Anisotropic structure of the central North American Craton surrounding the Mid-Continent Rift: Evidence from Rayleigh waves

Abstract

The subsurface of the central North American Craton has been imaged by body-, surface-, and full-waveform studies at varying resolutions. These studies offer tantalizing clues about the evolution of Archean and Proterozoic lithosphere. The oldest cratonic lithosphere may have been formed under a different or pre-plate-tectonic regime and, in this region, was later modified by orogenesis around the edges, hotspot passage, rifting and magmatism. We improve the resolution of seismic imaging across the Great Lakes region of North America by carrying out two-station phase velocity dispersion measurements at selected station pairs and inverting them for anisotropic phase velocity maps at periods 20–200 s. We also perform extensive resolution tests to identify robust features in the data. Isotropic features to note are the strong signatures of the Trans-Hudson Orogen, Superior Craton, and Mid-Continent Rift (MCR) at periods most sensitive to the lower crust and uppermost mantle, relatively low velocities near the Great Lakes region at periods most sensitive to the middle lithosphere, and extremely fast velocities in the western Superior at long periods, corresponding to the lowermost cratonic lithosphere. We note a strong contrast in seismic anisotropy across the MCR, with strong anisotropy to the north and weaker anisotropy to the south at shorter periods, consistent with observations from other data types and studies. Fast orientations are heterogeneous within the Superior craton at intermediate periods. At periods ⩾160 s, an increase in magnitude of the anisotropy, and coherence of the fast orientation, suggest an asthenospheric contribution to the signal.