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 orien- tation 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, correspon- dence between observed polarization direc- tion of the fast wave and the trend of Protero- zoic and Paleozoic structures associated with rifts and orogenic belts implies a lithospheric origin for the observed anisotropy. The larg- est splitting times (up to 1.6 s) are observed at stations located in the ocean-continent transi- tion zone, in which the fast directions are par- allel to the Gulf of Mexico continental margin. The parallelism and the geometry of the keel of the craton beneath the study area suggest that asthenospheric fl ow around the keel of the North American craton, lithospheric fab- rics developed during Mesozoic rifting, or a combination of these factors are responsible for the observed anisotropy on stations above the transitional crust.