Lithospheric structure of the North American Craton constrained by full waveform inversion

Abstract:

The North American Craton (NAC) is formed with Proterozoic terranes accreted to a shield assembled by several Archean cratons (e.g. Superior, Wyoming, Hearne, Rae, and Slave). While the eastern part of the NAC, i.e., the central and eastern contiguous U.S., is relatively less deformed, the western and southwestern parts of the NAC have undergone various alternation processes since Mesozoic due to the Farallon plate subduction. High-resolution images of the lithospheric structure and lithosphere thickness can help elucidate the dynamic processes that modify the NAC. Extensive studies including receiver functions, seismic tomography, and seismic anisotropy have investigated the lithospheric structure of the NAC since the deployment of the USArray stations. However, the resolved lateral variations of the lithospheric thickness of the NAC still varies a lot amongst these studies. Here, we present a new model of the NAC based on full waveform inversion (FWI) of mainly the USArray recorded earthquake waveforms, with an optimized initial model constructed by combining US.2016 and Crust1.0 in the crust and S40RTS in the mantle. Our FWI initially incorporates 388,376 high-quality frequency-dependent travel-time misfit measurements from 120 regional earthquakes for both body and surface waves. With an increasing number of iterations, the source parameters are re-inverted after every five iterations with the updated structural models. Additional lower magnitude (Mw = 4–5) events and shorter period seismic waves to the shortest period of 10 s are incorporated in the final stage of inversion. Our new model reveals the NAC lithosphere with about +2% shear wave speed anomaly and an average thickness of 200–250 km beneath the Superior Craton. The NAC lithosphere becomes thinner towards the eastern, the southern, and the southwestern margins with a thickness decreased to 100–150 km. The lithospheric thinning beneath the NAC margins indicates the deformation of the lithosphere and is likely controlled by the large-scale mantle convection, therefore relates to the further modification process of the NAC.