Title: Electrical resistivity imaging of continental United States from three-dimensional inversion of EarthScope USArray magnetotelluric data
Authors: Bo Yang, Gary Egbert, Huiqian Zhang, Naser Meqbel, Xiangyun Hu*
Source: Earth and Planetary Science Letters, 15 December 2021, 117244
Available online：22 October 2021
DOI: 10.1016/j.epsl.2021.117244
Link: https://www.sciencedirect.com/science/article/pii/S0012821X21005008?via%3Dihub
Abstract：
Long-period magnetotelluric (MT) sites, covering much of the continental United States (US) have been collected through the EarthScope USArray MT Transportable Array (TA) project. Previous regional studies using subsets of these data suggest large-scale variations in deep (sub-lithospheric) resistivity, often of significant amplitude. 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 (±1order 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 (13Ω.m), and subtle (±1/4order 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 cariations below 200km are consistent with known continential structure, and suggest deep roots (~250km) 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.
