Three dimensional inversion of EarthScope long-period magnetotelluric data in north-central USA

Abstract:

We present results from three-dimensional (3D) inversion of EarthScope long-period MT data from the north-central USA, to map resistivity from the crust to mantle asthenosphere in the area surrounding the midcontinent rift (MCR). We employed the Modular system for Electromagnetic Inversion (ModEM) for 3D modeling and inversion to invert the full impedance tensors and vertical field transfer functions from 222 stations with 26 periods (11 - 7281 seconds). We used a strategy that reduced the smoothing parameters for layers shallower than 2 km, to improve data fit, and better allow for near surface static shift effects. This inversion strategy pushed resistivity variations to the shallower layers with less smoothing, resulting in cleaner images of deeper structures. The MCR, the most pronounced feature in gravity and magnetic map of the North American continent, is clearly imaged at shallow depths, with conductive flanking sedimentary basins prominent. However, the most striking conductive anomalies are two elongate E-W conductive features in the crust, which are readily identified with two distinct Proterozoic suture zones, one aligned with the Niagara Fault zone (NFz), and the second with the Spirit Lake tectonic zone (SLtz). Vertical sections show that NFz anomaly dips to the south, extending from the surface through the Moho, and at least weakly into the mantle lithosphere. The imaged dip is consistent with the sense of subduction inferred for the Penokean orogeny (~1.85 Ga). Meanwhile, the SLtz anomaly dips to the north, consistent with the sense of subduction associated with Yavapai accretion (~1.75 Ga). This anomaly has little expression in the upper crust, but also extends into the mantle. The 1.1 Ga MCR cuts both anomalies vertically. The uppermost mantle layers are mostly resistive, with reduced (~20 ohm-m) resistivities at depths of about 200 km, consistent with seismic estimates of aesthenospheric depths. Our results provide further support for the suggestion of previous studies that high conductivities in Paleoproterozoic suture zones result from carbon and sulfides accumulated in a foredeep environment, subsequently emplaced, graphitized and concentrated by subduction related deformation.

B. Yang was supported by Natural Science Foundation of China (NSFC) grant No.41304110.