Crustal and uppermost mantle structure in the central U.S. encompassing the Midcontinent Rift

Abstract

Rayleigh wave phase velocities across the western arm of the Midcontinent Rift (MCR) and surrounding regions are mapped by ambient noise (8–40 s) and earthquake tomography (25–80 s) applied to data from more than 120 Earthscope/USArray stations across the central U.S. Receiver functions also are computed for those stations using harmonic stripping. Joint Bayesian Monte Carlo inversion is applied to generate 3‐D posterior distributions of shear wave speeds (Vsv) in the crust and uppermost mantle to a depth of about 150 km, providing an overview of the seismic structure of the MCR and adjacent structures. Three major structural attributes are identified: (1) There is a high correlation between the long‐wavelength gravity field and shallow Vs structure, but the MCR gravity high is obscured by clastic sediments in the shallow crust. This is consistent with an upper crustal origin to the MCR gravity anomaly as well as other anomalies in the region. (2) Thick crust (>47 km) underlies the MCR, but there is a gradient Moho in the northern part of the rift and a sharp Moho in the south. Thickened crust beneath the MCR is evidence for postrifting compression with pure‐shear deformation along the entire rift; along‐rift differences in lower crustal structure may signify magmatic underplating in the northern rift. (3) Crustal shear wave speeds vary across the Precambrian sutures (e.g., Great Lakes Tectonic Zone, Spirit Lakes Tectonic Zone). This reveals the importance of Precambrian sutures in the subsequent tectonic evolution of the central U.S.