Seismic Anisotropy in the Java‐Banda and Philippine Subduction Zones and its Implications for the Mantle Flow System Beneath the Sunda Plate

Abstract

AbstractThe Sunda Plate is a minor tectonic plate bounded by tectonically active convergent boundaries, below which are subducting: the Philippine Sea Plate to the east and the Indo‐Australian Plate to the south and west. It is thus an ideal tectonic setting for investigating the interaction between subduction and asthenospheric flow. To better understand mantle interactions within the two nearly perpendicular subduction zones, we characterize seismic anisotropy by conducting a source‐side sS splitting analysis, which allows us to improve spatial resolution of anisotropic fabrics, in particular underneath the backarc regions, which are poorly constrained by previous studies. In the backarc of the Java‐Banda subduction zone, a gradual fast‐axis rotation from trench normal in the west to trench parallel in the east is clearly observed. We attribute this rotation to the interactions between the 2‐D corner flow in the Java wedge and a squeezed asthenospheric flow by the highly arcuate Banda slab. In the backarc of the Philippine subduction zone, the fast‐axis direction transitions from trench normal in the central south to trench oblique in the north; the trench normal is attributed to the mantle wedge corner flow, whereas the trench oblique is likely deflected by the eastward subduction of the South China Sea Plate. Hence, the mantle flow system beneath the Sunda Plate is composed of various types of flow developed in the mantle wedges. Their interactions play an important role in influencing greatly the regional geodynamics in the upper mantle above the 670‐km discontinuity.