Seismic anisotropy in the mantle transition zone beneath Fiji-Tonga

[1] We demonstrate a rare, unequivocal case of anisotropy in the transition zone (TZ), evident from many horizontal-polarized shear-waves (SH) that arrive up to 3 s earlier than vertical-polarized shear-waves (SV). The anisotropic region correlates with deep earthquakes away from the active subduction zone and known boundaries of lateral heterogeneities in compression- and shear-wave speeds. We interpret the anisotropy as strong fabric associated with a large petrologic anomaly in the core of remnant lithosphere which is buoyant and stagnates in the TZ. Fast subduction of cold slab actually favors such a configuration, so slab penetration into the lower mantle does not uniformly occur.