Imaging the North American continent using waveform inversion of global and USArray data

The deployment of USArray during the last decade has produced dense sampling of the central part of the North American continent with broadband seismic data. Regional tomography is now mapping the deep structure of the continent in great detail, in particular beneath the western US where USArray initiated. At the scale of the entire continent, however, the resolution of seismic imaging is uneven, much poorer away from the footprint of the array than beneath it. Important questions regarding the deep structure, lateral extent and evolution of the North American Craton, most of it not covered by USArray, thus remain difficult to answer. Here we present a new model of the upper mantle beneath North America constrained by waveform fits of 717 thousand vertical-component, broadband seismograms, of which over 228 thousand are from the Transportable Array component of USArray, a few tens of thousands from other USArray-affiliated stations, and the rest from global networks and arrays. Automated, multimode waveform inversion was used to extract structural information from surface and S waveforms, yielding resolving power from the crust down to the transition zone. Our unprecedentedly large waveform dataset, with highly complementary USArray and global-network subsets within it, produces improved resolution for a variety of features in North American upper mantle, compared to other available models. The internal structure of the North American Craton is resolved in detail. The lithosphere beneath the 1 Ga failed Mid-Continental Rift shows wavespeeds not as high as beneath surrounding cratons; it was probably altered. The sharp northern boundaries of the cratonic lithosphere closely follow the coastlines, with North Americaʹs and Greenlandʹs lithospheric roots clearly separate. Sharp velocity gradients in western Canada indicate that the craton boundary at depth closely follows the Rocky Mountain Front at the surface. High velocities between the Great Bear Arc and Beaufort Sea provide convincing evidence for the recently proposed ‘MacKenzie Craton’, unexposed at the surface.