MAKING IT AND BREAKING IT IN THE UPPER MIDWEST: CONSTRAINTS ON CONTINENTAL ASSEMBLY AND RIFTING FROM EARTHSCOPE

The North American mid-continent presents a window into craton growth and stabilization as well as the 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. Focusing on the upper Midwest, I will discuss the implications of a three-dimensional resistivity model derived from EarthScope magnetotelluric data.

The resistivity model reveals the distribution of highly conductive Paleoproterozoic meta-sedimentary rocks in Minnesota, Michigan, and Wisconsin. The electrical signature of these rocks is unique throughout the mid-continent region. Their high conductivity is attributed to metallic sulfides and in some cases graphite. The former is considered a potential source of sulfur for various types of mineral deposits found in the region. A more detailed magnetotelluric survey and a recent reconnaissance airborne electromagnetic survey have been carried out to map these rocks in greater detail.

The Paleoproterozoic structural collage is interrupted by the 1.1 Ga Mid-continent Rift System (MRS). The resistivity model reveals a west-to-east asymmetry within the Lake Superior graben, with the Thiel fault speculated to be the dividing line. The asymmetry is taken to reflect the different geometric response of each half of the basin to compression during the Grenville orogeny. The orientation of the rift-bounding faults in relation to the stress field at the time is speculated to have resulted in a greater degree of compression in the western half of the Superior graben than in the eastern half.

The resistivity of the mantle lithospheric beneath the region is surprisingly heterogeneous. The spatial pattern of these variations bears little resemblance to the crustal imprint of past tectonic events or to the direction of North American absolute plate motion. I argue that these resistivity variations reflect differing degrees of hydration (metasomatism) preserved within the lithosphere. Lithospheric hydration in the upper Midwest is speculated to have occurred during MRS magmatism. This interpretation is consistent with geochemical and isotopic analyses of MRS basalts and their inferred mantle sources.