Collaborative Research: Bayesian Estimation of Mantle Viscosity Structure and Geodynamic Implications

合作研究：地幔粘度结构的贝叶斯估计及其地球动力学意义

• 批准号: 1622464
• 负责人: Maxwell Rudolph
• 金额: $ 19.3万
• 依托单位: Portland State University
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2016
• 资助国家: 美国
• 起止时间: 2016-09-01 至 2018-02-28
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1622464&HistoricalAwards=false
• 关键词: Collaborative; Research; Bayesian; Estimation; Mantle

Earth's mantle comprises more than 80% of our planet's interior, and convection in the mantle is linked to plate tectonics, the magnetic field, volcanic activity, and the gases in our atmosphere. Solid mantle rocks deform and flow over long time scales, and the viscosity (resistance to flow) of mantle rocks affects the rate of flow in the mantle, the energy budget of Earth's deep interior, and the speed with which tectonic plates move past one another. One of the best constraints on the mantle viscosity structure comes from modeling variations in Earth's long-wavelength gravity field. The team will combine mantle flow models, seismic images of the Earth's interior, and results from mineral physics to better constrain the depth variation of viscosity and its influence on mantle convection. The project will address the following scientific questions: (1) How does viscosity vary with depth, (2) How does viscosity structure affect behavior of buoyant, upwelling mantle material, and (3) How does uncertainty in seismic images of Earth's mantle affect our uncertainty in the viscosity profile? In addition to this research, the project contributes to the training and professional development of a graduate student and a postdoctoral researcher. Additionally, the team will work with a Portland-area high school teacher to develop curricular materials to teach concepts related to flow in the mantle from the Next Generation Science Standards.Full waveform whole-mantle tomography has recently provided improved measurements of lower mantle shear wave velocity anomalies. These images shed new light on the behavior of mantle upwellings and downwellings. Wide plumes, continuous from just above the core mantle boundary to the base of the lithosphere, are resolved beneath many of Earth's active volcanic hot spots, and plumes frequently appear to be deflected laterally below the transition zone, at a depth of 1000 km, a depth not coincident with known seismic discontinuities, but at which slabs stagnate, plumes are deflected, and changes in long-wavelength radial correlation structure appear in many tomographic models. Tomographic studies combining whole-Earth free oscillations with various other seismological observations (e.g. body wave travel times, surface wave dispersion, full waveforms) have recently improved constraints on the long-wavelength variations in wavespeed in the transition zone and mid mantle. Recent seismological data sets also suggest a deviation from simple scaling relationships between density and shear velocity, indicative of large-scale chemical heterogeneity in the lowermost mantle. The investigators will identify robust aspects of recent tomographic models, estimate uncertainties associated with their translation to density variations and employ a new inversion technique that incorporates these results in a probabilistic way to obtain improved constraints on the mantle viscosity structure. They will then use the solutions as the basis for numerical mantle convection simulations to evaluate the extent to which the inferred viscosity structures are compatible with available constraints on the style of convection and rate of heat transport in the mantle. In addition to training and mentoring of a graduate student and a postdoctoral researcher, the investigating team will work with a high school educator to develop curricular materials relevant to the Next Generation Science Standard HS-ESS2-3.

地幔占地球内部的 80% 以上，地幔中的对流与板块构造、磁场、火山活动和大气中的气体有关。固体地幔岩石会在长时间尺度内变形和流动，地幔岩石的粘度（流动阻力）会影响地幔中的流速、地球内部深处的能量收支以及构造板块彼此移动的速度。对地幔粘度结构的最佳约束之一来自于对地球长波长重力场变化的建模。该团队将结合地幔流模型、地球内部的地震图像和矿物物理学的结果，以更好地限制粘度的深度变化及其对地幔对流的影响。该项目将解决以下科学问题：(1) 粘度如何随深度变化，(2) 粘度结构如何影响浮力、上涌地幔物质的行为，以及 (3) 地幔地震图像的不确定性如何影响我们的研究粘度曲线的不确定性？除了这项研究之外，该项目还有助于研究生和博士后研究员的培训和专业发展。此外，该团队将与波特兰地区的一名高中教师合作开发课程材料，以教授下一代科学标准中与地幔流动相关的概念。全波形全地幔断层扫描最近提供了对下地幔剪切波的改进测量速度异常。这些图像为地幔上升流和下降流的行为提供了新的线索。宽阔的地幔柱从核心地幔边界上方一直延伸到岩石圈底部，在地球许多活跃的火山热点下方得到解决，并且地幔柱经常出现在过渡带下方横向偏转，深度为 1000 公里，深度与已知的地震不连续性不重合，但在许多层析成像模型中，板片停滞、羽流偏转以及长波长径向相关结构的变化出现在该深度。将全地球自由振荡与各种其他地震观测（例如体波传播时间、表面波色散、全波形）相结合的层析成像研究最近改善了对过渡带和中地幔波速长波长变化的限制。最近的地震数据集还表明，密度和剪切速度之间的简单比例关系存在偏差，表明最下地幔存在大规模的化学不均匀性。研究人员将确定最新断层扫描模型的稳健方面，估计与其转化为密度变化相关的不确定性，并采用一种新的反演技术，以概率方式合并这些结果，以获得对地幔粘度结构的改进约束。然后，他们将使用这些解决方案作为地幔对流数值模拟的基础，以评估推断的粘度结构与地幔对流类型和热传输速率的现有约束的兼容程度。除了培训和指导一名研究生和一名博士后研究员外，调查团队还将与一名高中教育工作者合作开发与下一代科学标准 HS-ESS2-3 相关的课程材料。