High-resolution seismic constraints to reveal mid-mantle processes

高分辨率地震约束揭示中地幔过程

基本信息

批准号：
NE/R010862/1
负责人：
Sanne Cottaar
金额：
$ 40.92万
依托单位：
University of Cambridge
依托单位国家：
英国
项目类别：
Research Grant
财政年份：
2018
资助国家：
英国
起止时间：
2018 至无数据
项目状态：
已结题

来源：
https://gtr.ukri.org/projects?ref=NE%2FR010862%2F1
关键词：
resolution seismic constraints reveal mid

项目摘要

The dynamics of Earth's mantle, the 2900 km layer sandwiched between the crust and core, have shaped the Earth's surface, as we know it today. For example, upwelling material in the mantle, known as mantle plumes, causes localized, increased volcanism at the surface, forming most of today's ocean islands. The initiation and early formation of continents has also been attributed to mantle plumes, along with more recent continent-sized volcanic outflows known as 'large igneous provinces'. Processes in the deep mantle have also been hypothesized to control the pattern of plate tectonics, and the supercontinent cycle over the history of the Earth. We currently do not have a full picture of the dynamical history of the mantle that explains all these observations. For example, we do not know whether the mantle convects as one layer or two layers, and thus if mantle plumes directly connect processes at the core-mantle boundary to the surface, and if the mantle is well mixed over time. We also do not know the nature of heterogeneity in the deeper mantle, nor how this influences the overall dynamics. Much of our knowledge of the deep Earth's structure and dynamics comes from global seismic tomography, which uses earthquake waves to make an image of seismic velocity variations in the mantle. The images of seismic tomography show features with fast seismic wave speeds, interpreted as cold downgoing slabs, and features of slow seismic wave speeds, interpreted as hot upwelling mantle plumes. Resolution of these images has been ever improving with the burgeoning increase in data and computational power. One of the most remarkable recent discoveries has been the ponding of some slabs and mantle plumes around a depth of 1000 km in the mid-mantle. An unanswered question lies here: What is happening at this depth that affects the convective motion?The downside of seismic tomography is that it broadens imaged features and underestimates their true amplitudes. This is partly because the periods of the waves used are relatively long, thus reducing the resolution of the heterogeneity imaged at depth. Here we propose targeted studies using higher-frequency waves than can be incorporated in seismic tomography to image the small-scale heterogeneities around 1000 km in the mid-mantle. Specifically, we will use waves that are reflected or converted by these heterogeneities and therefore have strong sensitivity to the boundaries of these features. The unique sensitivities of the different phases allow us to map the size, shape, velocity contrast, density contrast and sharpness of the anomalous heterogeneities. In a preliminary study using converted seismic waves beneath Europe, we mapped broad patches of heterogeneity consistently at 1000 km depth. We will expand this technique to map these features on a global scale and understand how they relate to the observed slabs and mantle plumes and to what degree they are clustered around 1000 km. Next, we need to target the heterogeneities with a combination of different reflected and converted waves. The questions about the nature and role of these mantle heterogeneities are fundamentally interdisciplinary. We will combine these high-resolution seismological constraints with experiments and calculations of the thermo-elastic behaviour of specific compositions under high pressures and temperatures. In this manner we will test a number of key hypotheses on deep Earth structure: Do the heterogeneities originate from the surface and are they introduced by subducting slabs? Or do they represent primordial material, either brought up from the deeper mantle or stagnating at this depth throughout the history of the Earth? Are there different types of heterogeneities present?By understanding the composition of the observed heterogeneities through targeted deep Earth imaging, we can determine its role in controlling the overall mantle dynamics.

如今，我们所知道的，地球地幔的动力学是夹在地壳和核心之间的2900公里层。例如，地幔中的上升材料（称为地幔羽流）导致地表的局部火山增加，形成了当今大部分海洋岛屿。大陆的启动和早期形成也归因于地幔羽，以及较新的大陆大小的火山流出称为“大火成岩省”。还假设深幔的过程可以控制板块构造的模式，以及地球历史上的超大陆循环。目前，我们还没有对地幔的动态历史的全面了解，这解释了所有这些观察。例如，我们不知道地幔对流是否为一层或两层，因此是否将地幔羽直接连接到核心屏蔽边界的过程与表面连接在一起，以及地幔是否会随着时间的推移充分混合。我们也不知道更深的地幔中异质性的性质，也不知道这如何影响整体动态。我们对深层地球结构和动态的许多了解都来自全球地震层析成像，该层层造影使用地震波来形象地震中的地震速度变化。地震层析成像的图像显示了具有快速地震波速度的特征，被解释为冷的板，以及缓慢的地震波速度的特征，被解释为热上升的地幔羽流。随着数据和计算能力的增加，这些图像的分辨率一直在改善。最近发现的最引人注目的发现之一是在中间的1000公里深处堆积了一些板和地幔羽。一个未解决的问题在这里：在这个深度影响对流运动的深度发生了什么？地震层析成像的缺点是它扩大了成像的特征并低估了它们的真实幅度。这部分是因为所使用的波的周期相对较长，从而减少了深度成像的异质性的分辨率。在这里，我们提出了使用高频波的有针对性研究，而不是在地震断层扫描中纳入地震层析成像，以对中间的1000公里左右的小尺度异质性进行成像。具体而言，我们将使用这些异质性反映或转换的波，因此对这些特征的边界具有强烈的敏感性。不同阶段的独特敏感性使我们能够绘制大小，形状，速度对比度，密度对比度和异常异质性的清晰度。在使用欧洲下方转化的地震波的初步研究中，我们在1000公里的深度下始终绘制了宽阔的异质性斑块。我们将扩展此技术以在全球范围内绘制这些功能，并了解它们与观察到的板和地幔羽之间的关系，以及它们在1000公里左右的何种程度。接下来，我们需要以不同的反射和转换波的组合靶向异质性。这些地幔异质性的性质和作用的问题在根本上是跨学科的。我们将将这些高分辨率的地震学限制与在高压和温度下特定组成的热弹性行为的实验和计算结合在一起。通过这种方式，我们将对深层结构的许多关键假设进行测试：异质性是否来自表面，它们是否是通过俯冲板引入的？还是它们代表原始物质，要么是从更深的地幔中提出的，要么在整个地球的历史中都在这一深度停滞不前？是否存在不同类型的异质性？通过通过靶向深层成像来了解观察到的异质性的组成，我们可以确定其在控制整体地幔动力学中的作用。