Geodynamics of divergent double subduction: A case study of the Molucca Sea microplate in Southeast Asia

Divergent double subduction (DDS), characterized by simultaneous subduction on opposing sides of a single oceanic plate, is pivotal in the study of plate convergence and collisional processes. However, its geodynamics remain poorly understood due to the limited and exclusive nature of current observations. The Molucca Sea microplate in Southeast Asia—where oppositely dipping slabs form an asymmetric, inverted U shape—provides an ideal case for studying the geodynamics of DDS. Although the present-day slab configuration is well documented, the formation and dynamic evolution of this asymmetry have not been fully resolved. In this study, we integrated plate kinematics with numerical geodynamic modeling to explore the deep processes driving the formation of the asymmetric DDS in the Molucca Sea microplate. Our results reveal that a major plate reorganization at ca. 25 Ma initiated Sangihe subduction along a transform plate boundary, driven by triple junction instability. Halmahera subduction on the eastern side began at ca. 15 Ma, dominated by the westward motion of the Philippine Sea plate, which challenges the view that DDS formation can occur without external forcing. Multiple upper-mantle flow patterns—including lateral escape, poloidal, toroidal, trench-normal, and corner flows—collectively maintain the asymmetric DDS geometry. Notably, trench-normal flow above the subducting Halmahera slab potentially promotes the ongoing retreat of the Halmahera Trench, significantly influencing the dynamic interactions between the Pacific and Tethyan tectonic domains. These findings offer new insights into the geodynamic controls on DDS within the complex convergent environment of Southeast Asia.