Investigating the Cause and Significance of Ultra Low Velocity Zones

调查超低速区的原因和意义

• 批准号: 1045788
• 负责人: Allen McNamara
• 金额: $ 30.31万
• 依托单位: Arizona State University
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2011
• 资助国家: 美国
• 起止时间: 2011-01-01 至 2016-12-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1045788&HistoricalAwards=false
• 关键词: Investigating; Cause; Significance; Ultra; Low

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.

在地质时间尺度上，地球的岩石地幔表现为粘性流体，其内部的对流驱动着地表的板块构造，并控制着热量如何通过地球内部传输。目前，对地幔对流的了解很少，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。我们最好的线索是来自地震观测的内部深处，这表明存在大规模的成分不均匀性。这将导致一种由热量和成分密度对比驱动的地幔对流类型。对这种对流的流体动力学性质的计算机建模揭示了可能的行为的广泛丰富性，导致了地球内部动力学的多个假设模型，每个模型对我们理解热量和质量传输都有显着不同的后果。地震学观测还揭示了一种不同类型的成分不均匀性，在非常小的尺度上，在地幔的底部，直接在地球的铁芯之上。我们的初步工作表明，这些在核幔边界的最小规模的地震观测可能提供了强大的约束地幔尺度对流的风格。我们将进行地球地幔内流体对流的数值模拟，以研究地幔动力学的多个概念假设，将大尺度和最小尺度的成分异质性地震观测联系在一起。通过仔细比较我们的数值模型的地震观测，我们将限制地球的复杂mantle convention.Work这里提出的固态动力学和更大的图片意义的超低速区（ULVZ），这是小规模的，地震检测补丁在地幔的基础。地震模拟推断，ULVZ有一个更高的密度比周围的地幔，在10%以上的顺序，它被假设，ULVZ是由非常高密度的小规模成分不均匀性的积累。此外，极低地震带大多位于非洲和太平洋下面的大得多的极低地震速度区域及其附近。我们已经进行了初步工作，表明这些最小规模的特征是在最低地幔（即，ULVZ）可能会对最大的地幔尺度动力学过程提供限制。建议的工作的主要动机是确定是否和如何观测ULVZ可能被用来约束大规模的，热化学对流在地球的地幔。这是一个重要的追求，因为发现控制地幔对流的大规模热化学对流的风格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（千米级）、全地幔地球动力学调查。我们将充分表征小体积的超高密度材料如何与大规模热化学对流相互作用，以产生与ULVZ观测一致的小规模高密度积聚。我们将研究目前争论不休的三种概念性热化学地幔模型（原始和瞬时堆、超级羽流）。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，我们将确定每个大尺度概念模型在ULVZ形状、大小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，可用于地震研究的ULVZ，预计其地理和形态特征将提供重要的限制地幔对流。此外，我们将调查是否是动态可行的，ULVZ的源可能是在地球表面，而不是核幔边界。此外，我们将调查的温度变化，预计在全球人口的ULVZ，这可能会提供深入了解为什么一些ULVZ显示部分融化的证据，而其他人不？t.