Structure and dynamics of the North American lithosphere imaged using waveform inversion of global and USArray data

English Abstract:

The North American continent has had a long, eventful tectonic history. The assembly of the stable cratonic core has undergone numerous collisions and accretion at its boundaries, major rifting episodes within it, as well as the loss of ancient lithosphere beneath parts of it, all of which are examples of cratonic dynamics and evolution. Seismic tomography offers rich evidence on the structure and evolution of the cratonic lithosphere. With the continued deployment of the USArray during the last decade, the North American continent has now been densely sampled with broadband seismic data from west to east coast. We present a new high-resolution model of the upper mantle beneath North America constrained by waveform fits of over 700,000 vertical-component broadband seismograms. 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, and improved resolution for a variety of features in North America.

We focus on central and eastern North America; in particular, the internal structure of the central cratonic core is resolved in detail, with clear delineation from the deformed continental margins. The northern and northeastern boundaries of the cratonic lithosphere closely follow the coastlines, with North America’s and Greenland’s lithospheric roots clearly separated. On the eastern margin of the continent, where multiple episodes of continental rifting are superimposed, the craton boundary coincides with the western extent of the Appalachian orogenic front, with significantly lower lateral velocity gradients than in the west.

Within the cratonic interior, the lithosphere surrounding the 1 Ga failed Mid-Continental Rift shows a reduction in wavespeeds compared to the surrounding craton, likely indicating thermo-chemical alteration of the sub-continental lithospheric mantle, in agreement with results from geochemical and petrological analyses of diamondiferous kimberlites and peridotites. We examine the spatial extent of the lithospheric mantle root and LAB variations across the continent, and compare them with respect to the spatial location of diamondiferous kimberlites. Finally, we discuss potential lithospheric control on the distribution crustal seismicity.