Lithospheric Structure of the North American Continent from Ambient Noise Tomography And Wave Gradiometry

Abstract:

In order to better understand the formation and tectonic evolution of the North American continent we utilize data from the EarthScope Transportable Array network to calculate a 3D shear-velocity model for the continental United States. This model was produced through the inversion of Rayleigh wave phase velocities calculated using ambient noise tomography and wave gradiometry, which allows for sensitivity to a broad depth range. Seismic velocities within this model highlight the diverse lithospheric character across the continent and preliminary results are consistent with potential active tectonics within the eastern US.

Preliminary observations show that seismic velocities in the shallow crust correlate with sediment thickness, as the slowest velocities are observed along the Gulf Coast where thick sediment is found and the fastest velocities are observed in the cratonic north-central United States. Mid-crustal velocities appear to largely correlate with regions of active tectonics with slow velocities observed in the Basin and Range province, the Rio Grande Rift, and along the Pacific Coast. However, anomalous low velocity regions are observed in Iowa, Indiana, and West Virginia that warrant further investigation. Velocities between 30-40 km depth appear to be largely controlled by crustal thickness with fast velocities observed in regions of thinner crust and slower velocities in thick crustal regions.

In the upper mantle there is a sharp contrast between relatively slow seismic velocities in the western US and fast velocities in the eastern US that correlates well with the eastern limit of Laramide deformation. However, there are several pockets of low velocities in the upper mantle beneath western Virginia and New England that may suggest active tectonic processes in the mantle beneath these regions. Finally, within the deeper mantle (>100 km depth) the fastest velocities are observed beneath the cratonic regions of the north central US and slower velocities along the continental margins.

This model highlights the complexities of the lithospheric structure of the continental US and preliminary results are consistent with active tectonics occurring in pockets beneath the eastern US lithosphere. Future work will focus on better resolving and understanding these Eastern US velocity perturbations.