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Investigating the long-term spatial stability of LLSVPs

研究 LLSVP 的长期空间稳定性

基本信息

批准号：
1722623
负责人：
Allen McNamara
金额：
$ 21.36万
依托单位：
Michigan State University
依托单位国家：
美国
项目类别：
Continuing Grant
财政年份：
2017
资助国家：
美国
起止时间：
2017-07-01 至 2022-06-30
项目状态：
已结题

来源：
https://www.nsf.gov/awardsearch/showAward?AWD_ID=1722623&HistoricalAwards=false
关键词：
Investigating long term spatial stability

项目摘要

The Earth's surface is highly dynamic, characterized by earthquakes, volcanoes, and active faulting caused by the movement of tectonic plates at the surface. It is well-understood that the driver of plate tectonics is convection within Earth's mantle, the layer between the core and the crust. Over the past several decades, there has been much advancement toward increasing our understanding of how tectonic plates form, move about the surface, and sink back into the Earth's interior; however, we still have fundamental, first-order questions regarding how the mantle convects and drives plate tectonics. Seismology provides the best clues to answer these questions, by using seismic waves caused by earthquakes to generate a 3D image of the interior. One puzzling observation that we find from seismology is the presence of 2 large structures in the lowermost mantle, beneath Africa and the Pacific. These structures rest directly on the Earth's core, and extend several hundred kilometers into the mantle. Seismic waves travel slower than usual through these structures, and they seem to have sharp boundaries, indicating that they may be composed of something somewhat different than the surrounding mantle rock. Interestingly, these structures underlie a large number of anomalous active volcanoes (called hotspots), such as Hawaii. By reconstructing plate motions back through time, we find that many extinct, ancient volcanoes were also formed above these same locations. Therefore, if the large structures beneath Africa and the Pacific are the cause of these hotspot volcanoes, they must have been in their present location for a long time, perhaps for the past 500 million years. This is contradictory to computational models of mantle convection that predict that such structures would be easily moved by changing tectonic plate motions. The PI hypothesizes that these deep mantle structures may have a different mineralogical grain size than the rest of the mantle, which could cause them to have a higher viscosity. The team will perform numerical fluid dynamics to examine whether the higher viscosity associated with these structures could lead to a stabilizing of mantle convection currents over geologic time to explain the long-lived creation of volcanoes over these geographic areas. More importantly, the investigators will examine how the dynamics of these African and Pacific structures can guide and control the long term patterns of mantle convection. Results from this study will provide critical insight into understanding how mantle convection causes and controls plate tectonics.Seismic tomography reveals the presence of large regions in the lowermost mantle (beneath Africa and the Pacific) that exhibit lower-than-average shear wave velocity, commonly referred to as the Large Low Shear Velocity Provinces (LLSVPs). It has been hypothesized that they are caused by large-scale compositional heterogeneity (e.g., thermochemical piles). Discovering the properties and dynamical implications of the LLSVPs has involved much active research in recent years due to their critical role toward understanding large-scale mantle convection, heat transport, and thermal and chemical evolution of the Earth. Interestingly and somewhat paradoxically, recent paleomagnetic research has hinted that LLSVPs may have remained in the same positions for hundreds of millions of years. This is perplexing because dynamical calculations indicate that they should be easily swept around by changing subduction patterns. This is a critically important issue to address, and here, the following question is explored: Can a reasonable dynamical/rheological conceptual model of thermochemical mantle convection explain how LLSVPs could remain spatially fixed (or very slow moving)? Most geodynamical calculations employ a temperature-dependent rheology, resulting in LLSVPs being weaker (due to their high temperature) and therefore, easier to be laterally pushed around by downwelling slabs. However, diffusion creep rheology is highly sensitive to mineral grain-size (in a power-law manner), so small changes in grain-size can lead to large changes in viscosity. If LLSVPs have remained compositionally distinct from the background mantle over geologic timescales, there is no reason to expect that they would have the same average grain-size as the surrounding mantle. If the average grain-size of LLSVPs is only slightly larger than that of the background mantle, LLSVPs would have an increased compositional viscosity. Preliminary work has shown that when combined with temperature-dependence of viscosity, this leads to a strong rind or envelope surrounding the LLSVPs, producing unusual thermochemical structures that dynamically behave quite differently than conventional, passive thermochemical piles. This type of thermochemical convection has not been explored, and the proposed work will test the hypothesis that grain-size induced compositional dependence of viscosity can lead to LLSVP structures that are less easily pushed around by subducting slabs and therefore, become more spatially-stable over geologic timescales. An extensive geodynamical study (through numerical modeling) will be performed to explore how grain-size dependence of rheology influences the fluid dynamical properties of the system, compared to traditional models. Additionally, grain-size evolution will be employed in a subset of models, to examine whether it is feasible for LLSVPs to have larger grain size than background mantle over the relevant geological timescales. In summary, numerical experiments will be developed and performed to determine how grain-size dependence of viscosity influences the spatial stability of thermochemical piles and whether this is a dynamically feasible mechanism to produce long-lived, stable positions of LLSVPs.

地球表面是高度动态的，其特征是地震、火山和由地表构造板块运动引起的活动断层。众所周知，板块构造的驱动力是地幔（地核和地壳之间的一层）内的对流。在过去的几十年里，我们对构造板块如何形成、在地表移动和沉入地球内部的理解有了很大的进步;然而，我们仍然有关于地幔如何对流和驱动板块构造的基本的一级问题。地震学提供了回答这些问题的最佳线索，通过使用地震引起的地震波来生成内部的3D图像。我们从地震学中发现的一个令人困惑的观察结果是，在非洲和太平洋下面的最低地幔中存在两个大型结构。这些结构直接位于地核上，并延伸数百公里进入地幔。地震波在这些结构中的传播速度比通常要慢，而且它们似乎有尖锐的边界，这表明它们可能是由与周围地幔岩石有些不同的东西组成的。有趣的是，这些结构位于大量异常活火山（称为热点）的下面，例如夏威夷。通过重建板块运动，我们发现许多死的古代火山也形成于这些相同的位置。因此，如果非洲和太平洋下面的大型结构是这些热点火山的原因，那么它们一定在现在的位置已经很长时间了，也许在过去的5亿年里。这与地幔对流的计算模型相矛盾，后者预测这种结构很容易通过改变构造板块运动而移动。PI假设这些深地幔结构可能具有与地幔其他部分不同的矿物粒度，这可能导致它们具有更高的粘度。该团队将进行数值流体动力学研究，以研究与这些结构相关的较高粘度是否会导致地质时期地幔对流的稳定，以解释这些地理区域火山的长期形成。更重要的是，研究人员将研究这些非洲和太平洋结构的动力学如何引导和控制地幔对流的长期模式。这项研究的结果将为理解地幔对流如何导致和控制板块构造提供重要的见解。地震层析成像揭示了最低地幔（非洲和太平洋下方）中存在大面积区域，这些区域表现出低于平均剪切波速度，通常被称为大低剪切速度区（LLSVP）。据推测，它们是由大规模的成分异质性（例如，热化学堆）。近年来，由于LLSVP对理解大尺度地幔对流、热传输以及地球的热演化和化学演化具有重要作用，因此发现LLSVP的性质和动力学意义涉及了许多积极的研究。有趣的是，最近的古地磁研究暗示，LLSVP可能已经保持了数亿年的相同位置。这是令人困惑的，因为动力学计算表明，它们应该很容易通过改变俯冲模式而被横扫。这是一个非常重要的问题，以解决，在这里，以下问题进行了探讨：一个合理的动力学/流变学概念模型的热化学地幔对流解释如何LLSVP可以保持空间固定（或非常缓慢的移动）？大多数地球动力学计算采用温度依赖的流变学，导致LLSVP较弱（由于其高温），因此更容易被下涌板块横向推动。然而，扩散蠕变流变学对矿物粒度高度敏感（以幂律方式），因此粒度的小变化可导致粘度的大变化。如果LLSVP在地质时间尺度上保持与背景地幔的成分不同，那么没有理由期望它们与周围地幔具有相同的平均粒度。如果LLSVP的平均粒度仅略大于背景地幔，LLSVP将具有增加的组分粘度。初步工作表明，当与粘度的温度依赖性相结合时，这会导致LLSVP周围的强烈外皮或信封，产生不寻常的热化学结构，其动态行为与传统的被动热化学堆完全不同。这种类型的热化学对流还没有被探索，和拟议的工作将测试的假设，粒度引起的粘度的组成依赖性，可以导致LLSVP结构，不太容易被俯冲板块左右推，因此，在地质时间尺度上变得更加空间稳定。将进行广泛的地球动力学研究（通过数值模拟），以探索流变学的粒度依赖性如何影响系统的流体动力学特性，与传统模型相比。此外，粒度演化将采用在一个子集的模型，以检查是否是可行的LLSVP有更大的粒度比背景地幔在相关的地质时标。总之，数值实验将开发和执行，以确定如何粒度粘度的依赖性影响的热化学堆的空间稳定性，这是否是一个动态可行的机制，以产生长寿命，稳定的位置LLSVP。