Three-dimensional electrical resistivity of the north-central USA from EarthScope long period magnetotelluric data

- We image the regional 3D electrical resistivity in the north-central US area.
- The 1.1 Ga Midcontinent rift is clearly imaged in the upper crust.
- Two Paleoproterozoic suture zones are imaged in the lower crust to the upper mantle.
- The lithosphere-asthenosphere boundary is imaged at ∼200 km depth.
- Asthenospheric resistivity suggests moderate levels of hydration.

We present initial results from three-dimensional inversion of long period EarthScope magnetotelluric (MT) transportable array data from 232 sites covering the north-central US. The study area covers the 1.1 Ga Mid-Continent Rift (MCR) system, which cuts across a series of Archean and Paleoproterozoic lithospheric blocks. The western arm of the MCR is clearly evident in shallow depth sections, with a narrow resistive core, flanked by elongate conductive basins. Other prominent upper-crustal features mapped include the moderately conductive Michigan and Illinois Basins, and extremely high conductivities in foreland basin rocks at the southern margin of the Superior craton. The most prominent conductive anomalies, in an otherwise relatively resistive mid-lower crust, are two elongate east-west oriented structures, which are closely aligned with previously inferred continental sutures. The first underlies the southern margin of the Superior craton just north of the Niagara Fault (NF), and can be associated with the ∼1.85 Ga Penokean Orogeny. A second, further south beneath Iowa and western Wisconsin, lies just south of the Spirit Lake tectonic zone (SLtz), and can be identified with Yavapai accretion at ∼1.75 Ga. Both of these conductive sutures are cleanly cut by the MCR, which is otherwise not clearly evident in the deeper parts of the resistivity model. The break in the anomalies is narrow, comparable to the surface expression of the MCR, indicating that rifting impacts on the entire crustal section were highly localized. Both suture-related anomalies are imaged as extending into, and perhaps through, the lithosphere as dipping diffuse zones of reduced mantle resistivity. Sense of dip of these structures (southward for the NF anomaly, northward for SLtz) agrees with previously inferred models for subduction and accretion, suggesting that a conductive phase (most likely carbon) has been thrust deep into the lower crust and uppermost mantle, providing a marker of the three-dimensional boundary between lithospheric blocks. Resistivities drop below ∼100 Ω m below ∼200 km depth, in rough agreement with the seismically determined lithosphere-asthenosphere boundary (LAB). There are modest lateral variations in this deep low-resistivity layer, but the reliability and significance of these are not yet clear.

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