Seismic anisotropy of the Archean crust in the Minnesota River Valley, Superior Province
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The Minnesota River Valley (MRV) subprovince is a well-exposed example of late Archean lithosphere. Its high-grade gneisses display a subhorizontal layering, most likely extending down to the crust-mantle boundary. The strong linear fabric of the gneisses results from high-temperature plastic flow during collage-related contraction. Seismic anisotropies measured up to 1 GPa in the laboratory, and seismic anisotropies calculated through forward-modeling indicate ΔVP ~5-6% and ΔVS ~3%. The MRV crust exhibits a strong macroscopic layering and foliation, and relatively strong seismic anisotropies at the hand specimen scale. Yet the horizontal attitude of these structures precludes any substantial contribution of the MRV crust to shear wave splitting for vertically propagating shear waves such as SKS. The origin of the regionally low seismic anisotropy must lie in the upper mantle. A horizontally layered mantle underneath the United States interior could provide an explanation for the observed low SWS. Key Points The Archean crust of the Minnesota River Valley is strongly anisotropic The horizontally layered crust of the MRV cannot split vertical shear waves The cause of low SWS in the MRV must be in the uppermost mantle ©2014. American Geophysical Union. All Rights Reserved.
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