Double subduction initiation in Molucca Sea: Insights of numerical modelling

The Molucca Sea subduction zone, located between the southern Philippines and northeastern Indonesia, forms a U-shaped divergent double subduction (DDS). Based on previous research, the subduction was initiated in the Sangihe region at 20 Ma, followed by the second subduction in the Halmahera region at 10 Ma. However, the mechanism of initiating the second subduction resulting in DDS remains unknown. In this study, we simulated a series of models by presetting a single subduction zone with the convergence of continents to examine the conditions for the initiation of the second subduction of DDS. By modifying parameters of plate age, subduction angle, and convergence rate. Our preliminary results indicate that plate age has little influence on the formation of DDS. A lower preset angle of the first subduction slab (< 30°) tends to favor the second subduction initiation, while a higher preset angle results in single subduction only.  Furthermore, we found that the convergence rate of two continental plates is a critical factor controlling DDS formation. The second subduction will initiate with a higher total convergence rate (> 7 cm/yr), particularly when the rate of the subducted plate exceeds 6 cm/yr. Our results are comparable to the existing asymmetric DDS under the Molucca Sea.