Anisotropic Properties of the Mid-lithospheric Discontinuity Beneath Central and Eastern North America

北美中部和东部下方岩石圈中部不连续面的各向异性特性

• 批准号: 1358325
• 负责人: Maureen Long
• 金额: $ 16.12万
• 依托单位: Yale University
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2014
• 资助国家: 美国
• 起止时间: 2014-08-15 至 2016-07-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1358325&HistoricalAwards=false
• 关键词: Anisotropic; Properties; Mid; lithospheric; Discontinuity

Earth's continents exhibit striking properties, including relatively thick and low-density crust and a strong, thick, long-lived mantle lithosphere. The continents have played a crucial role in controlling the Earth?s tectonic style and in the evolution of life, but we don't yet understand how continents form or why continental lithosphere remains stable over billions of years. Our understanding of the structure and dynamics of continents in general and the North American continent in particular is improving, enabled in large part by data from the EarthScope program. A major recent discovery is the presence of the so-called mid-lithospheric discontinuity (MLD): a sharp drop in seismic velocities at a depth of roughly 100 km, internal to the continental lithosphere. This feature has been extensively documented beneath North America and other continental interiors, but its origin remains unknown. The goal of this project is to probe the characteristics of the MLD beneath the central and eastern United States using EarthScope data and thus further our understanding of how continents form and evolve through time. An intriguing hypothesis is that the MLD corresponds to a sharp contrast in seismic anisotropy, perhaps associated with distinctive regimes of past deformation. This project is testing this hypothesis by studying the anisotropic properties of the MLD beneath eastern and central North America. In Phase I of the project, we are using anisotropic P-to-S receiver function (RF) analysis at long-running seismic stations, including stations of the USArray Reference Network. Characterizing the exact geometry of anisotropy across the MLD will allow us to determine if the MLD does in fact correspond to a change in the strength and/or orientation of anisotropy. In Phase II of the project, we will identify specific regions for a more in-depth analysis using Transportable Array and/or Flexible Array stations, perhaps including the Mid-Continent Rift, the Appalachians, the Interior Plains, and the New Madrid Seismic Zone, with the ultimate goal of understanding whether and how the MLD varies among different tectonic settings. We are addressing four specific science questions with this work: 1) Does the MLD beneath North America correspond to a change in the strength and/or orientation of seismic anisotropy? 2) Is the character of any change in anisotropy across the MLD consistent with inferences on MLD characteristics from previous work? 3) Do we see lateral variations in the anisotropic properties of the MLD within central and eastern North America, and do any variations correspond to geologic or tectonic provinces? 4) What physical mechanism(s) can explain the anisotropic properties of the MLD as a potentially ubiquitous feature throughout cratonic North America? The insights into MLD characteristics gained from this project will have important implications for understanding the structure and dynamics of continental lithosphere, the formation and preservation of continents, and continental deformation.

地球的大陆表现出惊人的特性，包括相对较厚和低密度的地壳以及坚固、厚实、寿命长的地幔岩石圈。大陆在控制地球构造风格和生命进化方面发挥了至关重要的作用，但我们还不了解大陆是如何形成的，也不了解大陆岩石圈为何在数十亿年的时间里保持稳定。我们对大陆尤其是北美大陆的结构和动态的了解正在不断改善，这在很大程度上得益于 EarthScope 计划的数据。最近的一项重大发现是存在所谓的岩石圈中部不连续面 (MLD)：大陆岩石圈内部约 100 公里深度处地震速度急剧下降。这一特征已在北美和其他大陆内部的地下被广泛记录，但其起源仍然未知。该项目的目标是利用 EarthScope 数据探索美国中部和东部地下 MLD 的特征，从而进一步了解大陆如何随时间形成和演化。 一个有趣的假设是，MLD 对应于地震各向异性的鲜明对比，这可能与过去变形的独特机制有关。该项目正在通过研究北美东部和中部地下 MLD 的各向异性特性来检验这一假设。在该项目的第一阶段，我们正在对长期运行的地震台站（包括 USArray 参考网络的台站）使用各向异性 P-to-S 接收器函数 (RF) 分析。表征 MLD 上各向异性的精确几何形状将使我们能够确定 MLD 是否确实对应于各向异性的强度和/或方向的变化。在该项目的第二阶段，我们将使用可移动阵列和/或灵活阵列站确定特定区域进行更深入的分析，可能包括中大陆裂谷、阿巴拉契亚山脉、内陆平原和新马德里地震带，最终目标是了解 MLD 在不同构造环境中是否以及如何变化。我们正在通过这项工作解决四个具体的科学问题：1）北美下方的 MLD 是否对应于地震各向异性的强度和/或方向的变化？ 2) MLD 各向异性变化的特征是否与之前工作中对 MLD 特性的推论一致？ 3）我们是否看到北美中部和东部MLD各向异性性质的横向变化，以及任何变化是否与地质或构造省份相对应？ 4) 什么物理机制可以解释 MLD 的各向异性特性作为整个北美克拉通潜在普遍存在的特征？从该项目中获得的对 MLD 特征的见解将对理解大陆岩石圈的结构和动力学、大陆的形成和保存以及大陆变形具有重要意义。