Investigating the Cause and Significance of Ultra Low Velocity Zones

在地质时间尺度上，地球的岩石地幔表现得像一种粘性流体，其内部的对流驱动着地表的板块构造，并控制着热量如何在地球内部传播。目前，人们对地幔对流知之甚少，发现它是如何运作的是更好地理解板块构造和地球热演化的一个重要目标。我们最好的线索来自于深层内部的地震观测，这些观测表明存在大规模的成分不均匀性。这将导致一种由热密度和成分密度对比驱动的地幔对流。这种对流的流体动力学性质的计算机建模揭示了可能行为的广泛丰富性，导致了地球内部动力学的多个假设模型，每个模型对我们对热量和质量传输的理解都有显著不同的影响。地震学观测还揭示了一种不同类型的成分不均匀性，在非常小的范围内，在地幔的最底部，直接在地球的铁芯上方。我们的初步工作表明，这些在核幔边界的小尺度地震观测可能对地幔尺度对流的类型提供强有力的约束。我们将对地幔内的流体对流进行数值模拟，以检验地幔动力学的多个概念假设，并将大尺度和小尺度的成分非均质性地震观测结合在一起。通过仔细比较我们的数值模型与地震学观测，我们将约束地球复杂地幔对流的类型。这里提出的工作是研究地幔底部的小型地震探测斑块——超低速度带（ULVZs）的固态动力学和更大的图像意义。地震模拟结果表明，超低震区的密度比其周围地幔高10%左右，并推测超低震区是由高密度的小尺度成分非均质性积累造成的。此外，ulvz大多位于非洲和太平洋下方的大尺度异常低地震速度区域及其附近。我们已经进行了初步的工作，表明在最底地幔（即ulvz）中观察到的这些最小尺度特征可能为最大的地幔尺度动态过程提供约束。提出这项工作的主要动机是确定ulvz的观测是否以及如何用于约束地幔中的大规模热化学对流。这是一项重要的研究，因为发现控制地幔对流的大规模热化学对流的类型，将对我们理解地球内部的热量和化学传输产生根本性的影响。我们将与积极从事ULVZ地震观测的地震学家密切合作，进行多分量、高分辨率（千米尺度）的全地幔地球动力学调查。我们将充分描述小体积超高密度材料如何与大规模热化学对流相互作用，从而产生与ulvz观测一致的小规模高密度堆积。我们将研究目前有争议的三种概念热化学地幔模型（原始和瞬态堆，超羽流）。对于每一个，我们将确定在地质时间尺度上维持ulvz所需的必要条件。最重要的是，我们将确定每个大型概念模型在ULVZ形状、尺寸和密度方面产生的独特特征。这项工作将导致可测试的预测，可用于ulvz的地震研究，预计其地理和形态特征将为地幔对流提供重要的约束。此外，我们将研究ulvz的来源可能在地球表面而不是核-地幔边界是否在动力学上可行。此外，我们将研究全球ULVZs种群中预期的温度变化，这可能会深入了解为什么一些ULVZs显示出部分融化的证据，而另一些则没有。

On geologic timescales, the Earth's rocky mantle behaves as a viscous fluid, and convection within it drives plate tectonics at the surface and controls how heat is transported through the planet's interior. Currently, mantle convection is poorly understood, and discovering how it operates is an important goal toward better understanding plate tectonics and Earth's thermal evolution. Our best clues are derived from seismological observations of the deep interior which indicate the presence of large scale compositional heterogeneity. This would cause a style of mantle convection that is driven by both thermal and compositional density contrasts. Computer modeling of the fluid dynamical nature of such convection reveals a wide richness in possible behavior, leading to multiple hypothetical models of Earth's interior dynamics, each having significantly different consequences toward our understanding of heat and mass transport. Seismological observations also reveal a different type of compositional heterogeneity, on a very small scale, at the very bottom of the mantle, directly above the Earth's iron core. Our preliminary work reveals that these smallest-scale seismic observations at the core-mantle boundary may provide strong constraints on the style of mantle-scale convection. We will perform numerical modeling of fluid convection within Earth's mantle to examine multiple conceptual hypotheses of mantle dynamics, tying together both the large-scale and smallest-scale seismic observations of compositional heterogeneity. By carefully comparing our numerical models to seismological observations, we will constrain the style of Earth's complex mantle convection.Work proposed here investigates the solid-state dynamics and bigger-picture significance of ultra low velocity zones (ULVZs), which are small-scale, seismically-detected patches at the base of the mantle. Seismic modeling infers that ULVZs have a much higher density than surrounding mantle, on the order of 10% greater, and it is hypothesized that ULVZs are caused by the accumulation of small-scale compositional heterogeneity of very high density. In addition, ULVZs are mostly located in and near the much larger-scale regions of anomalously low seismic velocity, beneath Africa and the Pacific. We have performed preliminary work that indicates that these smallest-scale features observed in the lowermost mantle (i.e., ULVZs) may provide constraints on the largest, mantle-scale dynamic processes. The primary motivation of the proposed work is to determine if and how observations of ULVZs may be used to constrain large- scale, thermochemical convection in Earth's mantle. This is an important pursuit because discovering the style of large-scale thermochemical convection that controls mantle convection will have fundamental consequences toward our understanding of heat and chemical transport in the Earth's interior. We will perform multi-component, high-resolution (km-scale), whole mantle geodynamical investigations, in close collaboration with seismologists who actively work with ULVZ seismic observations. We will fully characterize how a small volume of ultra-high density material interacts with large-scale thermochemical convection to produce small-scale, high- density accumulations consistent with observations of ULVZs. We will investigate the three currently-debated, conceptual thermochemical mantle models (primordial and transient piles, superplumes). For each, we will determine the necessary conditions required to maintain ULVZs over geologic timescales. Most importantly, we will determine unique signatures that each large- scale conceptual model produces in terms of ULVZ shape, size, and density. This work will lead to testable-predictions that can be utilized in seismic studies of ULVZs, in anticipation that their geographic and morphological characteristics will provide important constraints on mantle convection. In addition, we will investigate whether it is dynamically feasible that the source of ULVZs may be at the Earth's surface as opposed to the core-mantle boundary. Furthermore, we will investigate the temperature variations expected in the global population of ULVZs, which may provide insight into why some ULVZs show evidence for partial melt while others don?t.