Mantle flow entrained by the Hindu Kush continental subduction inferred from source-side seismic anisotropy

The intermediate-depth seismicity below the Hindu-Kush orogen is thought to mark the Indian-plate subduction with the bottom half of the slab currently breaking off. Unique features of this continental subduction are the near-vertical slab and the roughly stationary convergence boundary. How this subduction affects the mantle flow patterns remains to be understood. In this study we measured source-side shear wave splitting on the S waves from Hindu Kush intraslab events to sample the surrounding mantle. The observed fast polarization directions exhibit a circular pattern around the slab resembling that predicted for the toroidal flow driven by slab rollback. However, the rollback scenario is not favored because it hardly sustains in dynamic models without a considerable retreat of convergence boundary. We propose that the observed pattern is produced by the sub-vertical shear flow entrained by the steep descent of the slab and the ongoing breakoff. This scenario requires the existence of A-type or AG-type olivine fabrics with strong orthorhombic anisotropy in mid- to lower upper mantle, which is consistent with the global models of azimuthal and radial anisotropy. This interpretation circumvents the debate on the cause of trench-parallel anisotropy in some oceanic subduction zones where slab entrainment and rollback may coexist, and supports the notion that orthorhombic anisotropy of olivine may play an important role in shaping mantle anisotropy.