Complex Patterns of Mantle Flow in Eastern SE Asian Subduction Zones Inferred From P-Wave Anisotropic Tomography

We determine 3-D isotropic and anisotropic P-wave velocity models beneath eastern SE Asia by inverting a large number of P-wave arrival times selected from the ISC-EHB database. Our results reveal detailed structures of the subducting South China Sea (SCS), Negros, Molucca Sea, Philippine Sea, and Banda slabs, the previously subducted Proto-SCS slab and remnants of other paleo slabs, showing long-lived subductions of these oceanic slabs in eastern SE Asia. There is an obvious change in the subduction angle of the SCS slab at 18°N along the Manila trench. We suggest that the relatively low dip angle of the SCS slab to the south of 18°N was caused by subduction of an extinct mid-ocean ridge, and a slab tear is possibly formed at 18°N. This feature is supported by the resolved fast velocity directions (FVDs) of P-wave azimuthal anisotropy in the mantle wedge, which show that 3-D toroidal mantle flow may develop around the southern part of the SCS slab. Trench-normal FVDs are revealed in the deeper mantle wedge of the Sangihe subduction zone, which are associated with the deep subduction and stagnancy of the Molucca Sea slab. A nearly trench-parallel FVD is observed beneath the Molucca Sea slab, which may be caused by trench-parallel extension due to the retreating slab or reflect subslab mantle flow associated with the double-sided subduction. The subducting Banda slab exhibits a curved feature, which greatly affects the flow pattern in the mantle wedge.