Geophysical extent of the Wyoming Province, western USA: Insights into ancient subduction and craton stability

Abstract

A new 3-D resistivity model, estimated from inversion of magnetotelluric data, images crustal and upper-mantle structure of the Wyoming Province and adjacent areas. The Archean province is imaged as a coherent resistive domain, in sharp contrast to active tectonic domains of the western U.S. Prominent high-conductivity belts define the northern, eastern, and southern margins of the Wyoming Province and are interpreted as sutures marking the remnants of Paleoproterozoic orogens. The model results suggest the northern boundary of the Wyoming Province is located 150 km south of its traditional placement and adjacent to a composite orogen separating the Wyoming Province and Medicine Hat block. The eastern province boundary is clearly imaged along the Black Hills, whereas the western margin is obscured by Cenozoic extension and magmatism. An internal boundary within the Wyoming Province is interpreted to represent a Neoarchean suture; in stark contrast to Proterozoic sutures, though, it is not marked by a high-conductivity belt. This difference in conductivity is speculated to reflect changes in the subduction process through time. The absence of high-conductivity along Archean sutures appears to be global in nature and related to reduced continental freeboard in the Archean which limited continental weathering and the delivery of carbon-rich sediments to the seafloor. Although the entire Wyoming Province has been proposed to have undergone lithospheric modification that lessened its stability, the resistivity model suggests a thick lithospheric root remains in place except along its western margin. These results suggest that Archean cratons may be more resistant to lithospheric modification by influx of heat and fluids associated with extension and plumes than previously thought, and that metasomatism does not necessarily weaken the lithosphere and set a craton on the path to destruction.

Strong Physical Contrasts Across Two Mid‐Lithosphere Discontinuities Beneath the Northwestern United States: Evidence for Cratonic Mantle Metasomatism

“…These two pieces of evidence suggest that even the mantle lithosphere beneath the core of the Wyoming craton is unusually conductive compared to that of an intact craton, probably due to fluids or melts introduced through metasomatism. Compared to the core of the Wyoming craton, the regions near its boundary were shown to be underlain by mantle lithospheres with even lower resistivity (Bedrosian & Frost, 2023), evidence for stronger effects from metasomatism. Given that our station RSSD is located near the eastern boundary of the Wyoming craton (Figures 1a and 1b), a metasomatic origin of the MLDs beneath the station is generally consistent with the results of Bedrosian and Frost (2023).…”

“…Compared to the core of the Wyoming craton, the regions near its boundary were shown to be underlain by mantle lithospheres with even lower resistivity (Bedrosian & Frost, 2023), evidence for stronger effects from metasomatism. Given that our station RSSD is located near the eastern boundary of the Wyoming craton (Figures 1a and 1b), a metasomatic origin of the MLDs beneath the station is generally consistent with the results of Bedrosian and Frost (2023). Distinguishing between melts and volatile‐bearing phases as the cause of our observed velocity drops requires a quantitative analysis of the resistivity model, which is beyond the scope of this paper.…”

“…Nonetheless, their results showed that the mantle resistivity above ∼175 km depth beneath two locations in the core of the Wyoming craton is significantly lower than that beneath the Superior province in the same depth range (Figure 7a in Bedrosian and Frost (2023)). In addition, the mantle resistivity in this depth range beneath the two Wyoming locations is also lower than the one predicted for dry olivine, an end-member composition for an intact cratonic mantle lithosphere (Figure 7a in Bedrosian and Frost (2023)). These two pieces of evidence suggest that even the mantle lithosphere beneath the core of the Wyoming craton is unusually conductive compared to that of an intact craton, probably due to fluids or melts introduced through metasomatism.…”

“…Specifically, electrical-resistivity models may be used to distinguish between MLD models with hydrous minerals and melts as the cause for velocity reductions because the latter is likely significantly more conductive than the former (Section 3.4). Bedrosian and Frost (2023) presented a 3D electrical-resistivity model of the Wyoming craton and the surrounding region and argued that the lithosphere mantle below the Wyoming craton is largely unaffected by metasomatism as evidenced by the mantle high-resistivity body extending down to ∼200 km. Nonetheless, their results showed that the mantle resistivity above ∼175 km depth beneath two locations in the core of the Wyoming craton is significantly lower than that beneath the Superior province in the same depth range (Figure 7a in Bedrosian and Frost (2023)).…”

“…Bedrosian and Frost (2023) presented a 3D electrical-resistivity model of the Wyoming craton and the surrounding region and argued that the lithosphere mantle below the Wyoming craton is largely unaffected by metasomatism as evidenced by the mantle high-resistivity body extending down to ∼200 km. Nonetheless, their results showed that the mantle resistivity above ∼175 km depth beneath two locations in the core of the Wyoming craton is significantly lower than that beneath the Superior province in the same depth range (Figure 7a in Bedrosian and Frost (2023)). In addition, the mantle resistivity in this depth range beneath the two Wyoming locations is also lower than the one predicted for dry olivine, an end-member composition for an intact cratonic mantle lithosphere (Figure 7a in Bedrosian and Frost (2023)).…”