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

Abstract

AbstractWe 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.