What drives and resists plate sinking through the transition zone?

地幔环流在很大程度上是由冷密构造板块的下沉(‘俯冲’)驱动的。当这些冷板块到达上地幔和下地幔之间的过渡带(从400到800公里深)时，随着地幔矿物的相变，它们的发展受到地幔密度和粘度快速增加的阻碍。用地震波拍摄的地球内部的X射线图像(地震层析成像)显示，这一带只对一些板块形成了屏障，而其他板块似乎没有受到阻碍地通过。此外，当将地震图像与板块随时间的运动进行比较时，很明显，过去板块穿过下地幔，而今天板块的较浅部分被困在过渡区。这一点，以及板块运动中快速和缓慢俯冲阶段的证据(Goes等人，《自然》2008)表明，这是一个与时间有关的过程，板块物质可能会积聚到临界质量的物质，然后迅速涌入下地幔。理解地幔循环的这一基本部分很重要，因为它控制着地球冷却的效率，以及沉积物、地壳、流体和二氧化碳等不均匀物质混合或带回地球的程度。此外，突然进入下地幔的板块冲刷事件与大陆地壳形成、早期大气变化和板块运动重组有关。我们将使用俯冲的3D动态模型来研究过渡带中的板片行为，并评估所建模型的哪种机制与来自太平洋的观测数据相一致。以前的数值模型已经研究了板片强度、板片密度、与上板块的耦合和地幔相变等单个因素如何影响板片是直接穿过过渡区还是在那里失速。然而，这些单独的参数都不能解释观察到的板-过渡区相互作用的变化。也不清楚板块可能停滞多长时间，因此上下地幔混合在什么时间尺度上，以及板块运动和伴随的地表变形在什么时间尺度上变化。由于单一的性质不能解释板块-过渡区相互作用的可变性，地幔、下行和上盘性质之间的相互作用肯定是至关重要的。研究这些不同因素之间的相互作用需要3D动态模型，让板块运动和板块形态自由发展，这在数值上是具有挑战性的。近年来，我们的研究小组开发了这样的动力学模型，并阐明了板块密度、强度和宽度的组合如何控制上地幔板块的形态。最新一代的这些动力学模型是由位于达勒姆的PI小组开发的，现在能够模拟所有潜在的相关板块和地幔参数。通过这些模型，我们将探索一系列参数，以确定哪些组合会导致楼板积水和渗透。接下来，我们将比较模拟的失速和释放条件，以及从地震层析成像、板块运动历史和下行板块的地震中可以推断出的过去1亿-2亿年地球历史上主要俯冲带的条件。这将利用帝国理工学院Co-I小组在模型数据比较方面的专业知识，与分别在地震成像和板块运动重建方面的领先专家Spakman教授和Torsvik教授合作完成。有了这些新的模型和这个跨学科的团队，我们将能够回答过渡带如何捕获和释放俯冲板块的根本问题，这一过程在地球内部和板块构造演化中发挥着关键作用。

Mantle circulation is largely driven by the sinking ('subduction') of cold and dense tectonic plates. When these cold slabs reach the transition zone between the upper and lower mantle (from 400 to 800 km depth), their progress is hampered by rapid increases in mantle density and viscosity, as mantle minerals change phase. X-ray type images of the interior of the Earth made using earthquake waves ('seismic tomography') reveal that this zone only forms a barrier for some slabs, while others seem to pass through unhindered. Furthermore, when the seismic images are compared with plate motions through time, it becomes clear that slabs penetrated the lower mantle in the past, where shallower parts of the plate are trapped in the transition zone today. This, as well as evidence of fast and slow subduction phases in plate motions (Goes et al., Nature 2008) indicate that this is a time dependent process, where plate material may pond until a critical mass of material has accumulated, and then flush rapidly into the lower mantle.It is important to understand this fundamental part of mantle circulation as it controls how efficiently the Earth cools and how well heterogeneities like sediments, crust, fluids and CO2 are mixed into it, or brought back up. Furthermore, sudden slab flushing events into the lower mantle have been linked to periods of continental crust formation, changes to the early atmosphere and reorganisations of plate motions. We will investigate slab behaviour in the transition zone using 3D dynamic models of subduction, and evaluate which of the modelled mechanisms are consistent with observational data from the Pacific. Previous numerical models have investigated how individual factors like slab strength, slab density, coupling to the upper plate, and mantle phase transitions affect whether a slab goes straight through the transition zone or stalls there. However, none of these individual parameters can explain the observed variations of plate-transition zone interaction. Nor is it clear for how long slabs may be stalled, and hence on which time scales the upper and lower mantle mix, and on which time scales plate motions and accompanying surface deformation vary. As single properties do not explain the variability of slab-transition-zone interaction, the interplay between mantle, downgoing and upper-plate properties must be crucial. Studying the interaction between these different factors requires 3D dynamic models that let plate motions and slab morphology develop freely, something that is numerically challenging. In recent years, our groups developed such dynamic models and elucidated how combinations of plate density, strength and width control upper-mantle slab morphology. The newest generation of these dynamic models, developed by the group of the PI at Durham, are now capable of modelling all potentially relevant plate and mantle parameters. With these models, we will explore a wide range of parameters to determine which combinations lead to slab ponding and penetration. Next we will compare modelled conditions for stalling and release with those that can be inferred for major subduction zones throughout the last 100-200 million years of Earth history from seismic tomography, plate motion histories and earthquakes in downgoing slab. This will be done using the expertise in model-data comparison in the group of the Co-I at Imperial, in collaboration with partners Prof. Spakman and Prof. Torsvik, leading experts in seismic imaging and plate motion reconstructions, respectively. With these new models and this interdisciplinary team, we will be able to answer the fundamental question of how the transition zone traps and releases subducting slabs, a process that plays a pivotal role in the Earth's internal and plate-tectonic evolution.