Crustal and mantle seismic anisotropy beneath the ‘Great Lakes Mantle Divot’ and adjacent areas: Constraints from shear-wave splitting measurements

Abstract:

The recent availability of high-quality broadband seismic data from the deployment of the USArray Transportable Array stations from the Great Lakes to the eastern coast of North America provides a unique opportunity to investigate the origin of seismic anisotropy and to infer crustal/mantle structure and dynamics. Crustal anisotropy is quantified by using the sinusoidal moveout of the P-to-S converted phases from the Moho, and the integrated crustal/mantle anisotropy is measured using SKS, SKKS, and PKS (XKS) data recorded by 385 stations. A total of 3310 pairs of well-defined XKS splitting parameters are observed in the vicinity of the so-called Great Lakes Mantle Divot and adjacent areas. The measurements show systematic spatial variation of anisotropic characteristics. Fast polarization orientations subparallel to the absolute plate motion (APM) of North America are observed at most stations on and east of the Appalachians in the latitudinal range of 40° to 50°N with large splitting times (up to 1.7s). This area has a thin lithosphere relative to the continental interior to the west, indicating an asthenospheric origin for the observed azimuthally invariant splitting parameters which reflect a single-layer of anisotropy with a horizontal axis of symmetry. The dominant crustal anisotropy in this area is N-S oriented. Azimuthal varying splitting parameters are observed at most stations in the vicinity of the Great Lakes Mantle Divot, and can be adequately explained by a two-horizontal layer model. The lower layer has an APM-parallel fast orientation and the fast orientation of the upper layer is mostly consistent with the crust anisotropy. Based on the splitting measurements and previous seismic tomography studies, a model involving deflecting of asthenosphere flow by the undulating bottom of the lithosphere is proposed.