We present new shear-wave splitting data showing systematic lateral variations in upper-mantle anisotropy across the plate boundary in southernmost California (USA). Beneath the Peninsular Ranges batholith, fast polarization directions parallel the direction of former Farallon subduction, suggestive of a slab remnant. Near the eastern edge of the batholith, across the Elsinore fault, fast polarization directions change rapidly to align with the direction of San Andreas fault shear. We infer that the Elsinore fault penetrates the entire lithosphere and may represent a future localization of the plate boundary that is migrating west from the San Andreas fault. Beneath the Salton Trough and the Chocolate Mountains region, large splitting times, despite a very thin lithosphere, imply vertical melt pockets in the uppermost mantle aligned in the shear direction. Largest splitting times, ∼1.2 s, are seen closest to the Sand Hills fault that projects southeast from the San Andreas fault. Further east, in the southern Basin and Range province, fast directions align with North America absolute plate motion.