Shear‐wave splitting in the Rio Grande Rift

Fast directions and lag times associated with SKS shear‐wave splitting are determined for six digital stations in the Rio Grande Rift (RGR). The mean fast direction for three stations in the central RGR is about 45° oblique to the axis of the RGR. In contrast, fast directions approximately parallel to the axis of the RGR are found for two stations in the southern RGR (∼10°), and one station in the northern RGR (∼26°). The fast directions for southern stations are not aligned with the NE‐SW extension direction of the Oligocene to early Miocene stage of extension. The significant N‐S lateral variation in the SKS shear‐wave splitting parameters is difficult to explain by fossil anisotropy because the mantle structure beneath the RGR is underlain by hot material with low seismic‐wave velocities. Hence the fast direction of azimuthal anisotropy is probably related to the preferred orientation of olivine created by the present deformation in the upper mantle. Because none of the fast directions are parallel to the present E‐W extension direction of the RGR the dominant upper mantle flow direction is not perpendicular to the axis of the RGR. The pattern of the fast directions along the RGR provides evidence for a rising asthenosphere beneath the central to southern RGR and suggests that thermally driven small‐scale convection may be faster for flow parallel to the rift than perpendicular to it.