By inverting EarthScope long-period magnetotelluric (MT) data from the southeastern United States (SEUS), we obtain electrical conductivity images that provides key insights into the geodynamics of this region. Significantly, we resolve a highly electrically resistive block that extends to mantle depths beneath the modern Piedmont and Coastal Plain physiographic provinces. As high resistivity values in mantle minerals require cold mantle temperatures, the MT data indicate that the sub-
Piedmont thermal lithosphere must extend to greater than 200 km depth. This firm bound appears to conflict with conclusions from seismic results; tomography shows that velocities in this region are generally slightly slow with respect to references models. This observation has led to a seismically-informed view of relatively thin (<150 km), eroded thermal lithosphere beneath the SEUS. However, resolution tests demonstrate that our MT constraints are robust. Furthermore, narrow-band biases in
MT transfer functions from the SEUS due to geomagnetic pulsations associated with field-line resonances support the presence of bulk resistive lithosphere in this region. We show that, by considering anelastic prediction of seismic observables, MT and seismic (tomography, attenuation, receiver function) results are in fact consistent with thick (~200 km), coherent thermal lithosphere in this region. Our results demonstrate the danger of interpreting seismic results purely in terms of reference models and the importance of integrating different geophysical techniques when formulating geodynamic interpretations.