What drives and resists plate sinking through the transition zone?

是什么驱动和阻止板块通过过渡区下沉？

基本信息

批准号：
NE/J008028/1
负责人：
Jeroen Van Hunen
金额：
$ 25.51万
依托单位：
Durham University
依托单位国家：
英国
项目类别：
Research Grant
财政年份：
2013
资助国家：
英国
起止时间：
2013 至无数据
项目状态：
已结题

来源：
https://gtr.ukri.org/projects?ref=NE%2FJ008028%2F1
关键词：
What drives resists plate sinking

项目摘要

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.

地幔循环主要是由寒冷而致密的构造板块的下沉（“俯冲”）驱动的。当这些冷板片到达上地幔和下地幔之间的过渡带（深度为 400 至 800 公里）时，由于地幔矿物发生相变，地幔密度和粘度迅速增加，阻碍了它们的前进。使用地震波（“地震断层扫描”）拍摄的地球内部 X 射线图像显示，该区域仅对某些板块形成屏障，而其他板块似乎可以畅通无阻地穿过。此外，当将地震图像与随时间变化的板块运动进行比较时，可以清楚地看出，板块过去曾穿透过下地幔，而今天板块较浅的部分则被困在过渡带中。这以及板块运动中快速和慢速俯冲阶段的证据（Goes 等人，Nature 2008）表明，这是一个与时间相关的过程，其中板块材料可能会积聚，直到积累了临界质量的材料，然后冲刷了解地幔循环的这一基本部分非常重要，因为它控制着地球冷却的效率以及沉积物、地壳、流体和二氧化碳等异质性如何混合到其中或带回地球。此外，板片突然冲入下地幔的事件与大陆地壳形成时期、早期大气的变化和板块运动的重组有关。我们将使用俯冲 3D 动态模型研究过渡区的板片行为，并评估哪些建模机制与太平洋观测数据一致。先前的数值模型研究了板片强度、板片密度、与上板块的耦合以及地幔相变等各个因素如何影响板片是直接穿过过渡区还是在那里停滞。然而，这些单独的参数都不能解释观察到的板块-过渡区相互作用的变化。也不清楚板块会停滞多久，因此上地幔和下地幔混合在哪个时间尺度上，以及板块运动和伴随的地表变形在哪个时间尺度上发生变化。由于单一属性无法解释板片-过渡带相互作用的变化，因此地幔、下行和上板块属性之间的相互作用必定至关重要。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，这在数值上具有挑战性。近年来，我们的研究小组开发了这样的动态模型，并阐明了板块密度、强度和宽度的组合如何控制上地幔板片形态。最新一代的动态模型由达勒姆的 PI 小组开发，现在能够对所有潜在相关的板块和地幔参数进行建模。通过这些模型，我们将探索各种参数，以确定哪些组合会导致板积水和渗透。接下来，我们将把模拟的失速和释放条件与通过地震层析成像、板块运动历史和下行板片地震推断出的过去 100-2 亿年地球历史中主要俯冲带的条件进行比较。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，与分别是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。借助这些新模型和跨学科团队，我们将能够回答过渡带如何捕获和释放俯冲板块的基本问题，这一过程在地球内部和板块构造演化中发挥着关键作用。