Multi-Layer Seismic Anisotropy Beneath Greenland

Seismic anisotropy provides insight into past episodes of lithospheric deformation and the orientations of strain in the underlying asthenosphere. The Greenland mantle has played host to a rich history of tectonic processes, including multiple orogenies and plume-lithosphere interactions. This study presents new measurements of SKS splitting that reveal strong variations in fast polarization direction with back-azimuth that are consistent across Greenland, including at stations where splitting measurements have not previously been reported. We compared observed fast polarization directions to the predictions of two-layer models with olivine-orthopyroxene anisotropy. The family of models that provides acceptable misfits at 95% confidence indicates an upper layer olivine a-axis azimuth of 222-236 degrees, a lower layer olivine a-axis azimuth of 114-130 degrees, and non-zero a-axis plunges. These models are consistent with an upper layer of lithospheric anisotropy due to Proterozoic and Archean orogenic fabrics, and a lower layer of anisotropy corresponding to either asthenospheric flow aligned approximately parallel to the direction of absolute plate motion and plunging due to lithospheric topography or dipping lithospheric structures created by episodes of paleo-convergence.Plain Language Summary Measurements of seismic anisotropy (the direction-dependent variation in seismic wavespeed) provide useful information about the orientation of deformation in the Earth. We measured seismic anisotropy using shear waves refracted through the outer core and recorded by stations in Greenland. Due to new stations and data, this study includes more measurements of the effects of anisotropy than previously possible. We show that a model with two layers of anisotropy explains dominant patterns in the fast vibration direction of the shear waves as a function of the angle at which they approach each station. We suggest that the shallow layer reflects coherent deformation in the continental lithosphere of Greenland due to its history of plate collisions and that the lower layer reflects deformation in the asthenospheric mantle induced by the motion of the plate above or a second layer of lithospheric anisotropy.