The Seismic Properties of the Southeastern U.S. Asthenosphere

Abstract:

Asthenospheric properties vary widely beneath continents. Beneath the tectonically and magmatically active western U.S., a shallow asthenospheric layer with very low seismic velocities exists. The top of this layer is typically marked by a negative velocity gradient that is too vertically localized (< 30 km) to be explained by a thermal gradient alone. In some regions the positive velocity gradient at the lower boundary of the asthenosphere also appears to be localized enough to produce scattered waves. Beneath the thick, high velocity cratonic mantle lithosphere in the U.S., a low velocity asthenospheric layer is present, but it is deeper, its seismic velocities are higher, its velocity contrast with the overlying lithosphere is smaller, and this velocity gradient is typically gradual enough (> 50 km in depth) to be consistent with a purely thermal gradient. These contrasts in seismic properties point to lateral variations in asthenospheric viscosities and in the degree of coupling between the lithosphere and deeper mantle.

As the EarthScope Transportable Array completes its time in the eastern U.S., the asthenosphere beneath this region of Paleozoic orogenesis and Mesozoic rifting is coming into focus. In the southeastern U.S., we have analyzed data from the 85 broadband stations of the 2010-2014 SESAME EarthScope Flexible Array and adjacent Transportable Array and permanent stations. Rayleigh wave tomography reveals a low velocity asthenosphere beneath a lithospheric layer of intermediate thickness (typically 120-130 km). Minimum asthenospheric velocities lie between values in the western U.S. asthenosphere and those in the cratonic asthenosphere. Common conversion point stacks of Sp receiver functions show little coherent scattering from the top of the asthenosphere, indicating that the lithosphere-asthenosphere boundary is relatively gradual, with the exception of a few zones (such as one in the north of our region where the Appalachian orogen meets the craton). SKS splitting fast polarizations across the region follow a pattern with a 90˚ period in backazimuth. This result is consistent with different orientations of olivine a-axes in the asthenosphere versus the lithosphere, and will be compared to depth-varying azimuthal anisotropy from the Rayleigh wave tomography.