Does downgoing plate density govern the variability in subduction behaviour around the Pacific? A 3-D dynamic modelling study

俯冲是地球板块构造引擎的主要组成部分之一。在两个板块交汇的地方，温度最低、密度最大的板块将滑到另一个板块下方，并沉入下面的粘性地幔，同时带走玄武岩洋壳、沉积物和流体。流体和沉积物的再循环是地球上最具爆炸性的火山活动的根源，对大陆地壳的形成以及矿石的生产和浓缩至关重要。俯冲作用还产生了世界上最大的地震。然而，虽然在某些区域，两个板块似乎强烈耦合，运动引起了大地震，但其他地区没有震动。板块耦合也可能决定了为什么沿着一些俯冲带，像安第斯山脉和落基山脉这样的山脉带形成，而在其他山脉带后面，新的海洋，如菲律宾海和斐济盆地打开。火山弧的位置取决于板块下沉到地幔的陡峭程度。地幔中下行板块的形状也影响着它下沉到地幔深处的容易程度。尽管这些俯冲特征很重要，但目前我们还不了解是什么力量控制着板块耦合、俯冲板块形状或俯冲运动。来自致密下沉板块的引力通常被认为是板块构造的主要驱动力。然而，无论是在俯冲带观察到的运动，也没有下行板块的形状，也没有上部板块变形与下行板块的密度。有人提出，上覆板块或下行板块强度的强迫可以推翻下行板块密度的影响。然而，我们最近开发了一个纯密度驱动俯冲的二维模型，它表明下行板块密度的表达式可能是违反直觉的。例如，我们发现年轻的轻板块可以比老的致密板块更快地潜没，而且它们经常这样做（Goes等人，Nature 2008）。这一发现说明我们对俯冲力缺乏了解。在这里，我们提出了一个全面的调查，结合数值模拟与观测，探讨如何在下行板块密度和强度的三维变化决定俯冲行为。在项目的第一部分，我们将系统地记录板块运动和下行板块形状对板块结构时空变化的敏感性。在第二部分中，我们将运行一组太平洋“火环”的模型，这是世界上最大的俯冲带。我们的太平洋俯冲模型将由来自板块年龄最佳约束历史的密度驱动。在我们模拟的俯冲行为与观测到的现今下行板块形状和板块运动历史一致的地方，下行板块密度发挥了主导控制作用;在其他地方，额外的力量必须发挥作用。

Subduction is one of the main components of the Earth's plate-tectonic engine. Where two plates converge, the coolest and densest plate will slide below the other and sink into the underlying viscous mantle, taking with it basaltic ocean crust, sediments and fluids. The recycling of fluids and sediments lies at the root of the Earth's most explosive volcanism, and is crucial in the formation of continental crust, and production and concentration of ores. Subduction also produces the world's largest earthquakes. However, while in some zones the two plates appear strongly coupled and motions give rise to mega earthquakes, others converge without shaking. Plate coupling may also determine why along some subduction zones, mountain belts like the Andes and Rockies formed, while behind others, new oceans, like the Philippine Sea and Fiji Basin opened. Volcanic arc positions are controlled by how steeply the plate descends into the mantle. The shape of the downgoing plate in the mantle also affects how easily it can sink into the deep mantle. In spite of the importance of these subduction characteristics, at present, we do not understand what forces govern plate coupling, subducting plate shape or subduction motions. The gravitational pull from the dense sinking plates is generally considered to be the dominant driving force of plate tectonics. However, neither observed motions at subduction zones, nor downgoing plate shape, nor upper plate deformation correlate with the density of the downgoing plate. It has been proposed that forcing by the overriding plate or the strength of the downgoing plate can overrule the effects of downgoing plate density. However, we recently developed a two-dimensional model of purely density driven subduction, which demonstrated that the expressions of downgoing plate density can be counterintuitive. For example, we find that young light plates can subduct faster than old dense plates and what is more they often do (Goes et al., Nature 2008). This discovery illustrates our lack of understanding of subduction forces. Here we propose a comprehensive investigation that combines numerical modelling with observations, to explore how three-dimensional variation in downgoing plate density and strength determines subduction behaviour. In the first part of the project, we will systematically document the sensitivity of plate motions and downgoing plate shape to spatial and temporal variations in plate structure. In the second part, we will run a set of models for the Pacific 'Ring of Fire', location of the world's largest subduction zones. Our models of Pacific subduction will be driven by densities from the best-constrained history of plate ages. Where our modelled subduction behaviour is consistent with observed present-day downgoing plate shape, and the history of plate motions, downgoing plate density exerts the dominant control; elsewhere, additional forces must play a role.