In Southeast Asia, emerging subduction zones often appear to begin at the corners of small oceanic basins, which have a triangular-indenter continent–ocean boundary geometry. To investigate the influence of a triangular indenter on subduction initiation, we performed a series of three-dimensional numerical simulations with varying indenter angles and base lengths. The results show that the apex of the indenter constitutes the initial location of subduction, irrespective of the angle or the extent of the indenter. Smaller angle indenters are more likely to facilitate subduction initiation. At the same time, wide acute angle indenters are difficult to form. Our findings suggest that triangular indenter structures may facilitate subduction initiation in smaller basins; however, the role such indenters in subduction initiation is limited in larger basins. Our results emphasize the importance of accounting for the three-dimensional geometry of a subduction zone when examining its subduction dynamics and geological features.

The transition of a passive continental margin into a subduction zone remains a hypothesis because few geological cases have been reported. The North Sulawesi subduction zone is a 5-9 myr system in Southeast Asia that has evolved from a continental passive margin and has long been overlooked by studies of passive to active margin transitions. Here we compare geophysical data from the region with numerical simulation results. We find that the initial subduction of North Sulawesi must rely on horizontal forces, while the trench retreat depends on the negative buoyancy of the oceanic lithosphere. Furthermore, less space available for subduction leads to lower mantle flow caused by subduction and slower trench retreat. These new dynamical constraints indicate that the negative buoyancy of the oceanic plate is the key factor for subduction and trench retreat, even though the subduction initiation was induced.