Electrical Resistivity Imaging Of Continental United States From Three-dimensional Inversion Of Earth Scope Usarray Magnetotelluric Data

Long-period magnetotelluric(MT)sites,covering much of the continental United States(US)have been collected through the Earth Scope USArray MT Transportable Array(TA)project.Inversion of such a large area with approximately uniform data coverage would improve the resolutions on the deep lithospheric and asthenospheric structure of the contiguous US.Previous regional studies using subsets of these data suggest large-scale variations in deep(sub-lithospheric)resistivity,often of significant amplitude,which are suggestive of significant variations in mantle water content.However,given the inherent non-uniqueness of the MT method,and differences in inversion procedures used in different studies,it is unclear that these large-scale lateral variations in deep resistivity are truly required.Here,we present a range of three-dimensional(3D)continental-scale electrical resistivity models from 3D inversion of the MT TA data,with a focus on testing robustness and resolution of this deep structure.The main features of our initial model,obtained with no constraints,are quite similar to those from previous studies,with significant(± 1 order of magnitude)lateral variations in deep resistivity.We show that data fit,as measured by global normalized root-mean-square misfit,is not increased by replacing structures below 200 km depth with layer averages.A more careful examination of residuals leads to further refinements,with a slight improvement in data fit.These include a moderately conductive mantle transition zone(13 ?.m),and subtle(± 1/4 order of magnitude)lateral variations in resistivity below 200 km.In contrast to the initial results,these variations can be explained by reasonable variations in mantle temperature and hydration.Overall,our results suggest that the asthenospheric mantle at 200–275 km is nearly dry on average and at greater depths contains 150–300 ppm water.Lateral variations may be explained by this range of water content,or by temperature variations of 80–120?.Patterns of resistivity variations below 200 km are consistent with known continental structure,and suggest deep roots(?250 km)beneath cratons,and perhaps also beneath the mid-continent rift.Reduced resistivity,likely requiring greater hydration(or higher temperatures),occurs beneath the North-central part of the continental US,the oldest part of the continent,and beneath the Yellowstone hotspot.