Collaborative Research: Variations in Upper-Mantle Temperature, Deformation, and Melting Inferred from the Seismic Structure of the Atlantic Basin

该项目将研究大西洋盆地下地幔的热成分状态、熔融过程和变形。这些性质将从大西洋上地幔的地震速度、衰减和各向异性的新的高分辨率三维层析模型中推断出来。目前对海洋上地幔的了解大多来自太平洋盆地的区域地震模型。然而，大西洋和太平洋的扩张速度和板块速度的差异表明，两个盆地下面的地幔正在发生不同的动力过程。新模型将受到来自大西洋盆地内或边缘的事件和台站的基本和高模面波相位延迟和振幅的大量数据集的限制。本研究旨在解决三个关键问题：(1)是什么控制了地震结构作为深度和海底年龄的函数？这些特性能仅仅归因于温度吗，还是必须考虑成分和熔体？(2)沿轴和离轴地震异常的控制因素是什么？它们是否反映了一个可变的地幔源？温度和成分在其中起了什么作用？(3)与较慢板块速度相关的地幔结构是否比由较快扩张速率产生的地幔结构更弱、更多变？这些问题中的每一个都需要与快速扩张的太平洋上地幔进行比较，它们一起提供了一个机会来研究地震结构可能的传播速度依赖性以及可能产生它的机制。最终的地震模型将在矿物物理实验和其他数据集（如玄武岩化学、测深和大地水准面高度）的约束下，最终用于推断温度、成分、部分熔体含量和变形状态。研究的非技术描述根据板块构造理论，地球？地球坚硬的外壳被划分成相对缓慢移动的构造板块，这些板块在板块下方地幔（软流层）中热而弱的岩石内部的对流驱动下相互移动。我们对控制这种从板块内刚性岩石到板块下弱变形岩石的突然转变的过程知之甚少。仅仅是因为软流层的岩石更热吗？它们是部分熔融的，还是含有削弱它们的成分（水或其他挥发物）？由于地震产生的地震波对温度和其他减弱过程很敏感，我们可以利用地幔结构的地震成像来解决这些问题。区分这些过程将使我们更好地了解地球是如何？美国的构造体系随着时间的推移而发展和演变，也阐明了产生断层带和火山系统等地质灾害的弱化和融化过程。

This project will investigate the thermal and compositional state, melting processes, and deformation of the upper mantle beneath the Atlantic basin. These properties will be inferred from new, high-resolution, three-dimensional tomographic models of seismic velocity, attenuation, and anisotropy of the Atlantic upper mantle. Much of what is currently known about the oceanic upper mantle comes from regional seismic models of the Pacific basin. However, differences in the spreading rates and plate velocities of the Atlantic and Pacific oceans suggest that different dynamical processes are occurring in the mantle beneath the two basins. The new models will be constrained by large data sets of fundamental and higher-mode surface-wave phase delay and amplitude from events and stations located within or on the margins of the Atlantic basin. This research seeks to address three key issues: (1) What controls the seismic structure as a function of depth and seafloor age? Can these properties be attributed solely to temperature, or must composition and melt also be considered? (2) What controls along-axis and off-axis seismic anomalies? Do they reflect a variable mantle source, and what roles do temperature and composition play? (3) Is the mantle fabric associated with slower plate velocities weaker and more variable than that produced by faster spreading rates? Each of these questions invites a comparison with the faster-spreading Pacific upper mantle, and together they present an opportunity to investigate possible spreading-rate dependence of seismic structure as well as the mechanisms that might produce it. The final seismic models will ultimately be used to infer temperature, composition, partial melt content, and deformation state, aided by constraints from mineral-physics experiments and other data sets such as basalt chemistry, bathymetry, and geoid height. Non-Technical Description of ResearchAccording to the theory of plate tectonics, the Earth?s rigid outer shell is divided into tectonic plates that slowly move relative to one another, driven by convection currents within hot, weak rocks in the mantle beneath the plates (the asthenosphere). We have a poor understanding of the processes that control this abrupt transition from rigid rocks within the plate to weak deforming rocks beneath the plate. Is it simply that the asthenospheric rocks are hotter? Are they partially molten, or do they contain compositional components (water or other volatiles) that weaken them? Because seismic waves generated by earthquakes are sensitive to temperature and other weakening processes, we can use seismic imaging of mantle structure to address these questions. Distinguishing between these processes will allow us to better understand how the Earth?s tectonic system developed and evolved over time, and also illuminates the weakening and melting processes that produce geologic hazards such as fault zones and volcanic systems.