New Insights into the Tectonics of the Midcontintent of U.S.A. from EarthScope USArray Seismic, Gravity, Magnetic and Heat Flow Datasets

Abstract:

We examine crustal and upper mantle structure and physical parameters from EarthScope USArray receiver functions, seismic tomography, gravity, and magnetic anomaly data to understand the tectonic framework of the midcontinent of the U.S. (New Madrid Seismic Zone/Rift Complex, Illinois Basin, Tennessee-Illinois-Kentucky Lineament (TIKL), and Grenville Front). The neodymium (Nd) crustal formation boundary seen in geochemical and the long-wavelength magnetic anomaly data is not apparent in the crustal or mantle seismic parameters or density. This is not completely surprising since the Nd systematics and magnetization is controlled by accessory minerals rather than bulk physical parameters. The USArray station spacing is not sufficient to capture the density and corresponding magnetic variations associated with mafic plutons except where stations are located over the plutons themselves (e.g., the Bloomfield Pluton). With the standard Moho density contrasts, Vp/Vs based crustal densities, and mantle velocity-density relationships, significant long-wavelength residuals (misfit) between observed and modeled Bouguer gravity anomalies remain over the Illinois Basin, the TIKL, and the Grenville Front in Ohio, extending north into Lake Erie and New York. Jointly inverted USArray receiver functions and gravity based Vp/Vs and the crustal thickness suggest that the Moho in the western Illinois basin and the Grenville Front in Ohio are thicker than previously known (about 50 km). The bottom of the strong crustal magnetization layer is unusually thin (15-20 km) in the New Madrid Seismic Zone, which extends northward from there into Illinois, northern Missouri and Indiana. In the Rough Creek Graben, the magnetic layer thickness is about 40 km, which is consistent with the non-magmatic nature of that branch of the rift. These observation may also imply that the rift related basal crustal layer in the New Madrid Seismic Zone is non-magnetic or very weakly magnetic. The thinner magnetic layer is also present over much of the Eastern Granite-Rhyolite Province (except in its southeastern part).