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

地球的大陆表现出惊人的特性，包括相对较厚、密度较低的地壳，以及坚固、厚实、寿命较长的地幔岩石圈。大陆在控制地球上扮演了至关重要的角色？但我们还不知道大陆是如何形成的，也不知道为什么大陆岩石圈在数十亿年的时间里保持稳定。我们对大陆的总体结构和动力学的理解，特别是对北美大陆的理解，在很大程度上得益于“地球范围”项目的数据，正在不断提高。最近的一个重大发现是所谓的中岩石圈不连续（MLD）的存在：在大陆岩石圈内部大约100公里的深度，地震速度急剧下降。这一特征在北美和其他大陆内部被广泛记录下来，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的MLD特征，从而进一步了解大陆是如何形成和演变的。一个有趣的假设是，MLD对应于地震各向异性的鲜明对比，可能与过去变形的独特制度有关。该项目通过研究北美东部和中部MLD的各向异性特性来验证这一假设。在项目的第一阶段，我们在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接收函数（RF）分析。描述MLD各向异性的精确几何特征将使我们能够确定MLD是否确实对应于各向异性强度和/或方向的变化。在项目的第二阶段，我们将使用可移动阵列和/或柔性阵列站确定特定区域进行更深入的分析，可能包括大陆中部裂谷，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是否以及如何在不同的构造环境中变化。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强度和/或方向的变化？2) MLD各向异性变化的特征是否与以往工作对MLD特征的推断一致？3)在北美中部和东部，我们是否看到了MLD各向异性性质的横向变化，是否有任何变化与地质或构造省相对应？4)什么物理机制可以解释MLD的各向异性特性是整个北美克拉通地区潜在的普遍特征？该项目对MLD特征的深入研究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的变形具有重要意义。

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.