Characteristics of mantle fabrics beneath the south-central United States: Constraints from shear-wave splitting measurements

Abstract

New shear-wave splitting measurements at permanent broadband seismic stations in the south-central United States reveal the orientation and degree of polarization of mantle fabrics, and provide constraints on models for the formation of these fabrics. For stations on the stable North American craton, correspondence between observed polarization direction of the fast wave and the trend of Proterozoic and Paleozoic structures associated with rifts and orogenic belts implies a lithospheric origin for the observed anisotropy. The largest splitting times (up to 1.6 s) are observed at stations located in the ocean-continental transition zone, in which the fast directions are parallel to the Gulf of Mezico continental margin. The parallelism and the geometry of the keel of the craton beneath the study area suggest that asthenospheric flow around the keel of the North American craton, lithospheric fabries developed during Mesozoic rifting, or a combination of these factors are responsible for the observed anisotropy on stations above the transitional crust. ©2008 Geological Society of America.