MAGNETOTELLURIC INSIGHTS INTO THE LITHOSPHERIC ARCHITECTURE AND GEODYNAMIC HISTORY OF THE EASTERN UNITED STATES

EarthScope magnetotelluric (MT) data have provided new insights into the lithospheric architecture and tectonic history of the Appalachian domain. Most prominently, these data reveal two significant, large-scale geoelectric structures that provide new information about the evolution of this region. The first of these is a large-scale, margin-parallel conductive structure that extends from the mid-crust into the uppermost mantle and that follows the trace of the New York-Alabama magnetic lineament. With constraints from potential fields and radiogenic isotope data, this conductor is interpreted to represent a major Grenville-aged suture. This structure may represent the main boundary along which Grenville terranes were accreted onto Laurentia, and it also likely accommodated significant margin-parallel deformation during Grenvillian orogenesis. The other key feature illuminated by these MT data is a thick, coherent block of highly resistive lithosphere beneath the Piedmont and Coastal Plain physiographic provinces in the southeastern US. Here, the MT data require high resistivities (>300 Ωm) to at least 200 km depth. As dry mantle mineral conduction laws provide an upper bound on temperature for an observed resistivity value, these data require that lithospheric temperatures (<1330°C) persist to 200 km. Although seismic observations have been interpreted as showing relatively thin (<150 km), eroded thermal lithosphere beneath this region, both MT and seismic results are consistent with a thick (~200 km), coherent thermal lithosphere when considering the effect of finite grain size (anelasticity) on absolute seismic velocities and attenuation. This thick lithospheric block is interpreted to be the result of major mantle melting and depletion associated with formation of the Central Atlantic Magmatic Province. Preliminary MT inversions from the northeastern US suggest that this thick mantle root extends along the entirety of the eastern North American margin. These high resistivity values also effectively require that the constituent mantle is dry, so this structure is likely rheologically strong as well as compositionally buoyant due to melt depletion. Consequently, it may be resistant to recycling on long geological timescales and may represent a Mesozoic example of craton formation.