Surprises from the Magnetotelluric Component of the USArray in the Eastern United States: Perplexing Anticorrelations with Seismic Images and Puzzling Insights into Continental Dynamics

In addition to its broadband seismic component, the USArray has also been collecting long-period magnetotelluric (MT) data across the continental United States. These data allow for an unprecedented three-dimensional view of the lithospheric geoelectric structure of the continent. As electrical conductivity and seismic properties provide complementary views of the Earth, synthesizing seismic and MT images can reduce ambiguity inherent in each technique and can thereby allow for tighter constraints on lithospheric properties. In the western US, comparison of MT and seismic results has clarified some issues (e.g., with regard to fluids and volatiles) and has raised some new questions, but for the most part the two techniques provide views that generally mesh well together. In sharp contrast, MT and seismic results in the eastern US lead to seemingly contradictory conclusions about lithosphere properties. The most striking example is the Piedmont region of the southeastern United States; here seismic images suggest a relatively thin, warm Phanerozoic lithosphere, while MT images show a large, deep, highly resistive body that seems to require thick, cold, even cratonic lithosphere. While these MT results shed intriguing new light onto the enigmatic post-Paleozoic history of eastern North America, the strong anticorrelation with seismic images remains a mystery. A similar anticorrelation appears to also exist in the Northern Appalachians, and preliminary views of the geoelectric signature of the well-studied Northern Appalachian Anomaly suggest that synthesizing the seismic and MT images of that region may be nontrivial. Clearly, a major challenge in continued analysis of USArray data is the reconciliation of seemingly contradictory seismic and MT images. The path forward in addressing this problem will require closer collaboration between seismologists and MT scientists and will likely require a careful reconsideration of how each group interprets the physical meaning of their respective anomalies.