Distinct mantle flow patterns in the Sunda subduction zone revealed by a layered anisotropic structure

The nearly 6,000 km-long Sunda subduction-collision zone, comprising Sumatra, Java, Banda, and adjacent regions is an ideal locale for investigating interactions between plate (slab) kinematics and plastic flow in the asthenosphere, as well as probing mantle dynamics associated with large-scale anomalous volcanism. These interactions and relationships can be diagnosed by a layered anisotropic structure based on joint analyses of shear wave splitting and receiver functions. The mantle flow field characterized by the layered anisotropic structure exhibits distinct patterns from west to east. In the Sumatra subduction zone, slab advance drives mantle flow from the sub-slab mantle entering the mantle wedge through a slab window and flowing horizontally parallel to the trench, and its vertical component contributes to the significant amount of magma of the Toba supervolcano. By contrast, in the Java subduction zone, slab rollback makes mantle flow traverse a slab window in central Java, where a clockwise rotational pattern in the fast orientations is revealed. The vertical component of the flow field is responsible for the formation of the potassic volcanoes in central Java. To the east, in the Banda arc-continent collision zone which signifies one of the most seismically active and tectonically intricate zones, the mantle flow field is also strongly influenced by plate (slab) kinematics and shows trench-parallel flow pattern in both the sub-slab and wedge mantle. The modulated trench-parallel plastic mantle flows lead to the steepening of the southern portion of the asymmetric spoon-shaped Banda slab, diminishing the vertical viscous flow and the associated dynamic topography, and contributing to the uplift of the Banda basins.