Seismic anisotropy and mantle dynamics beneath the southeastern United States inferred from shear-wave splitting analysis

Abstract:

Systematic spatial variations of mantle azimuthal anisotropy are revealed by over 3000 pairs of high-quality shear-wave splitting parameters (fast polarization orientations and splitting times) recorded at ~400 USArray and other network stations in the SE United States (75˚-90 ˚ W, and 24˚ -40˚ N). The fast polarization orientations observed in the continental interior are subparallel to the absolute plate motion (APM) direction of the North American plate with apparent larger-than-normal splitting times, indicating a significant asthenospheric contribution. Fast orientations parallel to the edge of the North American craton are revealed along the southern and eastern margins of the continent. A portion of the eastern coastal area shows weak anisotropy, probably indicating the existence of vertical mantle flow. The majority of the splitting measurements can be satisfactorily explained by a model involving simple shear in the boundary layer between the lithosphere and asthenosphere. The model includes three flow systems. The first is related to the continental scale APM-parallel relative movement between the lithosphere and asthenosphere which creates the APM-parallel fast orientations observed in the continental interior. The second flow system is associated with the deflection of asthenospheric flow around the edges of the craton and is responsible for the edge-parallel fast orientations observed along the southern and eastern margins of the study area. The third system is sub-vertical, possibly caused by vertically deflected flow along the eastern root of the craton, similar to the mechanism proposed by Refayee et al. (2014, doi: 10.1016/j.epsl.2013.01.031) for the western edge. It could also be related to previously proposed upwelling or down-welling flow. This study demonstrates the essential role that shear-wave splitting measurements played in the investigation of mantle dynamics.