Mantle-driven diffuse deformation of the Indian Ocean lithosphere revealed by seismic anisotropy

Broad zones of diffuse deformation across the Indian Ocean floor mark the boundary region where the Indian, Capricorn, and Australian plates meet. Two hypotheses have been proposed to explain this deformation: (1) a slowdown in convergence due to the uplift of the Tibetan Plateau, and (2) an eastward push of the Capricorn plate driven by asthenospheric flow. If only the first mechanism were active, mantle shear beneath the Indian Ocean would be expected to align with absolute plate motion (APM), whereas the second hypothesis predicts predominantly east-west shear. Here we use P waves that convert to S upon surface reflection to determine the direction of mantle shear beneath the Indian Ocean. Fast polarization directions, which reflect the orientation of deformation, are east-west-aligned just west of the Ninetyeast Ridge and rotate parallel to APM near the ridge. We infer that previously proposed broad eastward asthenospheric flow associated with the Réunion hotspot extends to the Ninetyeast Ridge. These observations suggest eastward-directed mantle flow that drives the Capricorn plate, causing differential motion to neighboring plates, and leading to diffuse seafloor deformation.