INSIGHTS INTO NORTH AMERICAN EVOLUTION DERIVED FROM CRUSTAL STRUCTURE ACROSS THE ND-LINE AND TENNESSEE-ILLINOIS-KENTUCKY LINEAMENT

Here we investigate crustal structure across the Nd-line and Tennessee-Illinois-Kentucky Lineament (TIKL), which are Proterozoic geochemical and geophysical boundaries in the mid-continent and can be used to learn about the growth and evolution of the North American craton. The Nd-line marks the boundary between differing lower crustal and upper mantle sources, where the rocks have Nd-model ages greater than 1.55 Ga to the northwest of the boundary and less than 1.55 Ga to the southeast. The location of the Nd-line also correlates with the location of the change in long-wavelength magnetization. The TIKL is a magnetic anomaly that extends from the Grenville Front to the Nd line. This anomaly becomes apparent in short-wavelength magnetic variations, which are commonly attributed to structures in the shallow crust. Using data from EarthScope Transportable Array stations recorded between June 2010 and February 2012, we calculate receiver functions to investigate the thickness and structure of the crust in the mid-continent. These observations can be used to identify whether crustal structure changes across the Proterozoic geochemical and geophysical boundaries. Our observations reveal crust of similar thickness across the Nd-line and the TIKL, which suggests that any differences that may have existed during the formation of these crustal blocks has been removed. Instead, the patterns of crustal thickness coincide with current physiographic regions such as the Ozark Plateau, where the crust averages 43 km thick, and the Illinois Basin, where the crust thickens to close to 50 km. The crust thins to the north of the Ozark Plateau and is 41 km thick beneath northern Missouri where surface elevations diminish. The crustal thicknesses appear to reflect patterns of more recent lithospheric deformation, suggesting that the mid- and lower-crust of different age or origin assimilated during continental formation. Still, the features responsible for producing the Nd line and TIKL persist – possibly owing to their shallow extent.