From Archean to Present – Structure and Evolution of the North American Mid-Continent through the Lens of EarthScope Magnetotelluric Data

Abstract:

The North American mid-continent presents a window into craton growth and stabilization as well as a 1.1 Ga rifting event that nearly tore Laurentia apart. Unique to this region is the preservation of this tectonic collage, largely unmodified by subsequent tectonic events, which permits examination of if and how such events are preserved in the continental lithosphere.

I will present a three-dimensional lithospheric-scale resistivity model derived from EarthScope magnetotelluric data. I will discuss details of the resistivity model in relation to lithospheric sutures, defined primarily from aeromagnetic and geochronologic data, which record the southward growth of the Laurentian margin in the Precambrian. I will examine in detail the resistivity signature of the 1.1 Ga Mid-Continent Rift System as relates to rift geometry, extent, and segmentation. I further speculate that axial drainage along the southwest rift arm created an unrecognized expanse of (concealed) Precambrian deltaic deposits in Kansas akin to the Rio Grande delta that today drains multiple sub-basins within the Rio Grande Rift.

The resistivity model also reveals the distribution of highly conductive Paleoproterozoic metasediments in Minnesota, Michigan, and Wisconsin. The high conductivity is attributed to metallic sulfides and in some cases graphite. The former is considered a potential source of sulfur for multiple types of mineral deposits found in the region.

Finally, I consider the imprint left within the mantle following the 1.1 Ga rifting event. Throughout the mid-continent, variations in lithospheric mantle conductivity are revealed, most likely reflecting variations in hydration (depleted versus metasomatized mantle). The spatial pattern of conductivity variations bears little resemblance to past tectonic events or to the direction of North American absolute plate motion.