Moho depth (crustal thickness) variations under the northeastern midcontinent of North America, based on H-κ−c receiver-function analysis

Abstract

Previous studies of crustal thickness variation in the cratonic platform and bordering foreland basin of the USA Midcontinent emphasize that Moho relief exceeds topographic relief by an order of magnitude and exceeds structural relief of the Great Unconformity (the Phanerozoic-Precambrian contact) by a factor of 2 to 3. Consequently, the Moho displays significant local slopes. Unfortunately, traditional receiver-function analysis of depth to the Moho can lead to inaccurate results when applied to steeply sloping Moho. Therefore, to decrease measurement uncertainty due to slopes, we applied the recently developed H-κ-c receiver-function method (Li et al., 2018) to the northern and part of the eastern Midcontinent of the United States to produce a higher-resolution map of Moho-depth (i.e., crustal thickness) variation. Results for the central Midcontinent (including the Ozark Plateau and southern Illinois Basin) were reported in Xiao et al. (2022). Here, we extend our coverage eastward and northward, across the Michigan Basin, Grenville front, and western Appalachian Basin.

Our results emphasize that crustal thickness varies by almost 13 km in a region of North America where land-surface varies by less than 0.5 km and where relief of the Great Unconformity varies by a maximum of 7.5 km. In contrast to contemporary orogenic belts, crustal thickness does not correlate directly with either land-surface elevation or with sedimentary cover thickness (i.e., depth to the Great Unconformity). For example, the thickest crust of the study area occurs in the southwestern Illinois Basin, where land surface elevation is about 150 m, and the thinnest crust occurs in north central Indiana, where land-surface elevation is 250 m. There is a rough correspondence between crustal thickness and epeirogenic structures. In general, thinner crust underlies domes and arches, whereas thicker crust underlies basins, but there are exceptions. For example, while crust is relatively thin beneath the Kankakee and Cincinnati Arches, it is relatively thick beneath the Findlay Arch and the Wisconsin Arch, crust beneath the Michigan basin, overall, is thinner than that of the Illinois or Appalachian basins, and crust beneath the Canadian Shield is thinner than that beneath the cratonic platform. Notably, the Moho beneath the Canadian Shield and beneath the cratonic platform west of the Grenville front, displays roughly periodic long-wavelength north-south trending undulations. These undulations do not coincide with Precambrian tectonic fabrics or crustal boundaries, hinting that they developed after crustal assembly and could instead reflect variable degrees of thinning, underplating, or crustal delamination during failed Proterozoic rifting, or perhaps of crustal buckling associated with the Grenville collision.