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上地幔的热状态和成分状态、熔融过程和变形。这些特性将从新的、高分辨率的、关于大西洋上地幔的地震速度、衰减和各向异性的三维层析模型中推断出来。目前已知的关于大洋上地幔的大部分信息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，这两个盆地之下的地幔正在发生不同的动力学过程。新模型将受到来自大西洋盆地内部或边缘的事件和台站的基波和高模面波的相位延迟和幅度的大数据集的限制。这项研究试图解决三个关键问题：(1)是什么控制了地震结构作为深度和海底年龄的函数？这些特性是否可以完全归因于温度，或者必须同时考虑成分和熔融？(2)是什么控制了轴向和离轴地震异常？它们是否反映了可变的地幔来源，以及温度和成分扮演了什么角色？(3)与较慢的板块速度相关的地幔组构是否比较快的扩张速度产生的地幔组构更弱、更多变？这些问题中的每一个都引起了与扩张速度更快的太平洋上地幔的比较，它们共同提供了一个机会，来研究地震结构可能的扩张率相关性以及可能产生地震结构的机制。最终的地震模型将最终用于推断温度、成分、部分熔体含量和变形状态，并受到矿物物理实验和其他数据集(如玄武岩化学、水深测量和大地水准面高度)的限制。研究非技术性描述根据板块构造理论，地球-S刚性外壳被分成几个构造板块，板块下的地幔(软流圈)内的热而脆弱的岩石内的对流驱动着板块之间的缓慢运动。我们对控制从板块内的坚硬岩石到板块下的弱变形岩石的突然转变的过程知之甚少。这仅仅是因为软流圈的岩石更热吗？它们是部分熔融的，还是含有削弱它们的组成成分(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理解地球S构造体系是如何随着时间的推移而发展和演化的，也将阐明产生断裂带和火山系统等地质灾害的弱化和熔融过程。

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.