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

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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.