构和组成的整个沟，岛弧，弧后系统。在西南太平洋的~5S-35 S之间，汤加-克马德克海沟俯冲系统具有深的线性地形凹陷，白垩纪太平洋洋壳在该凹陷处俯冲到印度-澳大利亚板块之下。然而，在南纬25度左右，汤加海沟与路易斯维尔岭相交，路易斯维尔岭是一条线性海山链，倾斜延伸，并以地球上最快的板块会聚速度（~80毫米/年）俯冲。该洋脊的俯冲使海沟局部变形，由于倾斜俯冲的几何形状，碰撞点正以约118毫米/年的速度从北向南逐渐移动。因此，汤加的制度可分为三个部分。在27 S以南，太平洋板块的正常大洋岩石圈正在俯冲。在南纬26 - 25度之间，路易斯维尔海岭增厚的地壳和海山链进入俯冲带，海山要么被完整地俯冲，要么被削去并归入上覆板块。在24 S以北，路易斯维尔山脊已经俯冲，其对覆盖板块的长期影响在碰撞点之后被保留下来。两种完全不同的现象在空间上与这个交叉点相关。首先，目前的网站的脊俯冲的特点是一个明显的浅的沟槽和广泛的变形和隆起的弧;其次，一个静态的差距，在浅层地震活动观察到的碰撞点。因此，路易斯维尔海岭和汤加海沟交汇处的不寻常的构造环境使其成为研究俯冲输入、俯冲带行为和系统结构与动力学之间关系的独特位置。这项研究将提供整个碰撞区地壳结构的独特模型，并获得必要的直接观测结果，以参数化和约束岩石圈热-机械耦合粘塑性-弹性响应的数值模拟以及俯冲和上覆板块内的变形分布。这项研究所依据的观测和测量将在一艘研究船对碰撞区进行考察期间进行。最先进的设备将用于使用声波确定地壳和最上层地幔的结构，深度约为40公里的海底，记录这些信号，仪器部署在水深达6000米的海底，并拖在船后面。我们的目标是汤加岛弧海沟系统的一个500 x300公里的区域，该区域从弯曲应力使俯冲板块变形的外部隆起延伸，在山脊碰撞点穿过海沟并进入岛弧。由此产生的地壳、最上层地幔和海底图像将使我们能够确定地壳是如何构造、修改和变形的，以及板块边界系统是如何随着时间的推移对重要板块地形的俯冲作出反应的。

The plate tectonics paradigm provides the fundamental model for the destruction of oceanic lithosphere at subduction zones. But the dynamics of subduction zones are also responsible for the construction of arc lithosphere whose features include some of the largest and most active volcanoes on Earth and the majority of large earthquakes. The arcuate system of island arcs and deep sea trenches that comprise the SW Pacific is amongst the most structurally complex, geologically active section of the global subduction system and has the highest concentration of volcanic, seismic and associated tsunami hazard on Earth. Understanding the dynamics of this system is complicated by the diversity in the age, morphology and tectonic setting of the material that is entering the subduction zone, and yet it is the influence of this material which is a major factor in determining the architecture and composition of the entire trench, island arc, and back-arc system. Between ~5S-35S in the SW Pacific, the Tonga-Kermadec Trench subduction system has a deep, linear topographic depression at which Cretaceous Pacific oceanic crust is subducting beneath the Indo-Australian plate. However, at ~25S the Tonga Trench intersects with the Louisville Ridge, a linear chain of seamounts that runs obliquely to and is being subducted at the fastest rate of plate convergence on Earth (~80 mm/yr). Subduction of this ridge locally deforms the trench, and the point of collision is progressively moving north-to-south at ~118 mm/yr due to the oblique subduction geometry. The Tonga system can thus be divided into three parts. To the south of 27S, normal oceanic lithosphere of the Pacific plate is being subducted. Between 26S-25S, the thickened crust and seamount chain of the Louisville Ridge is entering the subduction zone with seamounts either being subducted intact or decapitated and subsumed into the overriding plate. North of 24S, the Louisville Ridge has been subducted and its long-term effect on the overriding plate is preserved in the wake of the point of collision. Two disparate phenomena appear spatially related to this intersection. Firstly, the current site of ridge subduction is characterised by a pronounced shallowing of the trench and extensive deformation and uplift of the arc; and secondly, a quiescent gap in the shallow seismicity is observed at the point of collision. The unusual tectonic setting of the intersection of the Louisville Ridge and the Tonga Trench therefore makes it a unique location in which to study the relationships between subduction input, subduction zone behaviour, and system architecture and dynamics. This study will provide unique models of crustal structure throughout the collision zone and obtain the necessary direct observations to parameterise and constrain numerical modelling of the thermo-mechanically coupled visco-plastic-elastic response of the lithosphere and the distribution of deformation within the subducting and overriding plates. The observations and measurements on which this study is based will be made during an expedition to the collision zone by a research ship. State-of-the-art equipment will be used to determine the structure of the Earth's crust and uppermost mantle to depths of ~40km sub-seabed using sound waves, recording these signals with instruments deployed onto the seabed in water depths of up to 6000m and towed behind the ship itself. Our target is a 500x300km region of the Tonga island arc-trench system that extends from the outer rise where flexural bending stresses are deforming the subducting plate, across the trench at the point of ridge collision and into the island arc. The resulting images of the crust, uppermost mantle and seabed will allow us to determine how the crust was constructed, modified and deformed, and how the plate boundary system is evolving over time in response to the subduction of significant plate topography.